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Phases of the Moon

Written by Patrick Miller, Christopher Keating, and Anahita Sidhwa, Audentes Publishing Co.

What is the relationship between the phase of the Moon and the time it rises or sets? Can you see a crescent at midnight? For how many hours will a full Moon be visible? In what part of the sky will you see a first quarter Moon if you went out at 9 p.m., and will its straight edge be toward your left or right?

The different phases of the Moon are caused by its revolutions about the Earth.
The different phases of the Moon are caused by its revolutions about the Earth.

The different phases of the Moon are caused by its revolutions about the Earth (see Figure 1). Review what is meant by the terms new moon, full moon, first quarter, third quarter, waxing and waning crescent, and waxing and waning gibbous. Here are some other facts about the motion of our nearest neighbor.

The Earth and the Moon rotate and revolve counterclockwise. The period of rotation and revolution for the Moon is the same, hence it presents the same face to us on Earth. The lunar "farside" has only been photographed and studied recently after space travel became a reality.

The synodic month is 29.5 days. This is the time it takes the Moon to go from one new Moon to the next. This is almost equal to one month. In fact, the word month comes from the word "moonth." So, in one week the moon goes through one-quarter of its cycle.

The Sun illuminates half the sphere of the Moon at all times. However, when the Moon is in its new phase, it lies in the same part of the sky as the Sun, and the illuminated face is away from Earth, so we don't see it. Since the Moon is in the same part of the sky as the Sun, it will rise with the Sun.

This photo of "Earthrise" over the lunar horizon was taken by the Apollo 8 crew in December 1968, showing Earth for the first time as it appears from deep space.
This photo of "Earthrise" over the lunar horizon was taken by the Apollo 8 crew in December 1968, showing Earth for the first time as it appears from deep space.

When the Moon is in its full phase, it lies on the opposite side from the Sun. Now its illuminated side faces the nighttime hemisphere of the Earth. Since it lies exactly opposite the Sun, it will rise 12 hours after the Sun.

At first and third quarter, the Moon, Earth, and Sun are at 90 degrees to each other. Since only half the Moon can be seen from Earth at any time, only half the illuminated disk will be visible in this position. Note that "first quarter" refers to the fact that one-quarter of the monthly cycle has been completed, while "third quarter" refers to the Moon being three-fourths of the way to a new moon.

The Moon does not orbit in exactly the same plane as the Earth. The angle between the orbital planes of the Earth and Moon is about 5?. Eclipses occur when the Moon crosses the path of the Earth's plane so that the Sun, Earth, and Moon are all lined up in the same plane. Since this angle is so small, for the purpose of this experiment we will consider the Moon, Earth, and Sun to all lie in the same plane. Actually, the solar system is pretty much in the same plane. This means the Sun, the planets, and the Moon will be seen to follow a similar path as seen from Earth. The path followed by the Sun is called the "ecliptic."

If the Sun, the planets, and the Moon follow a similar path, and we know the Sun rises in an easterly direction (but seldom exactly in the east), then we can reason that the Moon and the planets will also rise in an easterly direction. Likewise, they all set in a westerly direction.

We can talk of "moon rise" and "moon set" times just like we do for sunrise and sunset. Moon rise occurs about 50 minutes later every day. This is a useful fact to know, to be able to predict when to see the Moon.

Last Updated: 16 February 2011

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Last Updated: 16 Feb 2011