Planetary Motion
Year of Discovery: 1609
What Is It? The planets orbit the sun not in perfect circles, but in ellipses.
Who Discovered It? Johannes Kepler
Why Is This One of the 100 Greatest?
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.
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
pictures 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.
How Was It Discovered?
For 2,000 years, astronomers placed the earth at the center of the universe and assumed
that all heavenly bodies moved in perfect circles around it. But predictions using this system
never matched actual measurements. Scientists invented epi-circles—small circles that the
planets actually rolled around that, themselves, rolled around the great circular orbits for
each planet. Still there were errors, so scientists created epi-circles on the epi-circles.
Copernicus discovered that the sun lay at the center of the solar system, but still assumed that all planets traveled in perfect circles. Most epi-circles were eliminated, but errors in planetary plotting continued.
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.
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