NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Real accuracy of the method of lunar distances
From: Bruce Stark
Date: 2004 Jan 9, 15:39 EST
From: Bruce Stark
Date: 2004 Jan 9, 15:39 EST
I believe George was right when the concept of parallactic retardation first occurred to him. That is, it can affect the accuracy of GMT taken from the moon. But I never did understand what parallax in altitude had to do with it. The parallax in this problem comes, it seems to me, from the rotation of the earth. If you are at one of the poles the earth's rotation doesn't move you from west to east, and the effect disappears. But if you are at the equator, rotation moves you 900 nautical miles east every hour. To the extent the motion is perpendicular to the direction of the moon, parallax shifts the moon in the opposite direction. Nor do I understand what the lack of an effect on the cleared distance has to do with it. The difficulty is in measuring the distance we see. If not, why do we use scopes on our sextants? If we can only see and measure a shift of, say, 1' in the moon's apparent position, the accuracy of our resulting GMT will be affected if that 1' shift occurred in four minutes instead of two. Actually I hope I'm wrong in this, as some people might feel constrained to take lunars only at "appropriate" times. And experience suggests I am wrong. Seems to me the lunars I've taken when the moon is on or near the meridian have been as good or better than average. I intend to keep taking observations when it's convenient and comfortable to do so. Although I appreciate the discussions of these technical considerations, I'm not going to let them limit my pleasure. Bruce