Johannes Kepler inferred quantitative laws of planetary motion from astronomical observations. In Astronomia nova by Johannes Kepler, published in 1609, the problem of Mars's path led him to an ellipse with the Sun at one focus and an equal-area timing law.
The equal-area law states that the line from the Sun to the planet sweeps out equal areas in equal times. Together, a geometric orbit and a timing law specify where the planet moves and how its speed changes. Their later connection to a central force and an inverse-square law is explained in inferring an inverse-square force from Kepler laws; those dynamical deductions should be distinguished from Kepler's original empirical construction.[ref]
Astronomia nova by Johannes Kepler. Mars investigation and the first two planetary laws; digitized 1609 primary book
Portrait of Johannes Kepler
. Artist unknown; Photographische Gesellschaft Berlin reproduction, Smithsonian Libraries; file dates underlying portrait to 1620. Public domain (life+70 and U.S. pre-1931 publication tags). Source and usage information on the linked Wikimedia file page. Source.
Johannes Kepler (1571–1630) was a German mathematician, astronomer, and astrologer best known for his groundbreaking work in the field of astronomy. He is most famous for formulating the three laws of planetary motion, which describe the orbits of planets around the Sun. These laws are: 1. **Kepler's First Law (The Law of Ellipses)**: Each planet moves in an ellipse with the Sun at one of the two foci.
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