The movement of the moon forms a predictable pattern. Photo: Paul Chappell
Q. Does the moon rise and set always at the same place relative to a stationary observer?
A. Like the sun, we always consider the moon as rising in the east and setting in the west. In reality it generally rises either north or south of east and sets either north or south of west. The movement of the daily rise and set directions along the horizon form an interesting and predictable pattern.
Sunrise and set directions have a similar but simpler pattern that is easier to consider. As the Earth circles the sun it has a tilt of 23.5 degrees, which gives rise to the seasons: in the summer half of the year the sun appears in the southern part of the sky, while in the winter half it appears in the northern part. Hence, in summer the sun rises south of east and sets south of west and in winter it rises north of east and sets north of west.
The moon shows a similar pattern to the sun but, instead of taking a year, it runs through the pattern in a month of 27 days, which is the time the moon takes to circle the Earth. For half the month the moon rises south of east and sets south of west. During the other half it rises north of east and sets north of west.
Note that where the moon rises and sets has no connection to its phase. So on a given day a crescent moon, for example, can rise either north or south of east.
If that seems complicated, there are simple aspects to it. In summer, as mentioned earlier, the sun rises south of east and sets south of west. Since the full moon is always opposite the sun, in the warmer months the full moon rises north of east and sets north of west. Similarly in winter the full moon rises south of east and sets south of west.
Response from Dr Nick Lomb, Curator of Astronomy, Sydney Observatory and the Powerhouse Museum