How did Johannes Kepler modify Copernicus's heliocentric model? O Kepler invented the Astronomical Unit to properly place planets at the correct distances from the Sun. O Kepler used Galileo's observations of Jupiter's moons to extrapolate the orbital periods of the planets. O Kepler replaced circular orbits with elliptical orbits to better fit the orbit of Mars to Tycho Brahe's observations. O Kepler discovered that retrograde motion occurs whenever Earth passes an outer planet in its orbit