Kepler's law

noun

, Astronomy.

1. any one of three laws governing planetary motion: each planet revolves in an ellipse, with the sun at one focus; the line connecting a planet to the sun sweeps out equal areas in equal periods of time law of areas; or the square of the period of revolution of each planet is proportional to the cube of the semimajor axis of the planet's orbit harmonic law.

Word History and Origins

Origin of Kepler's law¹

After J. Kepler

Example Sentences

He determined that Kepler's law that areas were swept out in equal times implied that gravity acts in the direction of a line between the planet and the sun.

From Project Gutenberg

Only one kind of force would satisfy Kepler's requirement that the sun was a focus of an ellipse and still be consistent with Kepler's law that the square of a planet's period was proportional to the cube of its mean distance from the sun; that was the inverse square law.

From Project Gutenberg

The generalizations which can be collected on the subject from direct observation, even such as Kepler's law, are mere approximations: the planets, owing to their perturbations by one another, do not move in exact ellipses.

From Project Gutenberg

Jupiter, which shows the greatest effect, makes the circuit of his orbit in 4,333 days instead of 4,335, which it would require if Kepler's law were strictly true.

From Project Gutenberg

But in another respect the change is of the utmost importance, since it enables us to extend Kepler's law, which relates solely to the sun and its planets, to other attracting bodies, such as the earth, moon, and stars.

From Project Gutenberg