SCIENCE IN THE NEWS DAILY

An Astronomer's Astronomer: Kepler's Revolutionary Achievements in 1609 Rival Galileo's

from Scientific American

Four hundred years ago this year, two events marked what scientists and historians today regard as the birth of modern astronomy. The first of them, the beginning of Galileo's telescopic observations, has been immortalized by playwrights and authors and widely publicized as the cornerstone anniversary for the International Year of Astronomy. Through his looking glass, the Italian astronomer saw the mountains and valleys of the moon, the satellites of Jupiter, and sunspots--observations that would play a huge role in discrediting the prevailing, church-endorsed view of an Earth-centered cosmos.

The second event is not as well known, but is arguably equally important. It was the publication of Johannes Kepler's Astronomia Nova (The New Astronomy) in 1609, a treatise in which the German astronomer introduced the first two of his laws describing planetary motion.

The first law states that the planets travel in elliptical orbits around the sun and describes the sun's position as the focal point in that ellipse. The second law states that an imaginary line connecting a planet to the sun will sweep out a region of equal size in a given time period, wherever in the orbit that time period falls.