Kepler’s third law

Definition

Kepler’s third law states that the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. This law allows for the calculation of distances between planets and their stars based on their orbital periods.

5 Must Know Facts For Your Next Test

1. Kepler's third law can be mathematically expressed as P^2 = a^3, where P is the orbital period in years and a is the semi-major axis in astronomical units (AU).
2. The law applies to any object orbiting another object under gravity, not just planets around the Sun.
3. It was derived empirically by analyzing data from planetary observations made by Tycho Brahe.
4. Kepler’s third law helped confirm that planetary orbits are elliptical, not circular.
5. This law is crucial for understanding the scale of our solar system and other star systems.

Review Questions

• What does Kepler's third law state about the relationship between a planet's orbital period and its distance from the star?
• How can Kepler’s third law be used to calculate the distance between a planet and its star?
• Why was Kepler's third law significant in terms of understanding planetary orbits?

Related terms

Kepler's First Law: Describes that planets move in elliptical orbits with the Sun at one focus.

Orbital Period:

The time taken for an object to complete one full orbit around another object.

Semi-Major Axis: The longest radius of an ellipse, half of which represents the average distance between an orbiting body and its primary focus.