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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-star distances for arc error
From: Paul Hirose
Date: 2009 Jun 20, 11:54 -0700
From: Paul Hirose
Date: 2009 Jun 20, 11:54 -0700
The effect of refraction on the separation angle between stars has been discussed from time to time. Back in September 2000 I said this in another forum: http://www.i-DEADLINK-com/lists/navigation/0009/0065.html My tables are slightly mis-formatted on the web page. The column labels are altitudes, and the row labels are separation angles. The first table shows that refraction affects separation angle in a way nearly independent of altitude, if both stars are at the same altitude. For example, if two stars are the same altitude and 45° apart, refraction decreases their separation angle by .80' at 15° altitude and .86' at 75° altitude. Throughout the table, refraction has slightly increasing effect as altitude increases. Bowditch (1984) recommends selecting stars at the same altitude "to minimize refraction errors". But that's wrong. Instead of using stars at the same altitude and ignoring refraction, I believe it's better to use stars at the same *azimuth* and apply refraction corrections. For example, if the lower star is refracted 1.5' and the upper star .5', the refracted separation angle is 1.0' less than the unrefracted angle. I find star to star separation angles hard to measure consistently with a sextant. It's easier to test my accuracy with lunar distances. -- --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---