German astronomer Johannes Kepler (1571–1630) put forth the theory that planets, including Earth, rotate around the sun in elliptical orbits. But he had the help of research conducted by astronomer Tycho Brahe (1546–1601) to use as a basis for his conclusion.

In 1600 Kepler moved to Prague (Czech Republic), where he began working as an assistant to the flamboyant Brahe, a Danish aristocrat who a few years earlier (in 1576) had set up the first real astronomical observatory in history. Brahe’s benefactor in this work was none other than King Frederick II (1534–1588), ruler of Denmark and Norway, who was a patron of science. In the observatory, Brahe had made and recorded extraordinarily accurate observations of planetary positions. Even though he rejected the Copernican (sun-centered) view of the universe because it violated church beliefs, he also realized that his observations of the planets could not be explained by the Ptolemaic system. He soon put forth his own theory of planetary orbit. The Tychonic theory was something of a compromise between the two existing models (the sun-centered system of Copernicus and Earth-centered system of Ptolemy): Brahe’s model followed Copernicus’s theory in that it, too, had the planets orbiting around the sun; but it kept to the Ptolemaic belief that the sun orbited Earth (in this way, he accounted for a year). The theory was ignored.

But when Brahe hired Kepler in 1600, he turned over his observations to him and charged him with the task of devising a theory of planetary motion. As a mathematician, Kepler was the right man for the job, and he devoted himself to the effort for the next 20 years. At one point, Kepler had devised a scheme that almost matched Brahe’s observations, but not quite. Believing Brahe’s observations were perfectly accurate, Kepler threw out the scheme and started again. Finally, he gave up on using circular orbits and epicycles (smaller orbits centered on the larger ones) and began working with ellipses (ovals). When Kepler charted the planets’ orbits as ellipses, the results matched Brahe’s data. In 1609 Kepler published his first two laws of planetary motion (in the work Astronomia nova): a planet orbits the sun in an ellipse, not a circle (as Copernicus had believed); and a planet moves faster when near the sun and slower when farther away.

We also have Kepler to thank for a word that is in everyday use: satellite. After Galileo discovered the moons of Jupiter, Kepler used a telescope to view them for himself. He dubbed them satellites (satelles is from the Latin meaning “attendant”), a name that stuck. The celebrated astronomer also did pioneer work leading to the invention of calculus (late 1600s).