Kepler (full name Johannes Kepler) improved on the work of Copernicus and Galileo, Kepler recognised that the Earth and other Planets of the Solar System orbit the Sun in ellipses moving faster when they are nearer the sun. Kepler also worked out the relation between the distance of a planet from the sun and the period of its orbit.
Copernicus and Galileo were earlier Heliocentrists but both mistakenly believed the planets moved in perfect circles. Therefore the theories of Copernicus and Galileo didn't accurately describe the way the planets appear to move in the sky. Kepler's laws of motion finally showed Scientists how to understand the way planets move and Kepler's system predicted planetary motion better than the old Geocentric theory of Ptolemy.