Retrograde motion

Definition

Retrograde motion is the apparent backward movement of a planet against the background of stars. This phenomenon occurs when Earth, which moves faster in its orbit, overtakes and passes an outer planet.

5 Must Know Facts For Your Next Test

1. Retrograde motion was a significant puzzle for ancient astronomers who believed in geocentric models.
2. It typically affects the outer planets like Mars, Jupiter, and Saturn.
3. The phenomenon is best explained by the heliocentric model proposed by Copernicus.
4. Retrograde motion occurs periodically as Earth catches up to and passes these slower-moving outer planets.
5. The term 'retrograde' means 'moving backward,' but this motion is only apparent from Earth's perspective.

Review Questions

• What causes retrograde motion?
• Which model of the solar system provides a clear explanation for retrograde motion?
• Why did ancient astronomers find retrograde motion puzzling?

Related terms

Geocentric Model:

An ancient model of the universe with Earth at its center.

Heliocentric Model:

A model of the solar system that places the Sun at its center.

Epicycle:

A small circle along which a planet moves, whose center describes a larger circle (deferent) around Earth, used in Ptolemaic system to explain retrograde motion.