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 its two foci. This law is fundamental in understanding planetary motion under gravitational forces.

5 Must Know Facts For Your Next Test

1. Kepler's First Law describes the shape of planetary orbits as elliptical, not circular.
2. The Sun occupies one of the two foci of the ellipse, not at the center.
3. The closest point in a planet's orbit to the Sun is called perihelion, and the farthest point is called aphelion.
4. Ellipses are characterized by their eccentricity; an eccentricity of zero represents a perfect circle.
5. This law was derived from Tycho Brahe's extensive astronomical observations.

Review Questions

• What shape does Kepler's First Law state that planetary orbits take?
• Where is the Sun located in relation to a planet's elliptical orbit?
• What terms describe the closest and farthest points in a planet's orbit around the Sun?

Related terms

Ellipse: A geometric figure where the sum of distances from any point on its curve to two fixed points (foci) is constant.

Eccentricity:

A parameter that determines how much an ellipse deviates from being circular; defined as $e = \sqrt{1 - \frac{b^2}{a^2}}$ where $a$ and $b$ are the semi-major and semi-minor axes respectively.

Perihelion:

The point in an orbit where a planet or other celestial body is closest to the Sun.