What takes 24 hours to complete?

• A. Revolution
• B. Precession
• C. Orbit
• D. Rotation

Answer: (D)

Rotation is the turning of Earth on its axis, and that spin takes about 24 hours to complete. As Earth rotates, different areas face the Sun, creating the day-night cycle we experience each day. The other motions are different: revolution is Earth's orbit around the Sun, which takes about a year; precession is a very slow wobble of the axis over thousands of years; orbit refers generally to a path around a central body, which for Earth is the yearly path around the Sun. So the 24-hour period corresponds to Earth's rotation, not its revolution or precession or orbital path. For reference, the solar day is about 24 hours, while a sidereal day (relative to distant stars) is about 23 hours 56 minutes.