Earth's Moon - Revolve? Rotate?
The moon and Earth both revolve and rotate on their way around in our solar system, but what is the difference between them?
Rotation is when a planet or moon turns all the way around or spins on its axis one time. The axis of rotation is an imaginary line going from the North Pole to the South Pole. When a planet or moon travels around the body it is orbiting one time, this is a revolution. On Earth, a rotation is pretty short - it happens once a day! It is the rotation that makes the sun come up in the morning and set at night. On Earth, a revolution is quite a bit longer - one year! It takes Earth one year to go all the way around the sun.
The moon is a little different. The moon rotates once about every 27 days, and revolves once about every 27 days. So every time the moon goes around Earth it turns around one time. That is why the moon always looks the same - we only ever see one side of it! Another strange thing is that if we lived on the Moon and thought of days and years the same way we do on Earth, a day AND a year would be the same length! Since the rotation and revolution times are the same, the length of a day and the length of a year are identical.
Here is a test to see if you can tell the difference between a rotation and a revolution:
In the above picture, which object is rotating?
Which object is revolving?
What is the imaginary line that Earth rotates upon?
One rotation of Earth is a and one revolution of Earth is a