Next message: George Huxtable: "Re: Real accuracy of the method of lunar distances"
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
