ANSWER from Larry Kellogg on April 26, 2001:
The moon is a natural satellite of Earth. It goes around the Earth on an elliptical orbit in about 27 Earth days. It also rotates on its own axis in about 27 days. The Near Side of the moon always faces Earth as it pulls on Earth and Earth pulls on the moon.
The orbit of the moon sweeps out a plane that is tilted a little from the plane that the Earth would describe while going around the Sun (called the ecliptic plan) so when you are looking at a Full Moon, the moon could be level with a line drawn from the Sun through Earth, and then the shadow of Earth would fall on the moon, and you would have a Lunar Eclipse. Usually the moon is somewhere above or below the ecliptic at Full Moon and the Earth's shadow streams out into space but misses the moon during a full moon.
A New Moon happens about two weeks later when the moon has gone around the Earth and is now between the Sun and Earth. The moon has turned half way around and the Face of the Moon is now dark. (Night time on the near side of the moon) Sometimes you can dimly see the dark face of the moon from the light reflected back off of Earth. If it was the time of year when a New Moon happened at the same time the moon was crossing the ecliptic plane, well then the Moon's shadow could fall on Earth and you would have an eclipse of the Sun. (If it was day time on Earth at the moment this happened and you were in the Moon's shadow, the Sun would be block out.)
Just why the Moon is locked into a revolution about its axis that is the same length as it takes to go around Earth is something scientists wonder about. There are different ideas as to how the Moon was formed and these theories need to account for a two-week day-night speed of rotation for the Moon.
The Recession of the Moon and the Age of the Earth-Moon System
Copyright © 2000 by Tim Thompson
[Last Update: December 1999]
You can find information on the Phases of the Moon at the Lunar Prospector web site.
The Moon doesn't produce its own light, but looks bright because it reflects light from the Sun. Think of the Sun as a light bulb, and the Moon as a mirror, reflecting light from the light bulb. The lunar phase changes as the Moon orbits the Earth and different portions of its surface are illuminated by the Sun.
The Phases of the Moon
The diagram shows the Moon in different positions along its orbit around the Earth. The Sun is off in the distance, lighting the Earth-Moon system. At any position, half of the Moon is illuminated by the Sun (the light side of the Moon) and half is not (the dark side). Also, half of the Moon is visible to the Earth (the near side of the Moon) and half is not (the far side). As the Moon moves around the Earth, we can see different fractions of the illuminated half of the Moon.
The Orbit of the Moon
It takes the Moon 27.322 days to go around the Earth once. Because of this motion, the Moon appears to move about 13° against the stars each day, or about one-half degree per hour. If you watch the Moon over the course of several hours one night, you will notice that its position among the stars will change by a few degrees. The changing position of the Moon with respect to the Sun leads to lunar phases.
The force of gravity caused by an object gets weaker as you move farther away from that object. In this picture, the Earth is pulling on the Moon, and the Moon is pulling on the Earth. The Moon pulls more strongly on the side of the Earth facing the Moon than on the side facing away from the Moon. Because the gravitational force on one side of the planet is different from that on the other side, it is called a tidal force.
The Earth's Moon
If you have a Windows PC you may want to down load the program for Home Planet and you will be able see where the stars and the moon are for every night.
You can explanation of the differences in the size of the Moon at John Walker's Inconstant Moon.
More information about the moon.
The Lunar and Planetary Institute
In the Earth's night sky, you can gaze into the atmosphere to see the Moon, our own companion world. Read here