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    Re: Longitude by Sunset
    From: Marcel Tschudin
    Date: 2012 May 9, 13:00 +0300
    GeorgeB, you wrote:

    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.

    Sorry, but I still do question that you saw this at the apparent (sea) horizon. Could it be that your observations relate to an other, higher horizon?

    The photos which Greg sent from the setting sun are typical ones. Imagine that in these pictures the sun would be about 400'000 times less bright. This would then correspond to the brightness of a mean full moon (14 magnitudes difference) at the same location. Do you really think that you would still be able to see it there?

    I am really interested to hear how my observations compare with others.

    @ Greg:
    I suggest you try to watch the moon some time during the second part of this month (weather permitted). The moon does not necessarily have to be full. One can observe the "phenomena" also with a half moon because it is related to the brightness density and not the total brightness. May be you are even able to take some photos (using a tripod) before and during the first part of the dimming. Before the "real" dimming starts the shape of the moon is still clearly visible. During the dimming the moon looses its sharp borders and turns gradually into a blotch of light without sharp contours. As dimming continues the blotch gets fainter and smaller with only the brightest parts in it remaining faintly visible until they also disappear.




    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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