Astronomia nova

Title Page of Kepler's Astronomia nova (1609)

Astronomia nova, or to give its full title in English, New Astronomy, Based upon Causes, or Celestial Physics, Treated by means of Commentaries on the Motions of the Star Mars, from the Observations of Tycho Brahe, Gent., was written by Johannes Kepler and published in 1609. As its title indicates, in this book Kepler employed physical principles and Tycho Brahe's observations to develop his first two laws of planetary motion:

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.

Interestingly, the second law was found before the first. This is logic in so far that the dates for the second law follow out of the observations themselves, when you strict stay to them. And that is what Kepler did "in the long run", accepting the observations by Tycho as facts. When he started his calculations to determine the orbit of Mars he presumed a mysterious center from which the orbit would be a circle. After 70 calculations he concluded this as false. The movement was not (as presumed by earlier philosophers and astronomers, incliuding Copernicus) "circular and uniform".

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

• Kepler's Discovery, an online module on Kepler's scientific method

References

• Johannes Kepler, New Astronomy, translated by William H. Donahue, Cambridge: Cambridge Univ. Pr., 1992. ISBN 0-521-30131-9