The Moon takes just under a month to revolve around the Earth. The Moon revolves counterclockwise around the Earth which is the same direction that the Earth rotates. During the monthly trek the moon appears to change its shape. This phenomena fascinated ancient astronomers, who deduced that the Moon must shine by reflected sunlight. They also deduced by observing the Moons phases that the Moon must orbit the Earth.

The Moon orbits the Earth every 27.3 days and over the course of its orbit the moons apparent shape changes dramatically. The apparent shape of the Moon depends on how much its illuminated surface is visible from the Earth. The Earth the Moon has a completely dark half and a completely lit half and any point during its orbit.

Lets start our discussion of the Moon's phases with the New Moon phase. New Moon occurs when the dark hemisphere of the Moon is facing the Earth. The Moon is between the Earth and the Sun, thus they are in the same part of the sky. Thus, the moon is not visible during this phase.

If you wait about 7 days and observe the Moon you will observe the phase called waxing crescent moon and during this time you will see more and more of the Moon's illuminated suface. If you wait just a bit longer you will see the first quarter moon. The name of the phase reflects how much of the orbit the moon has traversed. There is a 90o angle between the Earth, Sun and the Moon at first quarter, which means that we can see half of the illuminated surface of the Moon.

Notice that these phases are correlated with their setting and rising times. Consider the first quarter moon again. You observe the Moon from the Earth which also means you are being carried around by the Earth. As you pass from day into night, the Sun is setting on your horizon and the time must be 6 P.M.. The first quarter Moon will therefore be seen on the meridian at sunset and you can also view it during the afternoon. However, the Moon lags behind the Sun on its daily path so the Moon will set at midnight and rise again at noon.

So for the waxing crescent moon we see less than half of the illuminated surface. Thus, the angle between the Earth, Sun and Moon is less than 90o therefore the Moon sets in the evening before midnight. When do you think the Moon rises?