Sun-Earth-Moon #2

The motions of the Moon around the Earth and of the Earth around the Sun are complex. The motions involved in revolutions are superimposed on the movements involved in rotations. The Earth and the Moon both turn on their own axis (rotation), but both also move around another object (revolution).

The rotation of the Earth (24 hours) explains the alternation of day and night.
The revolution of the Earth around the Sun (365.25 days), because of the inclination of its axis (not shown here), explains the changing seasons.

The fact that the rotation of the Moon on its own axis and its revolution around the Earth both require the same amount of time (29.5 days) explains why the Moon always shows its same side to the Earth.

In this animation, neither proportions nor time scales are accurately presented.

• To differentiate between the motions of rotation and revolution
• To show why the Moon always shows the same face to the Earth
• To distinguish between the terrestrial and the lunar orbits.

The period of rotation of an object is the time it needs to complete one full turn on its own axis. This should not be confused with the period of revolution, which is the time needed for an object to complete one orbit around a second object.
In the case of the Sun-Earth-Moon system, note the following combinations:

• 24 hours is the rotational period of the Earth on its axis. This motion is responsible for the alternation of day and night.
• 365 days is the period of the Earth’s revolution around the Sun. The inclination of the Earth on its axis as it revolves around the Sun explains the changing seasons.

The fact that the rotation of the Moon on its own axis and its revolution around the Earth both require the same amount of time (29.5 days) .This synchronous rotation explains why the Moon always shows its same side to the Earth.
This was not always the case.
Tidal forces imposed on the Moon by the Earth gradually slowed the Moon’s speed of rotation until this synchronization was attained.

Similarly, the rotation of the Earth is slowed down by tidal forces imposed on the planet by the Sun. This slowing results in a gradual lengthening of the day, which, it is estimated, lasted only 22 hours 400 million years ago.