Earthrise
The Moon
The success of the Apollo moon missions is one of the greatest triumphs in the history of science. In order to land on the moon and return safely to earth, the motions of the earth and moon had to be known accurately. This information was accumulated over a long period of time as people studied the changing phases of the moon.
The Lunar Cycle
On some nights the moon shines brightly enough for you to read a book by its light. But moonlight is not produced by the moon. The moon reflects light from the sun. As the moon revolves around the earth, different parts of the lighted side of the moon face the earth. The shape of the visible portion of the moon varies. These varying shapes, lighted by reflected sunlight, are called phases of the moon.
Phases of the Moon
When the moon is between the sun and the earth, the side of the moon facing the earth is unlighted. At these times the moon is in the new moon phase. During the new moon phase there is no lighted area of the moon visible from the earth. As the moon continues to move in its orbit around the earth, part of its lighted half becomes visible. When the size of the visible portion of the moon is increasing, the moon is waxing. When a sliver of the moon becomes visible from the earth, the moon enters the waxing-crescent phase. When the moon has moved through one-quarter of its orbit after the new moon phase, the moon looks like a semicircle. Half of the lighted side of the moon is facing the earth. When a waxing moon becomes a semicircle, it enters the first-quarter phase. When the visible portion of the moon is larger than a semicircle and still increasing, the moon is in the waxing-gibbous phase. The moon continues to wax until it appears as a full circle. At full-moon the earth is between the sun and the moon the entire half of the moon reflecting the light of the sun is visible from the earth. After the full moon phase, the portion of the moon visible from the earth decreases. The moon is then waning. After the full-moon phase, the moon enters the waning-gibbous phase. In this phase the visible portion of the moon is still larger than a semicircle, but this portion is decreasing in size. When the visible portion of the moon becomes a semicircle, the moon enters the last-quarter phase. When only a sliver of the moon is visible, the moon enters the waning-crescent phase. After the waning-crescent phase, the moon again moves between the earth and the sun. The moon becomes a new moon, and the cycle of phases begins again.
In the crescent phases before and after a new moon, only a small part of the moon shines brightly. However, the rest of the moon is not completely dark. It shines dimly from sunlight that reflects off the earth's clouds and oceans and then reflects off the moon. Sunlight reflected off the earth is called earthshine.
Although the moon revolves around the earth in 27.3 days, a longer period of time is needed for it to go through a complete cycle of phases. The period from one new moon to the next is 29.5 days. The cause of this 2.2-day difference is the orbiting of the earth and the moon around the sun.
Therefore the moon must go a little farther to get directly between the earth and the sun. This is the position of the moon in the new moon phase. About 2.2 days are needed for the moon to travel this extra distance.
The above Logo Program of the Moon Phases was created by the very talented sixth grade students during Spring 2001
Procedures
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