What is the Moon doing?
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The (fairly) simple answer
The Moon rises and sets every day, like the Sun. But the Sun always rises in the morning and sets in the evening; the Moon does it at a different time every day.
At New Moon, the Moon lies in the same direction
as the Sun. But
the Moon is orbiting around the Earth; every day, it moves eastwards
(further left
from the Sun) by about 12 degrees. This means that it increasingly
lags behind the Sun, by about 50 minutes a day.
- At New Moon, the Moon rises in the morning; it's at its highest, in the south, in the middle of the day and it sets in the evening - just like the Sun. Of course this is academic, since we can't see the Moon when it's New!
Over the next few days, as the Moon grows to a crescent, it moves further left, and lags more and more behind the Sun. Soon we can see it in the evenings, still above the western horizon when the Sun has already set.
- By First Quarter, the Moon is one-quarter of the way around its orbit (and half illuminated). It is now 90 degrees to the left of the Sun, and lags behind it by 6 hours. So it rises in the middle of the day, it's high in the south at sunset, and it sets in the middle of the night.
Over the next few days, as the Moon grows to a gibbous phase (more than half-illuminated), it continues to lag further behind, rising later each afternoon and setting later each night.
- At Full Moon, the Moon is opposite to the Sun - 180 degrees away, and 12 hours behind it. So the Moon rises as the Sun is setting; it's high in the south at midnight, and it sets in the morning, at sunrise.
Over the next few days, as it shrinks back to gibbous again, it rises later in the night.
- By Last Quarter, the Moon is 270 degrees to the left of the Sun - or 90 degrees to the right of it; and it lags 18 hours behind the Sun - or it's 6 hours ahead. So it rises in the middle of the night, it's high in the south at dawn, and it sets in the middle of the day.
Over the next few days, as the Moon shrinks back down to a "crescent", it rises later every night. Eventually we catch only a glimpse of it at sunrise, coming over the horizon just before the Sun. (Note that crescent strictly means "growing"; the Moon should really be called decrescent at this phase.)
- And by New Moon, once again we can't see the Moon at all.
If you know how many days it is since New Moon, multiply that by 50 minutes, to find out approximately how much the Moon is lagging behind the Sun.
(It will only be approximate, because the Moon's orbit is an ellipse rather than a circle, and it doesn't go round at a constant speed.)
Further complications
So the Moon crosses the sky about 50 minutes later every day. And you might think it would rise 50 minutes later every day, and set 50 minutes later too. But this isn't true - because the Moon doesn't spend the same length of time above the horizon every day.
The Sun goes through a seasonal variation, once a
year.
In December and January, the Sun is in the far south of the sky. It rises in the south-east; it spends only a few hours above the horizon (for northern-hemisphere observers), crossing the sky quite low down; and it sets in the south-west.
In June and July, the Sun is in the far north of the sky. It rises in the north-east ; it spends many hours above the horizon (for northern-hemisphere observers), crossing the sky quite high up; and it sets in the north-west.
The Moon follows the same cycle, but about twelve times faster; it gets round in about a month. The Moon is in the same place as the Sun at New Moon.
As we have seen, the time at which the Moon crosses the southern sky gets later every day, by about 50 minutes. So the time at which the Moon rises and sets will also vary from day to day. For example, when the Moon is heading northwards, it spends longer above the horizon each day. The time of moonrise gets later (because the Moon crosses the sky later), but it tries to get earlier too (because it is spending longer above the horizon). The net result can be that the time of moonrise gets only a few minutes later from one day to the next. Conversely, when the Moon is heading southwards, the time of moonrise can change by well over an hour from one day to the next.
But we can still predict how high the Moon will cross the sky, how long it will spend above the horizon, and where it will rise and set, by comparing its behaviour with that of the Sun.
To see how this works for each month, choose one of the links to the left.
And it gets worse....
The Moon doesn't follow the Sun's path exactly. To find out what difference that makes, try clicking here.