Craig C. Freudenrich
Johannes Kepler (1570-1630) was a German astronomer and mathematician. His story is a classic example of how theory and observation meld together in science. It begins with the night sky. If you observe a planet like Mars over the course of a year and note its position against the starry background, you will see that it (along with the other planets) moves in a "loopy" fashion. It moves forward for a time, then in reverse, and then forward again. This movement puzzled scientists from the time of the ancient Greeks through the Renaissance. Astronomer Nicholas Copernicus proposed that the planets orbited in circles around the sun. Copernicus contradicted the accepted idea of the Greek philosopher Ptolemy -- that the sun and planets orbited the Earth in circles. Copernicus' model worked about as well as Ptolemy's, but neither one was really correct.
During Kepler's time, there were only six known planets. Kepler believed that God placed the orbits of the six planets in the shapes of regular geometric solids (such as a sphere, cube or tetrahedron). He tried to make models where these solids were nested inside each other. The models never agreed, however, with his observations. He decided that his observations were in error and needed better data.
Kepler went to work with a Danish nobleman named Tycho Brahe who was the best observational astronomer of the time. Tycho's precise observations of the positions and movements of the planets were just what Kepler needed. Kepler and Tycho had an intense and jealous rivalry, however. Kepler was the better theorist, but Tycho was the superior observer and they argued constantly. Tycho often withheld key data. When Tycho died, Kepler convinced the surviving family members to give him Tycho's information.
For years, Kepler struggled to make various geometrical shapes fit Tycho's data, but to no avail. The theory was always off by small fractions of a degree. Then, he hit upon one shape that worked: the ellipse. Things fell into place. Kepler was able to work out three laws of planetary motion, which he published in 1609:
- The orbits of planets are elliptical with the sun at one focus.
- As planets orbit, they sweep out sections of equal areas in equal time intervals; planets travel faster when close to the sun and slower when farther away.
- The period of time it takes a planet to orbit the sun is related to its distance from the sun.
Astronomers use Kepler's laws today to calculate the orbits of newly discovered asteroids and comets and the paths of interplanetary spacecraft.
view of the sunrise from space (stockbyte/getty images)
Johannes Kepler, the German astronomer and contemporary of Galileo, attempted to reconcile the views of Copernicus with observations by other astronomers. In painstakingly recording the planetary movements, Kepler observed that planets did not move in circular orbits, as was previously believed. He discovered that planets actually traveled along elliptical paths.
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