TIGHAR
Amelia Earhart Search Forum => Celestial choir => Topic started by: Liz Smith on January 23, 2011, 02:13:59 PM
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Hi everyone,
I've posted some new ideas about possible navigation errors due to the International Date Line on my website - www.datelinetheory.com. As far as I can tell, the Date Line has not been considered before as a direct cause for Earhart and Noonan becoming lost and/or not reaching Howland Island.
I'm very curious to hear what forum members have to say about my theory, given all of the experience and expertise of the group. I'd be grateful to hear any comments or thoughts on the matter. I propose the theory as one possible scenario, not necessarily the only scenario possible and I remain open to all the possibilities of what happened to the fliers until conclusive evidence of the Electra is found.
Thanks for your time,
Liz Smith
The Date Line Theory
www.datelinetheory.com
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Hi Liz,
I commend your enthusiasm and your willingness to subject your theory to review by this forum.
I see a few problems.
1. You've fallen into the "would have" trap. As I've said in other threads on this forum, the phrase "would have" masquerades a guess as a fact. You say:
"Even though a date error can have the same effect on navigation as a chronometer error, it was not as much of a practical consideration for Noonan. He would have understood the implications of using the wrong date, but for the most part in his celestial navigation experience, flights had been conducted over the course of one day, and, if the flights were overnight, it was relatively straightforward - his charts were arranged such that he barely noticed the change of dates as a flight progressed. It would not be a major concern of his to get the correct date – it would be an obvious, simple matter of knowing what day it was. Noonan and Amelia Earhart were set to take off on July 2, 1937 and that is the date he would have used to look up figures in his charts. As the night progressed and it became July 3rd, he would continue down his charts in the almanac as usual."
2. You also say, "...his charts were arranged such that he barely noticed the change of dates as a flight progressed." With all due respect, you don't know how his charts were arranged. No one does.
3. Your theory is based upon the proposition that a navigator of Noonan's expertise and experience neglected to notice that his course would cross the International Dateline. During his career with Pan American he navigated the Clippers across the Pacific multiple times, crossing the Dateline on every trip. Accounting for the Dateline should have been routine for him.
4. Like many others, you also assume that Earhart and Noonan did not have the correct lat/long coordinated for Howland. This assumption is usually based on the fact that the strip charts prepared by Clarence Williams for Earhart's first world flight attempt show outdated coordinates for Howland. But Bill Miller, the Bureau of Air Commerce representative who later assisted Earhart in her preparations for the world flight, had the correct coordinates for the island. It seems inconceivable that he didn't give her the correct lat/long.
5. As an aside, the video on your "Final Thoughts" page is erroneously captioned "These are the last pictures taken of the female flyer Amelia Earhart. ..." That newsreel clip dates from preparations for the first world flight attempt in March 1937. Not even close to being "the last pictures."
6. Finally, any theory that holds that Earhart and Noonan crashed and sank at sea must somehow account for the post-loss radio signals that could only have been sent if the aircraft was on land and able to operate the right hand, generator-equipped engine.
In constructing his theory of how, why, and where Earhart went down, my old friend and adversary Elgen Long stood the scientific method of inquiry on its head. Rather than assembling the available evidence, then constructing and testing a hypothesis, Elgen began with the received wisdom that the Electra hit the water moments after the last transmission heard by Itasca. He then selectively interpreted the scant available evidence to build a case to support his theory.
I fear you may have followed a similar path.
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Hi Ric,
First, thanks so much for taking the time to take a look at the Date Line Theory and for your thorough comments. I'll try to address your comments above as best I can.
1. I understand the criticism about using the term "would have" - I've done my best to take the phrase out of my language on that page. In many cases, I replaced it with "could have" - which is what I'm really trying to say. What I'm suggesting is something that could have happened, not necessarily that it's definitely the way it did happen. I tried very hard in my explanation of the Date Line Theory to stick to that idea - this is one possibility for what happened among many possibilities.
2. This was a misunderstanding of language and so I've changed my wording here. I didn't mean to suggest knowing the way Noonan's charts (aka maps) were arranged in the plane, but rather the way the navigation tables and information are arranged in an almanac. I would not presume to know how Noonan had his tools arranged in the Electra. I should probably have stuck more closely to navigators' and aviators' specific uses of the terms "charts" and "tables." I interchanged them in this instance, and have now corrected that.
3. I feel that I did account for Noonan's expertise in my explanation. I point out that he did have experience crossing the Date Line, knew it was along their course and knew he needed to account for it. However, the timing of the flight was such that they crossed local midnight prior to the Date Line and changed local dates twice in the flight. As far as I can tell, Noonan had never dealt with that particular situation before. I also should clarify that my understanding is that Noonan was an excellent navigator and I have a deep respect for his abilities to celestially navigate. That is, in fact, near to the heart of the Date Line Theory - how could such an excellent navigator not find Howland Island? It makes me wonder if he made a simple mistake - in much the same vein that geniuses are inept at tying their own shoes. I find it hard to believe that such a skilled navigator could just "get lost" out there. Too many people underestimate Noonan's (and Earhart's) skills and experience. My theory suggests quite the contrary.
4. I do rely on Elgen Long's account that Howland was mapped as being ~6nm west of the actual Island. However, that fact is not essential to the core of the Date Line Theory. If they had the true coordinates of Howland Island, then the location they were heading for (if the Date Line Theory is correct) was 60nm west of Howland, rather than 66nm west of Howland. True or Mapped Howland coordinates do not change the effect of the Date Line. I chose to use the mapped Howland coordinates because I found Elgen Long's argument for it to be substantial.
5. Thanks for the note on the video caption. That caption is a quote from the source of the video which is labeled in the notes at the bottom of that page. I went ahead and edited the caption to reflect your comments. I took out the quotes for the first sentence and put it in my own words to avoid any errors in facts about the film clip.
6. I haven't seen substantial evidence that any of the post-loss radio signals were indeed from Earhart and Noonan. Therefore, I didn't include them as evidence in constructing the theory. I should note that while I am relatively convinced that Earhart crashed in the ocean and sank (mostly by the information as laid out by Elgen Long in his book), I'm not sold on any definitive re-construction of what happened. My main interest is in the effect of the International Date Line on the flight's navigation and I've shown that potential effect on my website. From there, many different things could have happened. In fact, the Date Line could have caused such an error, and they may have continued from that position southward, ending up at Gardner, as you suggest. This is one reason why I included both a straight-line and LOP approach to Howland Island in my diagrams. I don't wish to argue about the two different approaches - it seems there is evidence for both. I only wished to show that the Date Line could have been part of the chain of events in keeping them from reaching Howland Island. In my best estimation from reading the varied evidence, I came to the conclusion that crashing and sinking is the most plausible of outcomes from the Date Line error, so that is what I've put forward on the site.
As for your last critique, I would argue that I haven't taken the scant facts and twisted them to my theory. Quite the contrary. I read about the flight and the information we do have (thanks to you, Elgen, and others), and it made me wonder if the Date Line had an effect. So I started with the hypothesis - what if Noonan made an error when crossing the Date Line? What would have happened to their course and position with such an error? In following that hypothesis, it became apparent that such an error would put them ~60-66nm to the west of Howland (actual or as mapped). None of the substantiated evidence we have discounts that possibility, so I put it forth as a possibility. Building on that, I feel that the disparate radio calls (100 and 200 miles out) could potentially support the Date Line Theory, but (as I say on my website) I recognize that these radio calls don't don't make it so either and the discrepancy could be attributed to other explanations. As they say, correlation does not imply causation.
I hope that helps clarify the Date Line Theory in regards to your comments. Again, I do appreciate the feedback. Science and research are meaningless without peer review. I'd be curious to hear more commentary, especially on the celestial navigation calculations and how the Date Line theory works out for those in your forum experienced with using celestial navigation at sea.
clear skies,
Liz
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Let's back up a bit.
Earhart and Noonan left Lae, New Guinea intending to fly to Howland Island. They had enough fuel for roughly 24 hours of flight.
Radio transmissions received by Itasca reliably indicate that after roughly 19 hours of flight Earhart was under the impression that they had reached a point from which from she should be able to see the island ("We must be on you but cannot see you..."). The strength of the transmissions suggest that the aircraft was in the general vicinity of Howland (within perhaps 200 miles). Over an hour later, the aircraft was still aloft, still in the general vicinity, and running on a 157 337 line. The Electra never arrived at Howland.
We know that Earhart and Noonan never intended to use celestial navigation alone to find Howland, but the failure of the radio direction finding part of the plan left no alternative.
What bothers you is why a navigator of Noonan's obvious expertise and experience was apparently so far off course that he couldn't find Howland within the time constraints of the fuel supply. I agree with you that something must have happened to put him off course. You have suggested one possibility - that he made an error in his calculations by failing to account for crossing the Dateline. There is no evidence that he made such an error but that doesn't mean it couldn't have happened. He could have also mis-identified a star during the night, as he did on the Oakland-Honolulu flight. There is no evidence that he made such an error but that doesn't mean it couldn't have happened. There could have been any number of mishaps for which there is no evidence, but that doesn't mean they couldn't have happened. There is, however, documented evidence of something that happened that might logically put the aircraft significantly off course.
The discovery of the James Carey Diary (http://tighar.org/Projects/Earhart/Archives/Documents/Carey_Diary/Careydiary.html) in 2007 (Carey was the Associated Press reporter aboard Itasca) confirmed that during the first transmission heard from Earhart at 02:48 a.m. local time, Earhart was heard to say "sky overcast." Celestial navigation only works if you can see the sky. If an overcast was hiding the stars from Noonan during the night he could navigate only by dead reckoning. The forecast wind for that portion of the flight was East Northeast at 20 knots. The observed wind aloft at Howland was East Northeast at 31 knots. It appears that Noonan experienced stronger than expected crosswinds from the left during the night and was unable to correct his course using celestial observations, putting him significantly south of course in the morning. He could get a line of position once the sun was up but there was no way he could know where he was on the line.
Neither your Dateline Theory nor what we might call the Overcast Theory is a testable hypothesis, so one will ever be able to say that you are wrong. But when choosing between possible explanations, isn't it more reasonable to pick one that is supported by evidence than one that is based purely on speculation?
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Could you clarify your premise for me? Do you presume that Noonan used local time for his navigation, then for each individual celestial observation converted his local time to GMT for his celestial computations? (If Noonan set his chronometer to GMT at departure--1000L was conveniently 0000 GMT--and en-route elapsed time did not exceed 24 hours, he would not have changed GMT dates, and would have not have been exposed to the possibility of a "dateline error".) I did find your site very well done, but was confused by the basic premise.
Rick J
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I was about to make the same remark as Ricker. Liz you are dragging us into a strange paradox, on the one hand writing (on your web site) that '[FN's] chronometer was set to Greenwich Mean Time and did not imply a date change at all', on the other hand supposing that he had to change twice his reference day during the flight. But he hadn't ! He was on July 2nd GMT on the whole leg and would never have to shift for another page of his almanach. The only possible difficulty could have been to deal with longitudes above 180° or below -180°, but that occurs so often in celestial calculations, even far from the date-line, that it could not be a problem.
Something more important : I find it strange to make celestial navigation referring to the right ascension of the sun, i.e. taking the distant stars as the reference frame. OK, we would see the sun rise 4 minutes later every morning if the day was defined as the time needed for the Earth to come back to the same orientation with respect to the distant stars after one rotation. But the day is not defined that way (do you see the same constellations all year long in the evening ?) The day is precisely defined by the period of our coming back in front of the sun ! This implies that during 24 h, the Earth rotates a little more than exactly 360°. A little more that may be ... 60 miles at the equator (a day is actually the period of a ~361° rotation). As a consequence, because we have defined time that way, except for small variations the sun rises nearly everyday at the equator at the same time (Ric, you've been there, did you notice the sunrise to be delayed by 4 min everyday ??) The small variations, by the way, can be calculated from the tables, but I guess that their main source at the equator (nearly no seasonality) are the variations of the Greenwich hour angle, and it is just a matter of a few ' per day, exactly 2.8' for instance between July 2 and July 3 2010 (sorry I do not have 1937 at hand).
So if FN acidentally skipped by one page (or one line), from 2 to 3 July, he may have been wrong by 2 to 3 nautical miles, certainly not 60.
Christophe Blondel (who never uses celestial navigation in a glider)
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Wow, thanks for all the input everyone.
In response to Ric, I'm not really aiming to argue one possibility versus another or discount the enormous amount of work your group has done. I followed one potential idea down its path to see what might happen as a result, and that's what I've put forth (e.g. Fred's Howland as shown on my website). In an effort to extrapolate on that result and suggest a practical search area, I've chosen to use the calculations and ideas put forth by Elgen in his book as I felt his numbers were more substantiated. It doesn't mean they're right, just that I felt more swayed by his arguments and chose to apply them since we don't know what happened.
I think it's possible that the radio calls from Earhart support my idea, just as you think its possible they support yours. However, taking small fragments of information from those calls is hardly factual evidence. We don't know what else was said or what the context of each statement made by Earhart was, so it's really difficult to place any absolute value on them. There may also have been missed radio calls that amounted to "we're out of fuel and ditching" or "we're heading south in search of other islands" - we just don't know, so it's impossible to rely on them for evidence. (In a personal side note - I almost died in a head-on car collision years ago because the end of a sentence was cut off in a CB radio transmission, so I understand the value of not taking radio calls at face value.) Sometimes what's missing is just as important as what's there.
In response to Mr. Blondel and Mr. Jones, you make very good points and I'm re-assessing the conclusion based on what you've said to see how it affects the theory. The work I've done is based on astronomy calculations and is theoretical. As I stated on the website, I do not have practical experience celestially navigating. My hope in posting the information here and consulting with others familiar with celestial navigation is to test my hypothesis: could the international date line have affected the navigation of the flight and, if so, what kind of calculation error would that result in?
As far as I can tell, changing the date by one day and using the same time (hours/minutes) will move your location by 60 nautical miles. Could that have happened on the flight? Would Noonan have made that kind of error? Is it more likely than other scenarios? I'm asking the members of this forum about it to help clarify the factors involved. I don't wish to definitively declare what did happen, only to suggest ideas for what was possible. If, in effect, I have shown through these calculations that the date line most definitely did not affect the flight, then we still have learned something. As always in scientific research, a negative or positive outcome for an experiment isn't good or bad - what we aim for is that the result will add to our knowledge of the subject. I hope that I've added something valuable to the discussion by posing my hypothesis - whether or not Noonan actually made such an error.
I look forward to hearing more specifics - perhaps we can put the date line to rest if there is significant factual evidence that proves the contrary to what I've suggested is possible.
Once again, thanks again for taking the time to look into these ideas!
Liz
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Here is the July 2,1937 Nautical Almanac (https://4618319391870527131-a-1802744773732722657-s-sites.googlegroups.com/site/fredienoonan/resources/nautical-almanac-1937/almanac-1937-22.JPG?attachauth=ANoY7cqQDhtOyNLNzt6wJsqr2z8ZjE6ERUXFSq3vU2k0VrykZJ85jB2YxTq3tn25wB8aob0bKfgVLCD5VzLhqZYCZyFU7LIG4fJMjNpnz4h2FJJnpzjOq0oj4DmVlxhdhvO_VA33DOav8IvmYWRt9aL_H6cqVl2S8b7sVGIfMWk9K1crQd9CZlLmJIQHtshswoWJx6NXKwWcXdvyAxJx_PGMtmF-H_nxvWjfy-8c5Cb-LmgS9zzwplJa0PLjDbnZtRD4xQoq_9_m&attredirects=0)page for the sun. You can see how the GHA values change from day to day.
Rick J
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Do you presume that Noonan used local time for his navigation, then for each individual celestial observation converted his local time to GMT for his celestial computations?
Thank goodness you posted, Rick. (Having read the mini-bio for you on Ameliapedia, I was fervently hoping that you would!)
I know next to nothing about navigation, much less celestial navigation. But it struck me as quite odd that the procedure given for finding out where you are on the surface of the earth would require that you know what the local date/time was at that spot, which then presumes you know where you are ... like a dog chasing its tail. When it's so wellknown that FN had calibrated his chronometer in Lae and knew exactly the date/time at the Prime Meridian, and since by that standard it was never July 3 for him, I was quite puzzled why we should ever think that he was required to use the July 3 figures -- but only up to the International Date line -- and then somehow he forgot to drop back to July 2 for subsequent calculations. That made no sense at all.
But, of course, the idea that his eyes were tired and he mistakenly used the July 3 figures is certainly an intriguing possibility... even though the post-loss messages throw a monkeywrench into the works.
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Here is the July 2,1937 Nautical Almanac (https://4618319391870527131-a-1802744773732722657-s-sites.googlegroups.com/site/fredienoonan/resources/nautical-almanac-1937/almanac-1937-22.JPG?attachauth=ANoY7cqQDhtOyNLNzt6wJsqr2z8ZjE6ERUXFSq3vU2k0VrykZJ85jB2YxTq3tn25wB8aob0bKfgVLCD5VzLhqZYCZyFU7LIG4fJMjNpnz4h2FJJnpzjOq0oj4DmVlxhdhvO_VA33DOav8IvmYWRt9aL_H6cqVl2S8b7sVGIfMWk9K1crQd9CZlLmJIQHtshswoWJx6NXKwWcXdvyAxJx_PGMtmF-H_nxvWjfy-8c5Cb-LmgS9zzwplJa0PLjDbnZtRD4xQoq_9_m&attredirects=0)page for the sun. You can see how the GHA values change from day to day.
Rick J
Thank you Ricker. From this page one can see that at a given time in the day (typically Oh GMT), the Greenwich hour angle (GHA) of the sun varies by 2.8' or 2.9' (actually something in between, the variation being only due to rounding errors) between July 2 and July 3. The error due to taking the July 3 GHA instead of the July 2 one would thus be less than 3 nautical miles, quite the same order of magnitude as the one I estimated yesterday using the 2010 almanach. Since the sun was a bit further in the east at 0h on July 3 than on July 2, erroneously using the July 3 reference on July 2 would actually have made FN believe he was further in the east than where he actually was. But only by a few miles, for this does not result from the average 1°/day displacement of the sun with respect to the distant stars. It only appears because the increase of the sun's right ascension is not a linear function of time, due to the ellipticity of the Earth orbit and a seasonal effect. It is only a correction to the ideal circular-orbit model, which thus has a much lower magnitude than the 60 NM effect that Liz estimated.
To say it again in short words, since the sun rises everyday at the same time on the equator, you can know your longitude by measuring this time, without bothering too much about the date. Maybe if you take your almanach on January 2 instead of July 2 (that may happen too ...) you will get a total error of more than 100 NM, but the day-to-day variation, or possible error, is just a few arc-minutes, i.e. a few NM, to my opinion nothing more.
Christophe Blondel
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As an aside, when the colony was on Nikumaroro the sun rose every day at precisely 6 a.m.
This was quite puzzling to a visiting British administrator until he realized that every morning when the sun came up everyone set their clocks to 6.
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In sum it seems that FN may have gotten caught a bit short on opportunities to make-up for lost RDF once the problem was realized after a night of mostly dead-reckoning due perhaps to overcast.
I'm pretty sure that the recognition that RDF was not working came well AFTER dawn. Fred would have had only the sun and perhaps the moon to work with--all the stars and planets would have disappeared shortly after sunrise (I don't know where Mercury and Venus were that day).
Gee Marty, aren't you proud of me? I finally 'did a link' (I hope it worked...)! :)
Well done!
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Mrs.L.Smith , "local midnight" and "the date line" depend on definitions having not existed on board of Earhart´s aircraft since it was flown on Z (GMT) time schedule as usual. It was therefore for the navigator not needed to use any other almanac pages than the ones for July 2 , 1937 . The flight deployed ( by plans and by reality) between 0000 and 2400 GMT of one single day and it ended about 4 hours before sun´s sub point came to the Greenwich anti meridian for the start of July 3. Hence , the phenomenon that the crew " lived one day after they disappeared" is not valid , except for the facultative case that they survived until after that point of time. Flying an aircraft long range on " local" time is impossible since the time zones as well as Local Mean Time have administrative definitions only , and both , like the international date line which geometrically is the Greenwich anti meridian , have virtual quality only. Therefore neither the dateline , nor the local (zone or mean) time do not have any importance for navigation , let be the concerning remarks above. The 180 degrees meridian btw was crossed about 1725 GMT , or 6 1/2 before July 2 ended.
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Mrs.L.Smith , By recomputing the 0720 radioed fixed position near Nukumanu island(s) , proof comes up that the great circle route seen in many maps , books and internet sites does not at all apply to the July 2nd flight. And it has been published : European Journal of Navigation , vol. 9 , no.1 , april 2011 : "Frederick Noonan Precomputed a Running Sunset Fix for Amelia Earhart´s Flight from New Guinea to Howland , July 2 , 1937".
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I am writing this as a rookie when it comes to navigation. This comment may be logical, simple and WRONG. I recently read a book which I did not thoroughly understand, but one of the points explained was that while the earth takes 365.25 solar days to orbit the sun, in a year the stars do 366 rotations, that is an observer would see the stars revolve 366 times in one exact year assuming they could view them, presumably also during the day. So the star chart would not completely agree with the sun chart. The book said the "Ancients" knew this, think Stonehenge, so I pondered if the date used was wrong, is it possible that would throw off the plotted position by 1/366 of the earth's circumference? Like about 68 miles by simple division? I haven't read the link to that new dateline theory yet, but I will. I remember when a Mars satellite or lander went wrong because the NASA engineers used feet instead of meters in their calculations. So couldn't Fred make a similar mistake? I think Ric wrote about them being tired, so they took a day off in Lae, and/or Fred was too drunk. You have to remember in 1937 excessive imbibing was not then a "disease" and was likely to be covered up in most cases. Even if Fred was simply fatigued and stressed as he would not have been when sitting in a breezy office making calculations for Pan Am flights it could have led to a mistake date line or not. As has been said, possibly they were planning to depend on their radio and direction finder, which failed them. As soon as I take a course in celestial navigation and astronomy I will comment again.
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"Frederick Noonan Precomputed a Running Sunset Fix for Amelia Earhart´s Flight from New Guinea to Howland , July 2 , 1937".
"Sunset" doesn't sound right.
It should be "sunrise," I think.
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Moleski. No , sunset is correct , it here concerns the sunset fix near Nukumanu from wich it is derived that no great circle path has been flown.
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Moleski. Noonan also computed a running sunrise fix , the concerning observation with the ship´s sextant caused a local hour angle error by which the island did not run in sight at ETA. Publshed EJN, July 2008.
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Moleski. Howland would have come in sight at 1912 Z ,RDF failure became the navigation error not incurred. Hence, RDF failure became a problem , after sunrise with a 3min50sec time error which was not visible on the watch faces.
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Moleski . Correction, delete former. Noonan also precompted a sunrise fix and checked by ship´s sextant insted of bubble sextant at sunset. By this a 3min50s local hour angle error was incurred , not visible on the watch faces .See EJN July 2008 "Where to Find Amelia Earhart´s Lockheed Electra. RDF failure became a problem after sunrise ; without the time error Howland would have run in sight at 1912 Z.
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Noonan also precompted a sunrise fix and checked by ship´s sextant insted of bubble sextant at sunset.
Why would he use a ship's sextant if he had an operational bubble octant?
By this a 3min50s local hour angle error was incurred , not visible on the watch faces .See EJN July 2008 "Where to Find Amelia Earhart´s Lockheed Electra. RDF failure became a problem after sunrise ; without the time error Howland would have run in sight at 1912 Z.
So all of your speculation hinges on a time error induced by Noonan using a ship's sextant instead of a bubble octant. You also rely upon a highly skilled and experienced navigator not noticing a whopping 3 min 50 sec error.
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Mrs.L.Smith , By recomputing the 0720 radioed fixed position near Nukumanu island(s) , proof comes up that the great circle route seen in many maps , books and internet sites does not at all apply to the July 2nd flight. And it has been published : European Journal of Navigation , vol. 9 , no.1 , april 2011 : "Frederick Noonan Precomputed a Running Sunset Fix for Amelia Earhart´s Flight from New Guinea to Howland , July 2 , 1937".
I would be very interested in reading the article you cite (in the European Journal of Navigation , vol. 9 , no.1 , April 2011). Do you have a link to it in English? Or I could translate it if necessary. Thanks.
Rick J
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R.J. If you wish sned URL to hac.vanasten@gmail.com for electronic copy.
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To R.C. The error was a local hour angle fault not visible on the watch face(s). Noonan , from his time with PANAM carried a mariner´s sextant for accuracy , as also given by Dutton , 1928 , Navigation and nautical Astronomy , art.31.l.p.348 , on advantages and disadvantages of the bubble versus ship´s sextant : "In any case (by using the bubble sextant, Ast), the pilot must fly the plane as steady as his skill permits while the sights are being taken. Because or the errors of the bubble sextant, the best results are obtained by flying very low and using the ordinary (=marine , Ast) sextant on the horizon" . The difference between 175453 GMT and 175103 GAT is 3m50s , it was with this difference that the aircraft was steered on the offset track , i.e. 3m50s too early for 11 miles westerly deviation from the over Howland , assumed position, advancedsun position line.
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Noonan , from his time with PANAM carried a mariner´s sextant for accuracy
Not true. In a May 11, 1935 letter to P.V.H. Weems describing his navigation methods on the Pan Am survey flights, Noonan wrote:
"Two sextants were carried - a Pioneer bubble octant, and a mariners sextant. The former was used for all sights; the latter carried as a "preventer"."
"Preventer" is an old nautical term for a back-up. There is no reason to think than Noonan ever used the mariner's sextant if the bubble octant was available.
The letter was subsequently published in the May 1938 issue Popular Aviation.
it would appear that your entire theory is based upon a a false premise.
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Nikunau is shown in 01deg23´-S latitude in (p.e. Norie´s) tables of the era. The most probable route to Howland was : Lae-Gagan 533 mls ,Gagan-Nukumanu 341 , Nukumanu-Initial Point 165deg21´-E/01deg23´-S 458 , In.Pt.- Nikunau via Tabiteuea-Beru 766 , Nikunau - sunrise fix 348 (Nikunau-Howland = 498 mls loxo) , fix to In.Pt one line appr. 50 , In.Pt. to TOP (Turn off Pt) 102 , thence to Howland ass.posn. 30 miles. Total 2,628 miles planned. Initially , sunrise o.b. would have occurred earlier , but due to delay (weather off forecast) on the 1st leg to Nukumanu , 44 minutes , sunrise was seen later , i.e. at 175453 GMT , being the only reference occurrence after twilight. On the leg In.Pt. to Nikunau , every 20 minutes a navigation star came in A/c´s meridian so that one-star observation only was needed to stabilize piloting in the latitude by this composite sailing fashion.
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R.J. If you wish sned URL to hac.vanasten@gmail.com for electronic copy.
For those of us trying to digest your theory, perhaps you could attach the EJN article to a post in this forum? Here (http://tighar.org/smf/index.php/topic,128.msg3031.html#msg3031) is how you do it.
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The 0720z running sunset fix could have been difficult for Noonan. If the sun's azimuth was, for example, 291 degrees, and the heading of the aircraft was 081 degrees, the port vertical stabilizer or other empennage components may have blocked the sight line from the aft cabin window.
Rick J
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R.Jones . Angle for observation of sunset. A/c heading 079 T , sunset line 023-203 . Sight angle 079-23 = 56 degrees w.r.t. A/c´s axis , large enough for observarion through l.h. cabin door , magn.variation included ; window installed with special flat glass, no disturbance by vertical rudder. Proof by Noonan giving o.b. sunset coordinates for precomputed time point , by mathematics inviolably connected.
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Perhaps you would share with us the details of your derivation of the sunset azimuth you cite for the 071930 GMT coordinates in your paper.
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Proof by Noonan giving o.b. sunset coordinates for precomputed time point , by mathematics inviolably connected.
When did Noonan give sunset coordinates for a precomputed time point?
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R.Jones . Angle for observation of sunset. A/c heading 079 T , sunset line 023-203 . Sight angle 079-23 = 56 degrees w.r.t. A/c´s axis , large enough for observarion through l.h. cabin door , magn.variation included ; window installed with special flat glass, no disturbance by vertical rudder. Proof by Noonan giving o.b. sunset coordinates for precomputed time point , by mathematics inviolably connected.
I thought the sun set in the west, more or less. So if the terminator ran 023-203, the azimuth of the sun as it set would be halfway in between at 293T. They were heading easterly at 079T, so her six oclock was at 259T and the sun was at an angle of 34 degrees to the tail, or 144 degrees off the bow of the aircraft, not 56. This was about her seven oclock. Tough spot to shoot the sun if aircraft remains on course.
I'm going to wait for a more clear presentation of this hypothesis before investing further effort.
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Since comments were solicited on this theory, I'd like to add that evening twilight observations on the East Coast very often occur around midnight Greenwich time, so a difference of a few minutes during a busy time of day for a navigator in an Eastern ( GMT-5 or 4) ZD can force you to a different day in the almanac. It's a pain, but a well understood pain and since FN worked up and down the east coast of North and South America, it's hard to imagine (although impossible to disprove) him getting tripped up on what was a very normal part of his existence.
I liked the thoughtfulness and presentation of the concept. Thank you for the input.
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Reply 32 W.R. The 56 deg mentioned is tha angle between A/c´s heading and the sun line (Az 67 deg @ sundown) to port. Your 34 deg from tail side is correct , imho no first line problem since A/c could , if necessary , steered more northwards for a while without risk since Nukumanu was nearby. Occam´s razor works in all directions.
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Stays aloft that " local midnight" and "date line time" do not exist when navigating on GMT schedule , the latter as long as GMT2400 is not surpassed. July 02 Almanac page(s) was/were the only on stage for Noonan.
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According to your paper, Noonan preset the bubble sextant to +53', including 37' for refraction. How did he know the value to use for refraction correction?
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53´ was an error (later corrected for sunset , for sunrise valid). The sun is affected by refraction , as a result we see the 32`diam. disc a number of arcminutes higher than it is actually w.r.t. the horizon. A bubble sextant registers on the sun´s centre which is also the reference for sun´s altitude in nav tables. At sunrise and sunset the refraction generally is 37 arcminutes and we see the sun this " distance" above the horizon. By presetting the bubble sextant negatively with 37´ on its scale , the true sun is in the horizon at collimation of the apparent sun and the horizontal crosshair which is the artificial horizon of this intstrument. For using the mariner´s sextant at sunrise/sunset we must apply additional correction for sun´s semi diameter of 16 arcmin , giving a total scale correction of 37´+ 16´ = (-) 53´. With this latter figure (sometimes taken 50´.0) sunrise-set tables in almanacs have been set up , giving LMT for upper limb (U.L.) in the horizon. By presetting a ship´s sextant for 53´ we see the apparent sun´s U.L. come up whereas the true sun´s centre is 53 armin below the local horizon.Above altitude 1,000 - 1,500 meters (av. 3,000 ft) the local horizon is not visible (optical path through haze too long) around the world and a bubble sextant must be used for any astro ; since the mariner´s sextant has considerably better accuracy it can effectively be used if the horizon is clearly visible. With low sun the lower limb of the sun is due to distortion unservicable for relably astronav.
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reliable astronav. (typo)
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The question is: How did Noonan know how much refraction correction to apply? Where did he get that information?
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Refraction & Parallax (this latter for the sun negligible) are listed in Nautical Almanacs and p.e. in HO.no.208 "Corrections to Observed Altitude of Sun , Stars , and Moon" , the tables being derived from physical / astronomical formulas. It is known that Noonan always used H.O.208 "Navigation Tables for Mariners and Aviators" by J.Y.Dreisonstok , USN. Listings also appear in H.O.no.9 "Useful Tables".
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R.Brandenburg. For navigators , H.O.no.117 "Azimuths of the Sun" was in the era available. For precision I used the underlying formula from spherics : {cos Az = +/- sin d - (sin h . sin L)} / (cos h . cos L). Shorter formulas , giving sin Az , exist but the outcomes are ambiguous. For sunrise/sunset at June , December , 21st : Az = sin d / cos L . Results always for centre of sun . Az = azimuth from South , d = declination , h = elevation , L = latitude , all in degrees , arcminutes & -seconds.
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The lowest sun altitude for which H.O. 108 gives a refraction correction is 6 degrees (correction -8'). The lower limit in the 1937 Nautical Almanac is 6.5 degrees (correction -7.8').
So the question remains: how did Noonan know that he needed the refraction correction you claim he used?
Are you suggesting Noonan had a copy of H.O. 117? If so, what is your evidence?
Are you suggesting Noonan used -- or even was aware of -- the algorithm you used? What is your evidence?
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The 37 arcmin refraction of sun near the horizon is a by navigators universally known figure , besides that it is computed from spherics and appears in all textbooks & handbooks on navigation. Having H.O.no.17 (Azimuths of Sun) [I do not have my copy at hand here and do not readily remember the issue number , 117 , or 17] on board was for Noonan not needed : with the declination of sun near its maximum , the azimuth of the sun at sunrise/sunset in the equatorial region is always (90 - 23) = 67 deg .The formula is : cos Z = sin d / cos L = 0.39073 for which is found (now calculator , then log cos table) 67 degrees from North in the northern hemisphere , for d=23 and L= 0.
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The 37 arcmin refraction of sun near the horizon is a by navigators universally known figure ...
As usual, you have no evidence - just your own suppositions which you regard as facts.
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Proof by Noonan giving o.b. sunset coordinates for precomputed time point , by mathematics inviolably connected.
When did Noonan give sunset coordinates for a precomputed time point?
The position at 0730 GMT was given 159-07´-E / 04-33.5-E . Sunset at this coordinates pair was at 071930 GMT (071545 GAT) , the position has been recomputed step by step (EJN ,April 2011) via H.O.208 and by direct spherics , for the same point of time. It is therefore reasonable assumption that Noonan precomputed a running sunset fix , made the observation and informed Amelia to communicate the results at 0720 GMT (time gap 1/2 minute).
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In other words, Noonan did NOT give sunset coordinates for a precomputed time point. Your statement that he did is merely what you consider to be a "reasonable assumption."
You would run into much less trouble from this forum if you acknowledged that your conclusions are guesses based upon your many assumptions and speculations.
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You seem to be confused about navigation publications. H.O. 117 is Distances Between Ports. The tables of Azimuths of the Sun are in H.O. 71.
How do you know that the magnitude of the sun's refraction near the horizon was "universally known" by navigators? Is this an assumption or a verifiable fact?
How do you know that Noonan knew spherical trigonometry and was able to calculate the refraction? Is this an assumption or a verifiable fact?
Unless you can demonstrate otherwise by verifiable facts that Noonan knew more about bubble sextant refraction correction than was available in H.O. 208 and the 1937 Nautical Almanac, there is no plausible basis for concluding that he had a sunset line of position.
The 37 arcmin refraction of sun near the horizon is a by navigators universally known figure , besides that it is computed from spherics and appears in all textbooks & handbooks on navigation. Having H.O.no.17 (Azimuths of Sun) [I do not have my copy at hand here and do not readily remember the issue number , 117 , or 17] on board was for Noonan not needed : with the declination of sun near its maximum , the azimuth of the sun at sunrise/sunset in the equatorial region is always (90 - 23) = 67 deg .The formula is : cos Z = sin d / cos L = 0.39073 for which is found (now calculator , then log cos table) 67 degrees from North in the northern hemisphere , for d=23 and L= 0.
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B.Brandenburg. 1. H.O.no.71 . Yes , as already remarked I am on my vacation address here and did not remember the table´s exact number. 2. Refraction universally known. At sundown valids cos Az = sin d/cos L , since cos 0 = 1 > cos Az = sin d > cos Az = 0.39073 (sin table) > Az = 67 deg (cos table). How else would Earhart communicate a 157-337 position line ? Navigators of the era always carried a listing of (log) sine /cosine / tan , or the work could not be done.
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B.Brandenberg. 3. Refraction. R = cot (ha + 7.31/(ha + 4.4). ha = apparent altitude = 0 at sunset. R = cot (7.31/4.4) > R = 34.5 . In the era this figure was taken 37´ . Refraction is an important phenomenon and every navigator , sea or air , knew how to handle it. Rule of thumb: refraction somewhat greater than sun´s visible diameter : lower limb apparent sun on horizon = upper limb true sun in horizon.
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Navigators of the era always carried a listing of (log) sine /cosine / tan , or the work could not be done.
You may find it instructive to review the derivation of the equations underlying Dreisonstok's tables in H.O. 208.
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R.Brandenburg. Yes, I extensively worked them during my study of (sea and) air navigation. btw , these days , working with the formulae and the calculator is much shorter than by the tables formerly in use.
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I have reached the limit of my patience with this thread. I will invest no more time in it.
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I just wished I had even the smallest understanding. It looks very interesting but the proof is in the pudding as they say ;)
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It looks very interesting but the proof is in the pudding as they say ;)
After much (way too much) back and forthing it is apparent that there is no pudding. We'll move on.
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I just wished I had even the smallest understanding. It looks very interesting but the proof is in the pudding as they say ;)
OK, a tangential nit, but what they say is "the proof of the pudding is in the eating." (http://www.phrases.org.uk/meanings/proof-of-the-pudding.html (http://www.phrases.org.uk/meanings/proof-of-the-pudding.html)
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Unless you can demonstrate otherwise by verifiable facts that Noonan knew more about bubble sextant refraction correction than was available in H.O. 208 and the 1937 Nautical Almanac, there is no plausible basis for concluding that he had a sunset line of position.
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I agree with you on this point Bob. So it must also be obvious to you that for the same reason Noonan could not take a "sunrise" observation while approaching Howland until about a half hour after sunrise to allow the sun to climb above an altitude of 6 degrees.
Gary LaPook
The 37 arcmin refraction of sun near the horizon is a by navigators universally known figure , besides that it is computed from spherics and appears in all textbooks & handbooks on navigation. Having H.O.no.17 (Azimuths of Sun) [I do not have my copy at hand here and do not readily remember the issue number , 117 , or 17] on board was for Noonan not needed : with the declination of sun near its maximum , the azimuth of the sun at sunrise/sunset in the equatorial region is always (90 - 23) = 67 deg .The formula is : cos Z = sin d / cos L = 0.39073 for which is found (now calculator , then log cos table) 67 degrees from North in the northern hemisphere , for d=23 and L= 0.
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Hi everyone,
I've posted some new ideas about possible navigation errors due to the International Date Line on my website - www.datelinetheory.com. As far as I can tell, the Date Line has not been considered before as a direct cause for Earhart and Noonan becoming lost and/or not reaching Howland Island.
I'm very curious to hear what forum members have to say about my theory, given all of the experience and expertise of the group. I'd be grateful to hear any comments or thoughts on the matter. I propose the theory as one possible scenario, not necessarily the only scenario possible and I remain open to all the possibilities of what happened to the fliers until conclusive evidence of the Electra is found.
Thanks for your time,
Liz Smith
The Date Line Theory
www.datelinetheory.com
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I sent a series to Liz Smith pointing problems with suggestions but she never replied to me so I am not sure she got my emails. I am posting those comments here so that she may be able to get this information.
Gary LaPook
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To Liz Smith
To summarize my prior messages. In the very unlikely event that Noonan erroneously used the data for July 3rd near the end of the flight for star sights he would have calculated a longitude about 59 NM west of his actual position which would then cause him to overshoot Howland. This is exactly the opposite of what your theory predicts.
I also pointed out that any longitude error from the star sights would be cured by the subsequent sun observation as the error from using the wrong day's data for sun sights is only 2.7 NM, not the 60 NM error that you theorized.
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I read your website and it is quite interesting and I followed your computation but that is not the
way a navigator does it. We use Greenwich Hour Angle (GHA) to determine the positions of
bodies and this is directly tabulated in the Nautical Almanac and the Air Almanac for the sun,
planets and the moon. To determine the GHA of a star we find the GHA of Aries in the almanac
and add to it the Sidereal Hour Angle (SHA, the inverse of R.A.)
I an afraid that this is going to be embarrassing for you but you need to know about the errors in
your website. .
First, the date line is an imaginary line and you do not have to do anything when you cross it, you
can keep any date you want until you touch shore where it becomes convenient to get on the
same date they are using ashore. See the discussion at:
http://www.fer3.com/arc/m2.aspx?i=115629&y=201102
You talk about "local time" but flight navigators have no use for local time. It is different on a
ship where the ship’s clock is changed by even hours at convenient points, when traveling east or
west, so that the ship's watches can be adjusted to keep in synch with the approximate local
apparent solar time and lunch can be served at approximately local noon. Time zones are 15
degrees wide, 900 NM at the equator, 450 NM at 60 degrees latitude. A ship sailing westward
near the equator at 12.5 knots will cover 900 NM, one time zone, every 72 hours so will set the
clock back one hour at noon every third day and this will keep the ship’s clock approximately in
step with the position of the sun. The same ship traveling westward at 60 degrees latitude will
reset the clock twice as frequently. In the days prior to time zones the ship’s time was reset to
noon each day at the noon observation. As Captain Aubrey said when looking through his
sextant, “Make it noon” and then the hour glass was turned and eight bells were sounded starting
the first afternoon watch.
Flight navigators do not reset their clocks while in flight so none of that applies to them. Almost
certainly Noonan kept his chronometers set to GMT. By his reckoning he took off at 0000 GMT
on July 2nd and planned to arrive at Howland at about 1800 GMT July 2nd and these would be the
times he used to look in his Nautical Almanac, not much chance for confusion here, no reason to
look at the next date’s data in the Nautical Almanac. Likewise on a ship, the ship’s chronometers
will be kept on GMT. However, the ship’s navigator doesn’t take the ship’s chronometer on deck
to time his celestial observations so he may set his wristwatch to local ship’s time by comparison
to the chronometer. He then uses the “Zone Description” (Z.D.) to convert his watch time to
GMT. (I am disregarding any watch error or chronometer error in this discussion but the
navigator will allow for them as part of the normal computation procedure. See the top of the
form at:
https://sites.google.com/site/fredienoonan/other-flight-navigation-information/modern-bygrave-sl
ide-rule/NauticalAlmanacForm.pdf?attredirects=0 )
For the sake of argument, assume Noonan kept his watch on Lae time. The Zone Description at
Lae is minus ten hours (-10). This is the amount you must adjust your Lae watch time to find
GMT time and date. At the takeoff, Lae time was 1000 (10:00 a.m.) on July 2nd. Apply the -10
hour Zone Description by subtracting ten hours from Lae time and you find GMT of 0000 hours
and the date remains July 2nd. Eighteen hours later Lae time was 0400 (4:00 a.m.) July 3rd. and
you must subtract the same ten hours (since you have not changed your watch time, your watch is
still keeping Lae time, Z.D. is still -10.) Since you can’t subtract 10 from 4 you must borrow a
day, 24 hours, from July 3rd, and just like normal subtraction, when you borrow the day from July
3rd your reduce it by one day to July 2nd, You add the 24 hours to the time of 0400 making the
time 2800 (28:00) on July 2nd, and then subtract the ten hours (-10 Z.D.) and you still get 1800
GMT on July 2nd. This is the standard way navigators deal with time and Noonan had done this
same computation thousands of a times as a ship’s navigator. You can see how extremely
unlikely it would have been for Noonan to use the wrong date in the Nautical Almanac especially
since he almost certainly kept his chronometers set to GMT. It is also quite likely that Noonan
precomputed all of his landfall approach celestial data the night before they took off, or early in
the flight, for twenty minute intervals from 1800 GMT to 2400 GMT July 2nd so that he could
prepare a graph of the computed altitudes. See:
https://sites.google.com/site/fredienoonan/topics/precomputed-altitude-curves
To do all of these computations and to draw the graph would take less than one hour. If you look
at his chart notations on the California to Hawaii and on the Natal to Dakar charts you will see
he marked the times of the LOPs in GMT but Earhart kept her journal on the time at the
departure point.
Assuming that he did use the wrong date then he would have had a longitude error of 59.1
minutes of longitude, 59.1 NM at the equator, for LOPs derived from taking observations of
STARS, so you are almost correct about this. You point out the discrepancy between the “200
mile” and the “100 mile” out position reports, claiming that crossing the dateline between these
observations caused Noonan to use the wrong date data accounting for the discrepancy. But,
Noonan would have noticed such a large discrepancy himself since there had to be an error
somewhere and he would have checked his computations. Such a large divergence in positions
would have required a 120 knot wind shift between the fixes which is obviously impossible.
Where you are completely wrong is in claiming that using the wrong date would cause the same
longitude error (60 NM) when shooting the sun for the LOPs for the landfall approach. See:
https://sites.google.com/site/fredienoonan/topics/landfall-procedure
and: https://sites.google.com/site/fredienoonan/discussions/navigation-to-howland-island
All you have to do to see this is to look at the Nautical Almanac for July 2, 1937 which I posted
here:
https://sites.google.com/site/fredienoonan/resources/nautical-almanac-1937/almanac-1937-22.JP
G?attredirects=0
Look at the Sun’s GHA (Greenwich Hour Angle) column, the third column, and compare the
values for the same times but for July 2nd and July 3rd. For example, at 1800 GMT on July 2nd the
sun’s GHA is 89̊ 02.5' and for the same time on July 3rd the GHA is 88̊ 59.6'. The difference
between these values is only 2.9' so if Noonan had used the GHA for July 3rd then the error in
longitude would have been only 2.9' which is 2.9 NM at the equator, not the 60 NM that you
claim. This slight difference is caused by the eccentricity of the earth’s orbit which causes its
orbital speed to vary resulting in solar days that are not exactly 24 hours long. This resulted in a
change in the equation of time between July 2nd and 3rd ,as can be seen by looking in the first
column in the almanac, from 3 minutes, 50.2 seconds to 4 minutes, 01.4 seconds and this 11
second change results in the 2.9' change in GHA and in the 2.9' error in longitude.
I think where you go wrong is in not recognizing the difference between GMT and Greenwich
Sidereal Time. A sidereal day is only 23 hours, 56 minutes and 4 seconds compared to the 24
hour solar day. This 3 minute and 56 seconds difference accounts for the 59.1' shift in the
positions of the stars but doesn’t effect the position of the sun for navigational purposes since its
position is determined by GMT not GST.
I have posted many reference documents that you might like to look at concerning the navigation
of this flight.
See:
https://sites.google.com/site/fredienoonan/
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If you look at the format of the 1937 almanac you will notice that both July 2nd and July 3rd are on the same page. So Noonan would be doing his computations and as the hours progressed he moves down the page through the July 2nd values. Then, as he crosses the date line and for his next sight he suddenly drops down to the next day giving up his orderly progress through the July 2nd data? I don't think so.
To be continued
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2nd half of my comments previously sent to Liz Smith.
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I have attached the July 3, 2009 and the March 2, 2011 Air Almanac pages. If you compare the GHAs of the sun for July 2nd and 3rd you will see that they differ by only 2.8' making any error in the computed longitude from using the wrong date the same 2.8', 2.8 NM at the equator.
The calculation of GMT and Greenwich date that I described before is done every time a surface navigator (with his watch set to ship's time) takes a sight and has nothing to do with crossing the date line so Noonan would have done it thousands of times while he was a mariner. I will give you an example. Right now it is 7:03:22 p.m. (1903:22), March 1, 2011 in California. If I took a sight right now, to use the almanac I would have to find out the GMT and date for ight now. The Z.D. in California is + 8 now ( +7 when on daylight saving time). The math goes like this. 1903:22 plus 8 hours makes the time 2703:22 GMT, March 1, 2011. Since this exceeds 24 hours we know we must reduce the time by 24 hours, making 0303:22 GMT and carry a day and add that to March 1st making the Greenwich date March 2nd. Anytime you take a sight in California after 4 p.m.requires that you advance the date by one day to find the Greenwich date and do this calculation.
Going further by referring to the almanac pages I have sent you we can find the declination and the Greenwich Hour Angle (GHA) of the sun. The almanac gives the declination of the sun at 0300 GMT, March 2nd as 7° 22.4' south and the GHA is 221° 56.0'. Looking at the interpolation table for the extra 3 minutes and 22 seconds shows the GHA increased 51' making the GHA of the sun at 0303:22 GMT as 222° 47'. (No interpolation is necessary for the declination since it is listed every ten minutes in the Air Almanac. When using the Nautical Almanac, which only tabulates data every hour, you do the same type of interpolation for declination.) Note that neither Right Ascension nor Greenwich Sidereal Time, nor local sidereal time enters into this calculation.
I have also attached the Air Almanac pages for July 26 & 27, 2009. There are some dates during the year were the equation of time does not change from one day to the next so using the wrong date will have no effect on the computed longitude. Compare the the GHAs of the sun on these two pages and you will see that they are exactly the same so using the wrong date would have absolutely no effect on the resulting computed longitude.
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I wrote to you before about the errors in your website. I agreed with you that a one day error in
using the almanac for a a star sight would lead to about a one degree error in longitude (actually
59.1') but I didn’t point out the direction of that error. I am attaching three pages from the 2009
Air Almanac, July 2nd, July 3rd and the interpolation table for GHA. We can use the 2009 almanac
for computations for 1937 since these years are at the same point in the leap year cycle so, for the
sun there is very little difference in the data (a few minutes of GHA and declination), and for
stars there is a constant difference of 33.2' in GHA and calculated longitude. We can use these
pages to do a sample calculation to see how the error works out. Using your example of Antares
crossing your meridian at 1728 GMT on July 2nd. (This is not the way a navigator would find his
longitude since a navigator cannot measure when the body crosses his meridian. Your method
can only be used for a telescope at a fixed location with the polar axis aligned with the meridian.
Navigators have traditionally measured the sun’s height at local noon, when it reaches its highest
point in the sky, but this observation is for latitude, not longitude, because the exact time of noon
cannot be determined this way as the sun’s altitude changes very little in the minutes around
noon so the actual change in altitude is lost in the noise of the measurement. From a moving
aircraft or fast ship the point of highest altitude is never the point where the body crosses the
meridian as the movement of the vessel causes the altitude of the body to change and this effect
swamps out the actual change in altitude of the body. To find longitude navigators traditionally
measured the altitude of a body on the “prime vertical,” bearing straight east or west. ) We can
use your example, however, to demonstrate the effect of using an incorrect time or date on
finding longitude since an error in time has exactly the same effect on longitude for any LOP.
Using your example, we find our longitude by determining the longitude of the body when it
is crossing our meridian at which point it has the same longitude as we do. The longitude of
the body is called Greenwich Hour Angle (GHA) and we can find this number in the almanac for
any time and date. Looking at the page of the Air Almanac for July 2nd, 2009 to find the position
of the star Antares at 1728 GMT we first take out the GHA for “Aries” which is the reference
point, the zero or prime meridian, for the positions of stars. The distance west from Aries to the
body is called Sidereal Hour Angle (SHA, the inverse of Right Ascension.) and adding the two
quanities, GHA Aries and SHA of the body gives us the GHA of the body, its longitude, and our
longitude if it is crossing our meridian.
The GHA of Aries at 1720 GMT was 180̊ 52.8'. Then we must use the interpolation table to
account for the increase in GHA during the extra eight minutes. The earth turns two degrees in
eight minutes (see interpolation table) and we add this two degrees and we find the GHA of Aries
at 1728 GMT is 182̊ 52.8'. We then find the SHA of Antares from the same page which is 112̊
30' , the same as a R.A of 16 hours 30 minutes. ( I don’t know where you got your R.A. of 16h
46m since it is not that now and in 1937 the SHA was 113̊ 35.8', a R.A. of 16h 25m 37s. The
difference between 1937 and now is due to precession of the equinoxes.) Adding the GHA of
Aries to the SHA of Antares results in a total of 295̊ 22.8' for the GHA of Antares at 1728 GMT
on July 2, 2009. This would make our west longitude the same, 295̊ 22.8' . Since longitude is
measured only up to 180̊ east and west we would convert this west longitude into east longitude
by subtracting it from 360̊ resulting in a longitude of 64̊ 31.8' east. (Somehow you came up
with a west longitude so there is some error in your calculation.)
Then to see what happens when the wrong date is used for the calculation we can do the same
computation for July 3rd by starting with the tabulated GHA for Aries which is 181̊ 51.9' at 1720
GMT. To this we add the same 2̊ from the interpolation table and the 112̊ 30' SHA and the
result is 296̊ 21.9' GHA of Antares resulting in a west longitude of the same number, 296̊ 21.9'
west. Compare this computed west longitude with that for July 2nd and we find that the computed
longitude is 59.1' further WEST. (Normally we would convert this to an east longitude of 63̊
38.1' east but this does not show the change as clearly as talking about only west longitudes. The
longitude calculated on July 3rd is 59.1' less east so that means it is 59.1' further west.)
The crux of your theory is that if Noonan had erroneously used the data from the July 3rd page
that he would have calculated that his longitude was one degree, 60 NM, further east (and closer
to Howland) than he actually was and so turned too soon, 60 NM west, to search for Howland
which prevented him from finding the island. The computation I just did shows that using data
for a star sight that is one day after the true Greenwich date actually results in a calculated
longitude that is about one degree further west than the actual position of the plane, exactly the
opposite of your computation. This is because the GHA of Aries and of all stars increases (moves
further west) by 59.1' each day so all longitudes calculated from star sights also move further
west every day by the same amount.
In my previous email I showed that the longitude of a sun line would not be this much in error
but would be only 2.7'. Look at the GHA for the sun at 1800 GMT on both days and you will see
that the GHA on July 3rd is 2.7' less meaning that the sun was 2.7' further east at the same time.
So a sun line LOP would indicate to Noonan that he was 2.7' further east than his actual position
so would have him flying 2.7 NM further west along the LOP while searching for Howland. This
would not make a significant difference especially in light of the fact that Itasca was making a
smoke trail that blew downwind at least 10 NM, well past the erroneous coordinates for Howland
that many think he was using and much further than this additional 2.7 NM theoretical error.
If you have any questions, just let me know.
We know that Noonan set his chronometers to GMT (G.C.T.) from a letter that he sent to P.V.H. Weems describing a prior Pacific crossing. This letter was published in "Air Navigation" by Weems (1938.) See page 423.
https://sites.google.com/site/fredienoonan/resources/weems/weems-422-423.JPG?attredirects=0
We also know he used the Greenwich Hour Angle method of computation that I showed you in my prior posts. See page 425
https://sites.google.com/site/fredienoonan/resources/weems/weems-424-425.JPG?attredirects=0
You might like to look at how celestial navigation is actually done in flight at these links:
https://sites.google.com/site/fredienoonan/other-flight-navigation-information/ocean-navigator-article-1
https://sites.google.com/site/fredienoonan/other-flight-navigation-information/working-the-sight-in-flight
https://sites.google.com/site/fredienoonan/other-flight-navigation-information/in-flight-celestial-navigation
http://www.avweb.com/news/avtraining/IFR_bySunAndStars_200781-1.html
Gary LaPook
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Unless you can demonstrate otherwise by verifiable facts that Noonan knew more about bubble sextant refraction correction than was available in H.O. 208 and the 1937 Nautical Almanac, there is no plausible basis for concluding that he had a sunset line of position.
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I agree with you on this point Bob. So it must also be obvious to you that for the same reason Noonan could not take a "sunrise" observation while approaching Howland until about a half hour after sunrise to allow the sun to climb above an altitude of 6 degrees.
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The refraction correction tables in H.O. 208, H.O. 211, H.O. 214, Nautical Almanac for 1937, Weems Line of Position Book are available here:
https://sites.google.com/site/fredienoonan/topics/refraction
All are limited to altitudes above six degrees.
Also the correction table in H.O. 249 is also available but this set of tables was not published until 1951.
Gary LaPook
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I agree with you on this point Bob. So it must also be obvious to you that for the same reason Noonan could not take a "sunrise" observation while approaching Howland until about a half hour after sunrise to allow the sun to climb above an altitude of 6 degrees.
And your point is . . . .?
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A sunrise fix (actually 150 mls off) was possible at 175453 GMT by observing U.L. of sun with the marine sextant.
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I agree with you on this point Bob. So it must also be obvious to you that for the same reason Noonan could not take a "sunrise" observation while approaching Howland until about a half hour after sunrise to allow the sun to climb above an altitude of 6 degrees.
And your point is . . . .?
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Well considering so many things we have disagreed about in the past it is nice to see that there is something we can agree on.
gl
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It is indeed nice.
I do agree that Noonan would have to wait until the sun was above an altitude of 6 degrees to get a sun line on the approach to Howland.
He couldn't have had a sun line at 1744Z when Earhart reported "about 200 miles out", but he could have had a line on Aldebaran (Azimuth 071, altitude 27) which would give him a usable speed check relative to previous shots on that star. He could have had sun lines starting at 1818Z (alt 6.7). We don't know when They were 100 miles out, but splitting the time difference between 1744Z and 1912Z ("We must be on you . . . flying at 1,000 feet") puts them about 100 miles out at 1828Z. Getting a sun line after 1922Z would be problematic, depending on the density of the popping cumulus layer, typically at about 2,500 feet in that area.
Bob
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1744 / 1745 was the 200 mls out signal for (1740) A/c 300 mls off last checkpunt , sunrise o/b was 175453 GMT @ 150 mls off , FN estimating that 348 mls had been made good , though it was 337 mls only. Speed LOP established. Gspeed 1744 - 1815 GMT was normal 150 mph. 1815 GMT A/c 100 mls out in init.pt. for approach circuit , 1859 on LOP 157-337 30 mls off , line however, was 10 mls in error. Circling 1928-1930 , 30 mls off , & back to North.
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B,Brndnbg . Any "sun line" may deliver a "line of position" , only , for the sun´s elevation other than zero , the line does not run over your target´s coordinates. A sun line as of the Earhart incident can only have it´s origin from observed sunrise for a maximum 1 1/2 hrs distance off destination , your eastward speed given.
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R.G. The bubble sextant can be used for any observation concerning an object in the centre of the bubble , whereas the bubble in it´s course must be in the centre of the visible instrument field. In that event , the bubble´s horizontal diameter represents the actual horizon line. Crosshairs are imaginary and not engraved in an occular lens. In the Nautical Almanac , sunrise-sunset time is always given for LMT - U.L. in the horizon. So , if a navigator extracts sunrise time from the Almanac table , he must necessarily for verification use a mariner´s sextant to acquire a sight of any accuracy , since this specimen has a defined and fixed horizon field line , not trembling like the bubble in the other instrument. It was for U.L. and L.L. sights on sun and moon that a ship´s sextant was taken " in reserve" on board of long range flying aircraft (the gas filled , low flying air ships included).
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Moleski . Correction, delete former. Noonan also precompted a sunrise fix and checked by ship´s sextant insted of bubble sextant at sunset. By this a 3min50s local hour angle error was incurred , not visible on the watch faces .See EJN July 2008 "Where to Find Amelia Earhart´s Lockheed Electra. RDF failure became a problem after sunrise ; without the time error Howland would have run in sight at 1912 Z.
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I read you article. Why would using a marine sextant cause a 3m 50 s error but using a bubble sextant wouldn't cause the same error?
gl
why w
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A sunrise fix (actually 150 mls off) was possible at 175453 GMT by observing U.L. of sun with the marine sextant.
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By your calculations, where do you think the plane was at 175453 GMT?
What altitude was it at when it would have been taking the observation of the U.L. of the sun?
What height would the sextant read for the U.L. observation?
What would be the true height of the sun based on that observation?
If the height read on the sextant is different from the true height what do you do to the sextant height to arrive at the true height?
gl
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G.Lapk. 1. A/c @ 175453 precomp. 178-47´-W / 00-09´-N . Actual posn. 178-56´-W / 00-06´-N . Course 072T.
2. A/c @ altitude 1,000 ft for sharp horizon.
3. Sextant height . Observation was for pt.of time only so sextant preset for dip only @ 1,000 ft = + 33´ for index zero (´index error´of sextant known & applied corrn. for , fixed).
Note: H.O.208 gives 31´ dip @ 1,000 ft.
4. True elevation of sun @ sunrise see 1. Centre 53´ below line of horizon. 53´= 37´refrac + 16´ 1/2 diam.
5. If Hs vs, Hc unequal : preset sextant altitude Hs in reverse sense (-/+) of the difference. Normally Hc - Hs = intercept, towards or away w.r.t. pos.lne of body.
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G.Lapk. 3m50s error bubble vs marine sextant. Explanation too long for here. I will send the July 2008 EJN article as attachment. plse confirm reception.
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G.Lapk. 1. A/c @ 175453 precomp. 178-47´-W / 00-09´-N . Actual posn. 178-56´-W / 00-06´-N . Course 072T.
2. A/c @ altitude 1,000 ft for sharp horizon.
3. Sextant height . Observation was for pt.of time only so sextant preset for dip only @ 1,000 ft = + 33´ for index zero (´index error´of sextant known & applied corrn. for , fixed).
Note: H.O.208 gives 31´ dip @ 1,000 ft.
4. True elevation of sun @ sunrise see 1. Centre 53´ below line of horizon. 53´= 37´refrac + 16´ 1/2 diam.
5. If Hs vs, Hc unequal : preset sextant altitude Hs in reverse sense (-/+) of the difference. Normally Hc - Hs = intercept, towards or away w.r.t. pos.lne of body.
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In your 2008 article you mention setting the sextant to 25.2' which appears to be different than the 33' you state here. Is the 25.2' for a different kind of sextant? In the 2011 article you said to set the bubble sextant to 53' are you now saying to set the marine sextant to 33' or is that for a bubble sextant? To clear this up for me pleas state, for a sunrise observation, what would you set a marine sextant for? What would set a bubble sextant for?
gl
gl
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G.Lapk. The 2011 for bubble sextant mentioned 53´ presetting is for refraction + 1/2 diameter : refr. 37´ ; diam. 32´. By refraction the sun shows an apparant elevation which is greater than the true elevation. Also see figure 3 illustration , p.27 of EJN 2008. Since a bubble sextant needs no correction for "dip" , the artificial equator being parallel with the celestial equator , correction is for refraction (plus parallax for the moon) only.
For the marine sextant things are different. The marine sextant registers on the visible horizon which latter ´dips´ lower when observed from altitudes above sea level. Depending on what nav table is used , somewhat different figures are found , the difference coming from rounding as well of development of empiric magnitudes , and of temperature/pression corrections applied yes or no . 25´.2 and 33´.0 in 2008-2011 issues are for dip only (not refrac etc).
The general formula for dip is : Dp = sq.rt 2 H/R with H = altitude in meters , R = radius of earth. The outcome is in radians and should thence be multiplied by 57.296 to obtain dip in arcminutes.
Example : Altitude over sea 1,000 ft = 305 m . Dp = sq.rt. 2 x 305 / 6,400,000 m = 0´.009760 x 57.2296 = 0 deg 33´34" , or 33´ finished. For this same case H.O.no.208 says 31´ . In navigation , finishing is subtractive.
If we follow H.O.208 the marine sextant setting for a sunrise observation @ 1,000 ft should be : I. set the index screw to (+) 31´ by which II. the horizon will show up ahead with the instrument held horizontally. III . Apply green filter ´dark´ and wait until U.L. of sun just clears the horizon. IV . Note the time point in GMT for U.L. & horizon tangency. V . Consult your precompted sunrise-time-coordinates listing , the figures closest to observation time are the right ones. VI. Accuracy : 6 miles or better uncertainty for experienced navigator.
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G.Lapk. The 2011 for bubble sextant mentioned 53´ presetting is for refraction + 1/2 diameter : refr. 37´ ; diam. 32´. By refraction the sun shows an apparant elevation which is greater than the true elevation. Also see figure 3 illustration , p.27 of EJN 2008. Since a bubble sextant needs no correction for "dip" , the artificial equator being parallel with the celestial equator , correction is for refraction (plus parallax for the moon) only.
For the marine sextant things are different. The marine sextant registers on the visible horizon which latter ´dips´ lower when observed from altitudes above sea level. Depending on what nav table is used , somewhat different figures are found , the difference coming from rounding as well of development of empiric magnitudes , and of temperature/pression corrections applied yes or no . 25´.2 and 33´.0 in 2008-2011 issues are for dip only (not refrac etc).
The general formula for dip is : Dp = sq.rt 2 H/R with H = altitude in meters , R = radius of earth. The outcome is in radians and should thence be multiplied by 57.296 to obtain dip in arcminutes.
Example : Altitude over sea 1,000 ft = 305 m . Dp = sq.rt. 2 x 305 / 6,400,000 m = 0´.009760 x 57.2296 = 0 deg 33´34" , or 33´ finished. For this same case H.O.no.208 says 31´ . In navigation , finishing is subtractive.
If we follow H.O.208 the marine sextant setting for a sunrise observation @ 1,000 ft should be : I. set the index screw to (+) 31´ by which II. the horizon will show up ahead with the instrument held horizontally. III . Apply green filter ´dark´ and wait until U.L. of sun just clears the horizon. IV . Note the time point in GMT for U.L. & horizon tangency. V . Consult your precompted sunrise-time-coordinates listing , the figures closest to observation time are the right ones. VI. Accuracy : 6 miles or better uncertainty for experienced navigator.
I think I understand what you are saying about setting the marine sextant. Since you are at 1,000 feet you are actually looking down towards the visible horizon at an angle below the actual horizontal by 31'. Since you are using the visible horizon as your reference for measuring the altitude of the sun, by setting the sextant to 31' and then measuring the sun in relationship to the 31' below horizontal visible horizon reference, you are measuring an altitude that is actually exactly horizontal.
Then, since refraction is making the upper limb appear to be 37' higher than it actually, is you must subtract 37' from zero, horizontal, to find the true altitude of the upper limb making the upper limb actually negative 37'. Then, since you need to know the height of the center of the sun, not the upper limb, you subtract an additional 16' (half the diameter of the sun) to calculate what altitude you would have measured to the center of the sun making its actual altitude negative 53'. Have I got that correct?
I'm still confused about setting the bubble sextant to 53' and then using it to actually measure to the center of the sun. Does this then produce the same true altitude, -53', that you get using your procedure for measuring the upper limb of the sun with the marine sextant set to 31'?
gl
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A marine sextant measures the elevation of a celestial object relative to the horizon. A bubble sextant measures the elevation of a celestial object relative to the horizontal plane. There are two important differences:
- The relationship between the observed horizon and the horizontal plane varies depending upon the height of the observer.
- Refraction changes the elevation of the observed horizon relative to the true horizon that would be observed if no air were present.
When using a marine sextant to observe an object close to the horizon (e.g. the sun at sunrise) then refraction affects the observed horizon and the observed object. When using a marine sextant to observe an object overhead, refraction affects the observed horizon but not the observed object. When using a bubble sextant to observe an object near the horizon, refraction affects the observed object but not the observed horizon. When using a bubble sextant to observe an object overhead, refraction affects neither the object nor the horizon. The applicable tables take all of this into account.
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The bubble (in the) sextant also sensible for Coriolis , not applicable in the era with low A/c speeds.
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I've been pretty much at a loss trying to follow this whole sextant/navigation issue. Chris's post is very straight-forward and understandable. I plan to go back through this discussion and see if it makes more sense to me now. Thank you, Sir!
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Mrs.L.Smith , By recomputing the 0720 radioed fixed position near Nukumanu island(s) , proof comes up that the great circle route seen in many maps , books and internet sites does not at all apply to the July 2nd flight. And it has been published : European Journal of Navigation , vol. 9 , no.1 , april 2011 : "Frederick Noonan Precomputed a Running Sunset Fix for Amelia Earhart´s Flight from New Guinea to Howland , July 2 , 1937".
I would be very interested in reading the article you cite (in the European Journal of Navigation , vol. 9 , no.1 , April 2011). Do you have a link to it in English? Or I could translate it if necessary. Thanks.
Rick J
These may be what h.a.c. van asten is quoting: 2 pdf files authored by h.a.c. van asten published in the European Journal of Navigation. (http://davidkbowman.com/page54.html) It may help if this thread is still valid :)
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These may be what h.a.c. van asten is quoting: 2 pdf files authored by h.a.c. van asten published in the European Journal of Navigation. (http://davidkbowman.com/page54.html) It may help if this thread is still valid :)
Nice work, Chris--most helpful. Thanks!
Comments on the first article.
"While Noonan most probably used his Pioneer marine sextant to fix his position on the roads of Howland when flying at 1,000 feet altitude, with the sun in mean time whereas the true sun should be taken, it was, as we will presently see, this combination, although not being the flight's laesio enormis, that triggered the primary impulse to not sighting the island before the fuel ran out. Both theory and practice became antagonistic to safety because the bubble sextant has its reference line which is the artificial horizon, over sun's centre and the marine sextant at sunrise registers on the optical sun's upper limb in the horizon, since due to severe refractive distortion the lower limb falls into disuse" ("Where to Search for the Earhart Lockheed Electra"). (http://davidkbowman.com/Search_For_Earhart_Lockheed.pdf)
Some questions:
- Did Noonan have a "Pioneer marine sextant" on the flight? How do we know that he did? In other words, what contemporaneous sources are there that back up this assertion?
- What evidence is there that the sun sight was taken at 1000'?
- What evidence is there that Noonan used "mean time" rather than "true time"?
- What evidence is there that Noonan's marine sextant was not equipped with a bubble to provide an artificial horizon? If he did have a marine sextant modified to be used in flight, how might that affect this argument?
- How do we know that Noonan used only one observation with one instrument? Doesn't the concept of a "preventer" (a second instrument carried to check errors in the use of the primary instrument) suggest using both instruments to check the validity of an observation?
This is a keeper: "When opening a can of worms there is one way only to get them in again: take a greater can (this being Zimurgi's First Law of Evolving System Dynamics)."
And this: "navigators do in the long run not get lost: they go astray for a restricted period of time."
The conclusion of the article is in the final figure: they splashed and sank in a region roughly 85 miles north-northwest of Howland.
Comments on the second article.
"Whatever algorithm is followed, be it H.O.208, any logarithmic gonio table, or e.g. the Douwes-Borda formula, the here recomputed endogenous outcomes remain constant: the Earhart to Herald Tribune Offices, New York, June 30, 1937 cable, reading in part: “..In addition FN has been unable [electric breakdown at Malabar radio station. auth.] account radio difficulties to set his chronometers lack knowledge their fastness or slowness..” later outdated since the 071930 GMT, 0720 by radio communicated time-coordinates group is inviolably interconnected by mathematical precomputation with heliographic time as exogenous parameter which made a structural time error impossible: the on board chronometers [and Longines hack watch] must have been perfectly synchronous with a record of the Greenwich time and for that matter: a navigator would never reset two [of three] chronometers on his own initiative, knowing thereby activating Spode´s Law [3] in its deadly configuration" ("Frederick Noonan Precomputed a Running Sunset Fix for Amelia Earhart’s Flight from New Guinea to Howland, July 2, 1937"). (http://davidkbowman.com/wagner_noonan.pdf)
That is one long sentence. It has many component parts. But it hinges on the claim that Noon did not get an accurate time check before leaving Lae. The evidence is from a telegram on June 30, 1937. Although this was one of the contributing factors in their failure to depart on June 30, that particular problem was rectified by two independent time checks on July 1 and July 2 (http://tighar.org/wiki/Delayed_in_Lae). All of the computations that flow from misunderstanding of the historical record may be perfectly correct, but they are irrelevant because they are based on a faulty assumption.
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Moles. Comment 2nd article. The ´long sentence´ says that Noonan , after having no time check initially , must before take off have acquired a good record of the Greenwich time , since the 071930 /0720 GMT established position is connected to latitude , longitude and time for sunset , these figures being inviolably interconnected by spherics algebra , so that an erroneous chronometer setting o/b of A/c was impossible (there has formerly been rumour that a watch time error was the basis of breakdown).
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Molsk. 1. Noonan carrie most probably an A-5 Pioneer instrument. 2. For a sharp view of the local horizon it was necessary to fly as low as possible , for altitudes higher than 3,000 ft the horizon is invisible due haze , the world over. 3. The 071930 GMT sunset fix approves , together with radio messages @ GMT hours , that A/c was flown on GMT schedule . "True" , local and other schedules were impractical if not impossible. 4. Any sextant configuration would not have influence on astro observations and reductions. 5. No. One instrument at a time : checking instruments against each other not needed due to calibrations. A "preventer" was in this case a marine sextant for special observations , like on the local horizon. A marine sextant has greater accuracy (no trembling bubble) than artificial horizon configuration.
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If a "preventer" is used for special observations, and not as a backup to the octant, what is it that the preventer prevents? I'm not being flip. I'm just trying to understand the terminology.
LTM,
Mona
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M.Kendr.The word "preventer" has been used for a marine sextant , having been on board of Clipper aircraft besides the bubble sextant. The marine sextant could be used for observations at low altitude with the local horizon traced sharply ; generally the mar.sext. has better accuracy (no trembling bubble) than the bubble specimen. Before becoming an air navigator , Noonan made a long career at sea ; he was licensed "All Oceans" and it is probably therefore he had great confidence in the marine sextant.
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If a "preventer" is used for special observations, and not as a backup to the octant, what is it that the preventer prevents? I'm not being flip. I'm just trying to understand the terminology.
LTM,
Mona
I think the consensus on terminology is that FN used "preventer" to mean backup or secondary unit. The primary unit was the Pioneer bubble octant, a device designed for aviation use with a built-in artificial horizon, so all you had to be able to do was correctly identify a star, see it and take a sight (no view of the natural horizon needed). The secondary or "preventer" was a marine sextant that had been designed and built (by Brandis) to be used on a ship with the natural horizon. This unit was later modified (google "Byrd sextant" for an example) with a homemade artificial horizon consisting of a spirit level and a small lens. My belief is that this lens is the "inverting eyepiece" Gallagher reported finding.
Hindsight allows for all kinds of theoretical suppositions and bloviation on what FN would have done or what someone armed a priori with the knowledge that the flight was not going to be supported by ADF could do simply because those things are possible, but there is no contemporary evidence of intent to do anything other that what had previously been done: use celestially augmented DR to get within range of the radio transmitters on or near Howland and ride the beam in.
I doubt the "preventer" ever came out of the box until they landed.
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Let me try re-phrasing my question: why did Fred call his marine sextant a "preventer"?
Mona
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Let me try re-phrasing my question: why did Fred call his marine sextant a "preventer"?
Mona
To prevent getting lost if there was a problem with his primary celestial navigation tool, the Pioneer octant.
Preventer (http://en.wikipedia.org/wiki/Preventer) is also a sailing term for a piece of safety gear. FN went to sea in the days of sail and probably had a pretty solid nautical vocabulary. That's where a lot of aviation terminology came from anyway. The Brandis was a backup, but it was still as good as it ever was. It's just that there were newer, better devices available by the time of the flight.
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Let me try re-phrasing my question: why did Fred call his marine sextant a "preventer"?
Mona
A somewhat archaic use of the word "preventer", taken from nautical usage, is a secondary or backup item (Webster-dictionary.net (http://www.webster-dictionary.net/definition/Preventer) 3rd definition) If we buy that usage, then his marine sextant was a secondary or backup item to have on hand in case his primary sextant failed him.
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Ah, so it's a nautical term. Thank you both.
Mona
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Ah, so it's a nautical term. Thank you both.
Mona
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The most common use of the term "preventer" at sea today is a reference to a line led forward from the boom when sailing before the wind, with the boom let out all the way to one side, to prevent accidental gybes (due to a sudden change in the wind direction)which would cause the boom to swing all the way around to the other side, possibly knocking someone's head off along the way and/or damaging the boom or rig.
gl
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Ah, so it's a nautical term. Thank you both.
Mona
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The most common use of the term "preventer" at sea today is a reference to a line led forward from the boom when sailing before the wind, with the boom let out all the way to one side, to prevent accidental gybes (due to a sudden change in the wind direction)which would cause the boom to swing all the way around to the other side, possibly knocking someone's head off along the way and/or damaging the boom or rig.
gl
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"PREVENTER
Any rope used as an additional security for another. A preventer backstay, for example, supplements a backstay."
A Visual Encyclopedia of Nautical Terms Under Sail. Crown Publishers, New York.