Unit 4: The Earth, the Moon, and the Sun

Unit 4: The Earth, the Moon, and the Sun

The motions of the Moon around the Earth and 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 turn on their axis (rotation) but 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 Moon's rotation on its axis and its revolution around the Earth require the same amount of time (29.5 days) explains why the Moon always shows its same side to the Earth.

Learning goals

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