Brahe, Copernicus, Kepler, and Galileo

Danish astronomer whose accurate astronomical observations formed the basis for Johannes Kepler's laws of planetary motion.

Copernicus was a Polish astronomer who advanced the heliocentric theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time as the Ptolemaic view of the universe, which was the prevailing theory for over 1000 years, was deeply ingrained in the prevailing philosophy and religion. (It
should be noted, however, that the heliocentic idea was first put forth by Aristarcus of Samos in the 3rd century BC, a fact known to Copernicus but long
ignored.)

A German astronomer and mathematician, Johannes Kepler is considered a founder of modern astronomy.  Johannes Kepler used simple mathematics to describe how planets move. Kepler was an assistant to the most accurate astronomical observer of the time, Tycho Brahe. Kepler was able to use Brahe's data to show that planets move in ellipses around the Sun (Kepler's First Law), that planets move proportionally  faster in their orbits when they are nearer the Sun (Kepler's Second Law), and that more distant planets take proportionally longer to orbit the Sun (Kepler's Third Law). Kepler lived from 1571 to 1630, during the time of discovery of the telescope. Kepler was one of the few vocal supporters of Galileo's discoveries and the Copernican system of planets orbiting the Sun instead of the Earth (basically provided the facts that proofed the theory).

Follow this link to an explanation of Kepler's Laws

 
Galileo was an Italian astronomer and physicist. The first to use a telescope to study the stars. Discoverer of the first moons of an extraterrestrial body (the moons of Jupiter), Galileo was an outspoken supporter of Copernicus's heliocentric theory. In reaction to Galileo, the Church declared it heresy to teach that the Earth moved and imprisoned him. The Church clung to this position for 350 years; Galileo was formally exonerated in 1992.