NavList:

Brad,

You ask a question that is deeper than it might first appear.

As you know, the "normal" refraction correction for the observed altitude of an single body depends only on its altitude, not on your position or time.

Now it might not be obvious, but the result of refraction on the observed arc distances between two bodies (like star-star distances, or moon-sun distances), can be expressed in terms of these two normal refraction corrections. In lunar distance sights, using these two normal refraction corrections for the individual bodies to correct the lunar distance is called "clearing the lunar distance." See chapter 8 in my book, particularly figures 8.2 and 8.3 (Expanded Edition is best). Figure 8.3 shows that the key to the "clearing" problem is that the relative bearing angle between the two bodies is constant -- is the same for the observed and the corrected star-star distance. None of this depends on the observer's position or on the time.

Hope this answers the question,
JK

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