Astronomia nova

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Astronomia nova (A new astronomy), written by Johannes Kepler and published in 1609, set out the evidence for what came to be known as Kepler's laws of planetary motion. This book contained (in somewhat different form) the first two laws:

1. that planets move in elliptical orbits with the sun at one focus, and
2. that planets do not move with constant speed along this orbit but their speed varies so that the line joining the centers of the sun and a planet sweeps out equal parts of the ellipse in equal times.

To these two laws Kepler added a third a decade later, in his book Harmonices Mundi (Harmonies of the world). The third law sets out a proportionality between the third power of the average distance of a planet from the sun and the square of the length of its year.

Futher links: http://library.thinkquest.org/C0116544/ http://www.library.usyd.edu.au/libraries/rare/modernity/kepler4.html