Orbit

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Orbit->> LAWS OF MOTION

Early in the 17th century, the German astronomer and natural philosopher Johannes Kepler deduced three laws that first described the motions of the planets about the sun: (1) The orbit of a planet around the sun is an ellipse. (2) A straight line from the planet to the center of the sun sweeps out equal areas in equal time intervals as it goes around the orbit; the planet moves faster when closer to the sun and slower when distant. (3) The square of the period (in years) for one revolution about the sun equals the cube of the mean distance from the sun's center, measured in astronomical units.

The physical causes of Kepler's three laws were later explained by the English mathematician and physicist Isaac Newton as consequences of Newton's laws of motion and of the inverse square law of gravity. Kepler's second law, in fact, expresses the conservation of angular momentum. Moreover, Kepler's third law, in generalized form, can be stated as follows: The square of the period (in years) times the total mass (measured in solar masses) equals the cube of the mean distance (in astronomical units). This last law permits the masses of the planets to be calculated by measuring the size and period of satellite orbits.

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