Planetary Motion


The planets orbit the sun not in perfect circles, but in ellipses.Image

  • Who Discovered it: Johannes Kepler
  • Year of Discovery: 1609

How was it Discovered?

Even after Copernicus simplified and corrected the structure of the solar system by discovering that the sun, not the earth, lay at the center of it, he (like all astronomers before him) assumed that the planets orbited the sun in perfect circles. As a result, errors continued to exist in the predicted position of the planets.

Johannes Kepler was born in Southern Germany in 1571, 28 years after the release of Copernicus’s discovery. Kepler suffered through a troubled upbringing. His aunt was burned at the stake as a witch. His mother almost suffered the same fate. The boy was often, sick and had bad eyesight that glasses could not correct. Still, Kepler enjoyed a brilliant—but again troubled—university career.

In 1597 he took a position as an assistant to Tycho Brahe, famed German astronomer.
For decades Tycho had been measuring the position of the planets (especially Mars) with far greater precision than any other European astronomer. When Tycho died in 1601 he left all his notes and tables of planetary readings to Kepler.

Kepler rejected the epi-circle on an epi-circle model of how planets moved and decided to work out an orbit for Mars that best fit Tycho’s data. It was still dangerous to suggest that the sun lay at the center of the solar system. The all-powerful Catholic Church had burned Friar Giordano Bruno at the stake for believing Copernicus. No other scientist had dared come forth to support Copernicus’s radical notion. Still, Kepler was determined to use Copernicus’s organization for the universe and Tycho’s data to make sense of the planets.

Kepler tried many ideas and mathematical approaches that didn’t work. His bad eye-sight prevented him from making his own astronomical sightings. He was forced to rely entirely on Tycho’s existing measurements. In bitter frustration, he was finally driven to consider what was—at the time—unthinkable: planetary orbits that weren’t perfect circles.

Nothing else explained Tycho’s readings for Mars. Kepler found that ellipse (elongated circles) fit far better with the accumulated readings. Yet the data still didn’t fit. In desperation, Kepler was forced to consider something else that was also unthinkable at that time: maybe the planets didn’t orbit the sun at a constant speed.

With these two revolutionary ideas, Kepler found that elliptical orbits fit perfectly with Tycho’s measured planetary motion. Elliptical orbits became Kepler’s first law. Kepler then added his Second Law: each planet’s speed altered as a function of its distance from the sun. As a planet flew closer, it flew faster.

Kepler published his discoveries in 1609 and then spent the next 18 years calculating detailed tables of planetary motion and position for all six known planets. This was also the first practical use of logarithms, invented by Scotsman John Napier during the early years of Kepler’s effort.

With these tables of calculations (which exactly matched measured planetary positions) Kepler proved that he had discovered true planetary motion.

Kepler discovered the concept of the ellipse and proved that planets actually follow slightly elliptical orbits. With this discovery, science was finally presented with an accurate picture of the position and mechanics of the solar system. After 400 years of vastly improved technology, our image of how planets move is still the one Kepler created. We haven’t changed or corrected it one bit, and likely never will.