1619

Three Laws of Planetary Motion

Johannes Kepler (1571–1630)

While there is significant overlap, astronomers today can be generally characterized as either observationalists, those who primarily collect data from telescopes or space missions, or theorists, those who primarily try to develop models or theories to explain existing observations. Most astronomers (and astrologers) from antiquity through the Middle Ages were observationalists who dabbled in theory. Theoretical astronomy had been primarily considered to be the realm of philosophers, not physicists.

The Renaissance German mathematician, astrologer, and astronomer Johannes Kepler changed that paradigm and arguably became the world’s first theoretical astrophysicist. Kepler worked with data from others—most notably Tycho Brahe and Galileo—in his quest to develop a single unifying model of the cosmos. A deeply religious man, Kepler believed that God had designed the universe in an elegant geometric plan, and that the plan could be reasoned out through careful observations.

Kepler believed in Copernicus’s heliocentric cosmology, and also believed that a Sun-centered solar system was entirely consistent with biblical writings. Kepler’s book Astronomia Nova (New Astronomy; 1609) described the orbits of Mars and the other planets as elliptical, not circular (first law), and asserted that the planets change speed in a way that allows them to sweep out equal areas in equal time as they orbit (second law). Later, in Harmonices Mundi (Harmony of the Worlds; 1619) he showed that a planet’s orbital period squared is proportional to its average distance from the Sun cubed (P²  a³; third law). Through Kepler’s patience and persistence, the harmony that he sought among the worlds was finally revealed.

A 1610 portrait of Kepler by an unknown artist.

Kepler’s laws were not widely appreciated until observationalists had verified their precise timing predictions during eclipses and planetary transits (the predictions were right), and ultimately until Isaac Newton found in 1687 that Kepler had discovered the natural consequences of a universal law of gravitation.

Johannes Kepler struggled to find divine perfection in the orbits of the known planets by trying to match them with the shapes of the so-called perfect solids (i.e., cube, tetrahedron [pyramid], octahedron, icosahedron, and dodecahedron) in this illustration from his book Mysterium Cosmographicum (1596).