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I have somewhat of an interest in sighting the new moon. I recently heard a lecture by someone who had written a program to provide the conditions for the first new moon sighting for any location for any given month. The time that the program uses as the basis for the sighting (not that the moon would necessarily be visible at that time, but the earliest time it potentially could be) was the first moonset following the new moon (of course if sighting were possible it would have to be a bit before the time of moonset).

The lecturer explained that sunset would theoretically have been the better time to use. However, he found that there were a few days in which despite the new moon having occurred, the moon sets before the sun. He gave November 13, 2023, as an example. (e.g. in London, England, the new moon occurs at 9:27, the moon sets at 15:56, and the sun sets at 16:12). From what I can tell, this will be the case in most locales in the Northern Hemisphere east of New York. The lecturer himself was a programmer, not an astronomer (he had just happened to stumble on this phenomenon while designing the program) so he was not able to provide much additional information.

I am interested in understanding a bit more about this phenomenon. What are the conditions causing it, and how often does it happen somewhere in the world? Thank you!

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  • $\begingroup$ The moon moves about 1/2 degree per hour. It takes the Earth about 4 minutes to rotate 1 degree. So the Moon's motion alone could account for about half the difference, in addition to what James explains in his answer. $\endgroup$ Commented Oct 28, 2023 at 14:32
  • $\begingroup$ @GregMiller If I understand correctly, the apparent motion of the moon is significantly slower than the rotation of the earth, so this phenomenon will also depend on the amount of time elapsed since the new moon (ie the less the moon has moved east of the sun following conjunction, the more likely it will set earlier than the sun depending on how far to the north or south it is)? $\endgroup$ Commented Oct 29, 2023 at 3:17

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This is because the New Moon is defined as the moment that the moon is at the same ecliptic longitude as the sun. It may be several degrees above or below.

The situation for moonset on Nov 23 is shown below:

enter image description here

The moon has passed new moon. You can see it is to the left of the sun, when considering the diagonal red lines. But you can also see that is below the sun in the sky. It is on the horizon (the horizontal line passing through the moon) whereas the sun is above the horizon. Sunset will occur a few minutes later than moonset.

When the geometry is like this, then moonset on the day of the new moon will occur slightly before sunset. It's not uncommon, probably occurring in about 20-30% of moonsets on the day of New Moon.

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  • $\begingroup$ Thank you for your reply. I realized that aside from a solar eclipse the moon would be above or below the sun. Somehow, I did not associate that with a different setting time. So If I understand correctly, this phenomenon of moonset occurring before sunset following a new moon will depend on the "ecliptic latitude" of the moon (if that is a term?), ie how far south or north of the sun it is , in the Northern or Southern hemispheres respectively. Also, am I correct in assuming that it is more likely to occur at higher latitudes? $\endgroup$ Commented Oct 29, 2023 at 3:23
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    $\begingroup$ That's right. I suppose it would be more common at higher latitudes, as the setting angle would be lower. $\endgroup$
    – James K
    Commented Oct 29, 2023 at 7:33

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