Between 1615 and 1621 Johannes Kepler published the seven volumes of Epitome astronomiae Copernicanae while living in Linz. In the Epitome he outlined his most famous discovery, the three laws of planetary motion. Among his other achievements Kepler provided an account of how vision occurs, developed an explanation for the behaviour of light in a telescope, offered a theoretical foundation for astrology, and his Astronomiae Pars Optica is considered the foundation of modern optics.
In 1600 in a prelude to the upheavals of the Thirty Years War the Catholic authorities in Graz moved against Protestants in the town. Kepler, a Lutheran, found his school forcibly closed and, after refusing to convert to Catholicism, Kepler and his family were banished from Graz upon pain of death should they return. Responding to Tycho Brahe’s earlier invitation Kepler made for Prague where Brahe set him to working planetary observations. Brahe’s unexpected death in 1601 led quickly to Kepler being appointed Imperial Mathematician to Rudolf II, Holy Roman Emperor.
Brahe had bequeathed his years of observational data to Kepler who swiftly found that they did not support his earlier work in the Mysterium Cosmographicum (1596), which defended the Copernican system and proposed that the distance between the planets and the sun could be understood by a model that contained the five Euclidean solids nested within the sphere of Saturn’s orbit. Careful study of Brahe’s observations led Kepler, after initially rejecting the correct answer due to arithmetical mistakes, to the conclusion that the planets did not move in circular orbits.
After trying several formulae based on various oval-like curves Kepler applied the formula for an ellipse, first codified by Apollonius of Perga (c. 292 BC – c. 190 BC), finding that it matched Brahe’s observations perfectly. “Ah, what a foolish bird I have been,” Kepler opined.
Kepler had discovered that Mars moved around the Sun in an ellipse, not in a circle. His first law of planetary motion stated that the orbit of a planet is an ellipse with the Sun as one of two foci around which the planet orbits. His second law, that planets sweep out equal areas in equal times, derives from the observation that planets speed up when closest to the Sun and slow down when further away. His third law, discovered several years later, demonstrates the precise connection between the size of a planet’s orbit and the period for it to go once around the Sun. In 1622 Kepler demonstrated that the third law applied to the four brightest moons of Jupiter as well as the planets, becoming the first person in history to understand how the planets move. Within the century Sir Isaac Newton’s law of universal gravitation provided the explanation as to why planetary bodies moved in an elliptical path.
For a far better explanation of the three laws of planetary motion than I could ever hope to give here’s the great, and sadly missed, Carl Sagan
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