moonless night and lunar phase
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 ?
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?
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.
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.
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:
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.
When you say 'once per month' you don't meant to suggest there is only one moonless night per month, right?
Depending on your horizon and the quality of your instruments, you may have three moonless nights. Check http://en.wikipedia.org/wiki/Hilal_(crescent_moon)
@Jeremy: By "once per month" I mean "once per lunar phase cycle the Moon passes close by the Sun".
So when the moon is in full moon phase, it could not make moonless night? And sometimes the moon may arise late in the night，before it arises, there is no moon in the sky and the sky is dark too. However a moonless night means there is no moon during a whole night, right?
@questionhang: Correct. The Moon is in the direction of "up" during the night when it is full. So far as the Moon never appearing during the whole night, there may be up to about an hour of a very small sliver of moon slightly after sunset or slightly before sunrise. Any more than that and I wouldn't call it a 'moonless night'.
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).
@dotancohen perhaps you should edit your answer to clarify because right now, someone who didn't already know the answer might misunderstand. As it is, I first read your answer as suggesting just one night per month as moonless. That is clearly not the case, and it indeed is not what you intend to say. Perhaps modify to say that during the period of the month that the moon is near the sun, nights will be moonless, and there may be three such consecutive nights in a lunar month.
Unless there's an existing definition, I would argue that it's a moonless night if the moon is only up when the sun's elevation is greater than -18 degrees. In other words, the moon isn't up except during astronomical/nautical/civil twilight (and daytime of course)
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.