# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Longitude by Sunset
From: George Brandenburg
Date: 2012 May 8, 14:03 -0700

Marcel,

Thanks for finally summarizing this so well! All I would add is that if you want to treat the difference between sunset and moonset as an effective lunar distance measure, then you certainly need to include the parallax correction for the moonset.

As for witnessing actual full-moonsets, I'm not usually up at that hour. But I have seen plenty of bright full-moonrises, and I would conclude that if you can see one you can see the other. Of course here I mean the actual moonset and not the "geometrical" one, which can't be seen.

Cheers,
GeorgeB

From: Marcel Tschudin
Date: 8 May 2012 15:08
OK, I think to be finally not only able to "see" the parallax "problem" for the observer of the moon but also to actually understand it:

(1) If the moon would be bright enough an observer would see it setting or rising like the sun. There is indeed "nothing" related to the question whether the moon is sufficiently bright to be seen rising/setting.

(2) Due to the parallax the observer sees the moon set earlier than it would be without parallax. We know that when the LL of the sun (which has a negligible parallax) touches the apparent horizon its *geometrical* position is already below it. The setting of the *geometrical* sun happened therefore at some moment *before* while one still observes the sun setting.

(3) For the moon the combined effect of (non-negligible) parallax and refraction is different. When the parallax exceeds the refraction their difference has the combined effect of a negative refraction. If we could see the LL of the moon touching the apparent horizon its *geometrical* position would not be below it as in the case of the sun but rather above the position where the moon is seen.

(4) Mr. van Asten's statement relates to the setting of the *geometrical* and not to the visible moon during the visible moon set. During the visible moon set the *geometrical* moon remains above the apparent horizon. The *geometrical* moon passes the apparent horizon only some times *after* the visible moon set.
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