NavList:
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 Jul 04, 00:05 -0700
From: Paul Hirose
Date: 2009 Jul 04, 00:05 -0700
Tonight I shot the angle between Spica and Arcturus six times with my Astra IIIB (3.5x scope and traditional split horizon glass). The extreme values were 1.0' apart, so I may have to take back my earlier remark that "I find star to star separation angles hard to measure consistently with a sextant." This was based on a few measurements attempted soon after I got the Astra. My results were so disappointing, I dismissed the technique as bunk, at least with my eyesight. Based on tonight's results, I suspect my inexperience was to blame. Though I'd owned bubble sextants for years, the Astra was my first marine sextant. Now I'm more skillful at positioning both bodies in the split image horizon glass and swinging the arc. That last is important, I've found. The pair of stars should appear to collide instead of passing each other as the sextant is rocked. Contrary to what I expected, attempting to keep the stars in contact makes it harder to judge coincidence. When viewing a single star to check index error, that last technique is all you have, since swinging the arc has no effect. My six observations tonight don't prove anything. It's easy to get lucky a few times in a row. However, I think the results are pretty good for someone who has done maybe a dozen of these in his life. With more practice I may be able to judge star coincidences as well as I can judge anything else through a sextant. Conditions tonight were hardly ideal. The wind was enough to make the stars dance maddeningly most of the time. My arms soon became tired of holding the sextant for long periods, waiting for the image to settle down. I have several tripods, but to make the experiment more challenging I did not use anything to steady myself. My opinion of this method for checking sextants has improved enough that I intend to try it some more. -- --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---