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Kepler's laws

Three laws of planetary motion introduced by the astronomer Johannes Kepler (1571-1630): (i) Each planet moves in an orbit that is an ellipse with the Sun at one focus (See conic). At the time, this was of particular significance because it superseded the concept of circular orbits, and gave the first real indication that the Sun controls the movement of the planets.

(ii) A line joining the Sun to a planet sweeps out area (as shaded in the diagram opposite) at a constant rate. This is a consequence of the law of conservation of angular momentum.

(iii) The square of the period of the orbit is proportional to the cube of the length of the major axis of the ellipse. This resulted in the first accurate prediction of the relative distances of planets from the Sun (since their orbital periods were well known).

Kepler\'s laws were explained later in the 17th century by Newton\'s law of gravitation.

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Kepler's laws

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