The Moon

A full moon image. The moon is a small world, with a diameter of about 2000 miles (the earth's diameter is 8000 miles). It has no water and no atmosphere and is a very dead world.

The moon orbits the earth, taking about a month to complete a revolution. That seems slow, but it is fast enough that if you go outside and look at the moon and then go back the next night at the same time, you will see the moon has moved a noticeable amount. In fact, it moves about 13 degrees in 24 hours. Because it moves in its orbit from west to east, the moon rises about 50 minutes later each night. Don't confuse this west to east orbital motion with the east to west motion during the night due to the earth's rotation.


As the moon orbits the earth, it seems to change its shape. We call this "going through phases." The moon is not actually changing, though. It only seems that way.

The moon does not produce any light of its own. It simply reflects light from the sun. To understand how this creates phases, look at the diagram below. Here we see the moon's orbit around the earth as we look down on the North Pole. The moon moves counterclockwise in its orbit. Notice that no matter where the moon is in its orbit, the side facing the sun is lighted and the other side is dark. Consult this table as you read about the phases below.

Phases of the Moon diagram.
A new moon images. Let's start with the position labeled new. As seen from the earth, the moon would be invisible, since the dark side is facing the earth.
A waxing crescent moon. As the moon moves counterclockwise from this position, it appears as a crescent. Each night the crescent grows larger. These are called waxing crescent phases (waxing means growing).
A first quarter moon image. When the moon has gone a quarter of the way around in its orbit, then from the earth we would see the right half of the moon lighted and the left half dark. This is called first quarter (some people refer to this as a half moon, but that's not correct).
A waxing gibbous moon image. Next the moon goes through a series of waxing gibbous phases, in which more than half of the side facing earth is lighted.
A full moon image. Then full moon occurs. The entire side facing earth is now lighted.
A waning gibbous moon image. Next is a series of gibbous phases again. This time, though, the left side of the moon's face is lighted and the right side is dark. These are called waning gibbous phases (waning means dying or going away).
A third quarter moon image. Three fourths of the way around in its orbit is last quarter or third quarter. This looks just like first quarter except the light and dark areas are reverse.
A waning crescent moon image. Finally comes a series of waning crescent phases. This will then lead back to new moon again.


The moon is also responsible for both lunar and solar eclipses.

An image of a lunar eclipse. A lunar eclipse occurs when the moon passes through the earth's shadow. The moon produces no light of its own. It simply reflects sunlight. So when the moon passes into the earth's shadow it goes dark. Interestingly, though, it doesn't go completely dark. A small amount of light passing through the earth's atmosphere is bent just enough to dimly light the moon with a pale reddish color.

Lunar eclipses occur infrequently. Some years have none, but there can be as many as three in one year.

An image of a solar eclipse. A solar eclipse occurs when the moon passes between the earth and the sun. The sun is about 400 times larger than the moon, but it's also about 400 times farther away. So the son and moon appear the same size in our sky. Therefore, then the moon passes exactly between the erath and moon, the sun's disk is completely covered, allowing us to see the corona, the layer of hot gasses surrounding the sun.

Solar eclipses occur more often than lunar eclipses. There are at least two solar eclipses each year and there can be as many as five, but that's very rare. Unfortunately, a particular solar eclipse can only be seen from a small part of the world. To see a total eclipse, you must somewhere along a narrow, less than 200 mile path. So eclipses are rarely seen from a given location.

An image of an eclipse in progress. The next total solar eclipse visible from South Carolina will be in 2017.