Question

Explain the difference between rotation and revolution. Then use these terms to explain how Earth, the moon, and the Sun move relative to one another.

Answer 1

Rotation is an object spinning around its own axis. Revolution is an object moving along a path (an orbit) around another object.

How that applies to the Earth–Moon–Sun system:

- Earth
- Rotation: Earth spins on its axis once about every 24 hours (a sidereal day is ~23 h 56 m). This rotation causes day and night because different parts of Earth face toward or away from the Sun.
- Revolution: Earth orbits (revolves around) the Sun once every ≈365.25 days (one year). Because Earth’s axis is tilted about 23.5° relative to its orbital plane, this revolution produces the seasons.

- Moon
- Revolution: The Moon orbits (revolves around) Earth about once every 27.3 days (relative to the stars). This orbit is why the Moon changes position relative to Earth and Sun.
- Rotation: The Moon also rotates on its own axis, and it does so in the same time it takes to orbit Earth (synchronous rotation). That is why we always see nearly the same face of the Moon from Earth.
- Note: the Moon and Earth both orbit their common center of mass (the barycenter), which lies within Earth but not at its exact center.

- Sun
- Apparent daily motion of the Sun across our sky is not the Sun rotating around us but Earth’s rotation on its axis.
- The Sun itself rotates on its axis (about 25 days at the equator, longer near the poles because the Sun is gaseous).
- The Sun (with the whole solar system) also revolves around the center of the Milky Way galaxy on a timescale of hundreds of millions of years.

Putting it together: Earth rotates to produce day/night and revolves around the Sun once per year. The Moon revolves around (and rotates with) Earth, causing phases and the same lunar face to be visible. All three bodies also participate in larger motions (Sun’s rotation and the solar system’s galactic orbit).