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How to define moonless night? There is no moon at all during some night?
How to calculate and know whether the moon would appear during one night?

If the lunar phase is full moon, is it possible that the moon will not appear ?

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  • $\begingroup$ Tonight, June 21, I actually saw the moon go down beyond the horizon; before midnight. There is NO moon in the night sky. How often does this happen? $\endgroup$ – user7516 Jun 22 '15 at 7:39
  • $\begingroup$ Hello Mary, and welcome to Astronomy SE! If you want to ask a followup question, it's better just to ask an new question than post as a comment. However, this is a normal part of moon phases. During the new moon phase, the moon will be up almost entirely during the day. The closer the phases are to a full moon, the more time it will be out during the day. This is because the phase and the part of the day the Moon is in the sky are both defined by the relative positioning of the Moon, Earth, and the Sun. $\endgroup$ – Mitch Goshorn Jun 22 '15 at 8:56
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    $\begingroup$ Moon sets before midnight for half the lunar month, 28 point something days. It's well worth the effort to go outside every night for several months and watch what the moon is doing in the sky. It's a very regular process. $\endgroup$ – Wayfaring Stranger Jun 22 '15 at 13:46
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A moonless night is, as you suspect, a night in which the Moon does not appear visible in the sky. This happens once per month, when the Moon is near the Sun. Due to the proximity of the Moon and the Sun in the sky, at that time the Moon is the smallest sliver possible, and therefore not a full moon.

This is because it is actually the Sun that illuminates the Moon, and when the Sun and the Moon are in the same direction in the sky we are seeing the non-illuminated side of the Moon. Note the direction of the sunlight in this image:

Moon phases

Obviously, the direction of the sunlight is the direction of "up" during the day. If you look at the horizon slightly after sunset or slightly before sunrise, you might actually catch a glimpse of the sliver of Moon before it set or rises slightly after or before the Sun.

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  • $\begingroup$ When you say 'once per month' you don't meant to suggest there is only one moonless night per month, right? $\endgroup$ – Jeremy Nov 24 '14 at 10:01
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    $\begingroup$ Depending on your horizon and the quality of your instruments, you may have three moonless nights. Check en.wikipedia.org/wiki/Hilal_(crescent_moon) $\endgroup$ – Envite Nov 24 '14 at 11:39
  • $\begingroup$ @Jeremy: By "once per month" I mean "once per lunar phase cycle the Moon passes close by the Sun". $\endgroup$ – dotancohen Nov 24 '14 at 11:44
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    $\begingroup$ Because the light from the moon is relative to its position with the Earth and Sun, a full moon will be up almost entirely during the night (rising approximately as the sun sets) while a new moon will be present in the sky during almost the same time as the sun (setting and rising approximately with the sun). $\endgroup$ – Mitch Goshorn Nov 24 '14 at 17:26
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    $\begingroup$ Typically you get 2 days or so on either side of the new moon where you have to look hard to find the moon during twilight. It's entirely missing the rest of those nights. Moon rises and sets about 50 minutes later each night, so that two day figure would keep it within a 100 minute twilight. $\endgroup$ – Wayfaring Stranger Jun 22 '15 at 13:43

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