NavList:

Dear Modris,

To your explanations why more light gives more accurate Lunars I can add two which are related to the functioning of  human eye.

1. When it is dark the pupil of your eye becomes larger, and this does not allow to focus it as well as with smaller pupil. (I think this explanation was suggested by Frank when the question was discussed long ago).

2. Our eye retina is equipped with two types of light-sensitive cells. They are called rods and cones. Rods are responsible for vision in low light (night vision) They are optimized for detecting motion. While cones provide sharp detail in bright conditions (day vision), optimized for high resolution.

I suppose that when you observe a star-to-star distance, you moistly use your rods, while for a Lunar you can use cones since the Moon is bright enough. THis also explains why day time Lunars are most accurate.

I notice that old Soviet manuals recommend to check the arc errors using star-to-star observations, ignoring the refraction correction! This is not the best way, but the reason may be in that these manuals were written in pre-computer (and pre-electronic calculator!) era, and the authors thought that explaning Lunars reduction (or refraction correction for star-to-star) will be too complicated for Soviet seamen:-) They just give a little table of star distances covening the whole range of angles, and give a recommendation not to use the stars with altitude less than 60 degrees.

Alex.