Kepler's Laws

First published: 2024

Brief summary

NASA overview of periodic orbital motion and Kepler's laws.

Article

Kepler’s first law states that planets travel around the Sun in elliptical orbits with the Sun at one focus. A circle is a special case of an ellipse, and the eccentricity of an orbit describes how much its shape differs from a circle.

The second law states that a line joining a planet and the Sun sweeps out equal areas during equal intervals of time. A planet therefore moves faster when it is closer to the Sun and more slowly when it is farther away.

The third law relates orbital period to orbital size. For bodies orbiting the same central mass, the square of the orbital period is proportional to the cube of the orbit’s semimajor axis, so more distant planets take longer to complete an orbit.

Kepler derived these laws from observations of planetary positions, particularly measurements of Mars. Newton later showed that the laws follow from gravitational attraction and the laws of motion, providing a general framework for planets, moons, comets and artificial satellites.

Source details and credits

• Source / publisher: NASA Science
• Source type: Educational article
• URL type: WWW
• Credits: NASA Science
• URL: https://science.nasa.gov/solar-system/keplers-laws/