The great contribution of Tycho Brahe was to .
a. discover that planets orbit the Sun in elliptical orbits with varying speed
b. discover four moons orbiting Jupiter, thereby lending strong support to the idea that the Earth is not the center of the universe
c. observe planetary positions with sufficient accuracy so that Kepler could later use the data
d. discover the laws of planetary motion offer the first detailed model of a Sun-centered solar system, thereby beginning the process of overturning the Earth-centered model of the Greeks