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Author Topic: Seeking Comments on New Date Line Theory  (Read 105560 times)

h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #60 on: April 30, 2011, 01:29:46 PM »

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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Gary LaPook

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Re: Seeking Comments on New Date Line Theory
« Reply #61 on: April 30, 2011, 08:34:10 PM »


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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Bob Brandenburg

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Re: Seeking Comments on New Date Line Theory
« Reply #62 on: May 01, 2011, 08:59:47 AM »

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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h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #63 on: May 01, 2011, 12:27:38 PM »

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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h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #64 on: May 04, 2011, 11:37:41 PM »

 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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h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #65 on: May 07, 2011, 12:11:00 AM »

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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Gary LaPook

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Re: Seeking Comments on New Date Line Theory
« Reply #66 on: May 07, 2011, 01:30:52 AM »

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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Gary LaPook

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Re: Seeking Comments on New Date Line Theory
« Reply #67 on: May 07, 2011, 02:04:58 AM »

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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h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #68 on: May 07, 2011, 02:23:07 PM »

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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h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #69 on: May 08, 2011, 02:23:40 AM »

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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Gary LaPook

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Re: Seeking Comments on New Date Line Theory
« Reply #70 on: May 08, 2011, 04:40:21 AM »

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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h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #71 on: May 08, 2011, 07:45:40 AM »

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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Gary LaPook

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Re: Seeking Comments on New Date Line Theory
« Reply #72 on: May 08, 2011, 03:40:53 PM »

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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Chris Owens

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Re: Seeking Comments on New Date Line Theory
« Reply #73 on: May 08, 2011, 10:13:38 PM »

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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h.a.c. van asten

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Re: Seeking Comments on New Date Line Theory
« Reply #74 on: May 08, 2011, 11:34:49 PM »

The bubble (in the)  sextant also sensible for Coriolis , not applicable in the era with low A/c speeds.
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