Kepler’s first law

Definition

Kepler's First Law states that the orbit of a planet around the Sun is an ellipse with the Sun at one of the two foci. This law is also known as the Law of Ellipses and describes the shape of planetary orbits.

5 Must Know Facts For Your Next Test

1. Kepler's First Law was formulated by Johannes Kepler in the early 17th century.
2. The elliptical shape means that planets are sometimes closer to and sometimes farther from the Sun during their orbit.
3. The point where a planet is closest to the Sun is called perihelion, and the point where it is farthest is called aphelion.
4. The eccentricity of an orbit determines how elongated it is; an eccentricity of 0 corresponds to a perfect circle.
5. Kepler's laws were based on precise observational data collected by Tycho Brahe.

Review Questions

• What does Kepler's First Law state about planetary orbits?
• What are perihelion and aphelion in relation to Kepler's First Law?
• Who provided the observational data that Kepler used for his laws?

Related terms

Elliptical Orbit:

An orbit shaped like an ellipse, characterized by its eccentricity.

Perihelion:

The point in a planet's orbit where it is closest to the Sun.

Eccentricity:

A measure of how much an orbit deviates from being circular.