Advanced search  
Pages: [1]   Go Down

Author Topic: Two minutes could mean 30 miles  (Read 29282 times)

Friend Weller

  • TIGHAR member
  • *
  • Posts: 156
Two minutes could mean 30 miles
« on: January 17, 2019, 08:46:46 AM »

An interesting article that could shed some light on the difficulties Fred may have had to deal with in shooting his sunrise sight on the morning of 02 July 1937 and his subsequent navigational calculations.

Kenton E. Spading

  • TIGHAR member
  • *
  • Posts: 14
Re: Two minutes could mean 30 miles
« Reply #1 on: January 17, 2019, 09:08:51 PM »

Thank you for posting this.   It is very interesting.  The cited dissertation by Teresa Wilson (now at the U.S. Naval Observatory) is here.

I extracted the following from the sky and telescope link.

Ms. Wilson suggests that typical sunrise and sunset estimates are often off by 1 to 5 minutes.  Her dissertation delves into the assumptions navigators and others make about the refraction of the sun’s light through earth’s atmosphere.   Most sources today assume an angle of refraction at the horizon of 34 arc-minutes although she found references to this dating back centuries.  Excerpts from the article.

Using a single refraction angle doesn’t account for different meteorological conditions from location to location.  Nor does this approach take into account how the observer’s altitude might change things.  Summer showed the largest discrepancies in sunrise-sunset predictions, probably due to the pronounced refractive effect that the large temperature difference in the atmosphere has during those months.

There are many factors involved but generally Ms. Wilson discovered that sunrise and sunset times cannot be reliably predicted to better than 2 minutes.

Two minutes might sound inconsequential, but as Ms. Wilson noted, if GPS fails, sailors will use celestial navigation (and before GPS e.g. Noonan they all utilized celestial).

Quoting Ms. Wilson; “Most sailors will tell you that they can get their position with celestial navigation to within 1 nautical mile. But if part of that calculation involves sunrise-sunset 1 minute of time turns into 15 nautical miles of error."

A better understanding of how the atmosphere bends light on the horizon would affect moonrise predictions, too.

I did not find a reference to how refraction might be affected by a volcanic eruption/ash/floating pumice which occurred at Rabual, Papua New approximately 1 month before the Earhart flight. 


Kenton E. Spading

  • TIGHAR member
  • *
  • Posts: 14
Volcanic Ash, Celetial Navigation, Two minutes could mean 30 miles
« Reply #2 on: February 04, 2019, 08:17:52 PM »

Greg George and I have been kicking this around.  The refractive index of moist air is approximately 1.0003 at 99% relative humidity.

The RI of 5% H2SO4 is 1.340 which is a 25% change in the observe celestial angle.   It should be possible to estimate the error based on plume concentrations measured after recent Rabaul eruptions.   It could be enough to alter navigational observations.

 The RI affect of ash depends on particle size - most remaining ash particles are very small as the largest particles have fallen out.   Gas and microparticulate can persist for months or years after an eruption.

Also, volcanic eruptions increase the drag coefficient on aircraft that are flying in the vicinity of the eruption.   It also messes up radio signals and compasses.
Pages: [1]   Go Up

Copyright 2024 by TIGHAR, a non-profit foundation. No portion of the TIGHAR Website may be reproduced by xerographic, photographic, digital or any other means for any purpose. No portion of the TIGHAR Website may be stored in a retrieval system, copied, transmitted or transferred in any form or by any means, whether electronic, mechanical, digital, photographic, magnetic or otherwise, for any purpose without the express, written permission of TIGHAR. All rights reserved.

Contact us at: • Phone: 610-467-1937 • Membership

Powered by MySQL SMF 2.0.18 | SMF © 2021, Simple Machines Powered by PHP