TYCHO BRAHE, Danish Astronomer (1546 - 1601) --- he made very detailed observations of the changing positions of the planets in the sky as seen from earth.

JOHANNES KEPLER (1571 - 1630) -- He tried various possible orbits for earth and other planets, until Brahe's observations reduced to consistent results for all planets.

ISAAC NEWTON (1642 - 1727) created theoretical physics. His law of gravity and laws of motion for the first time allowed a complete understanding of the empirical rules discovered by Kepler.

ALBERT EINSTEIN (1879 - 1955) --- created a more general theory of gravitation, in 1915, which has been the basic description underlying all further progress in understanding the nature of the universe, since. [It is also the basis of the GPS navigational system.]