Renaissance/Early Modern

Johannes Kepler

1571–1630

“One cannot leave to the astronomer absolute license to feign whatever hypotheses he pleases. It is necessary that he form his hypotheses not capriciously but with reason.”

Epitome of Copernican Astronomy, Book IV

Johannes Kepler was a German mathematician who served as imperial mathematician in the Holy Roman Empire. Working from Tycho Brahe's observations, he framed the three laws of planetary motion in the and related works. Planets move in ellipses with the sun at one focus. Equal areas are swept in equal times. Period and mean distance stand in fixed proportion. Astronomy, he insisted, belongs to physics. Geometrical devices that merely save the appearances were no longer enough. Newton built celestial mechanics on these empirical constraints.

In the Conversation

2 of the Great Ideas

Eternity & Infinity

Angel

The planets move by natural physical forces, not by angelic intelligences; the celestial movers of Aristotle and Aquinas are an explanation that explains nothing.

Natural Science

Astronomy

Astronomy is part of physics; the planets move in ellipses, not circles, and their motions are governed by physical causes, not mere mathematical devices.