to test copernicus’s model, tycho took meticulous measurements of the positions of the planets, and he hired kepler to interpret the data. kepler found that the geocentric model did not accurately predict the planets’ positions, but neither did copernicus’s heliocentric model. he found that copernicus’s insistence that the planets’ orbits were perfectly circular was the problem. if the orbits were instead ellipses, then the heliocentric model would have been able to very accurately predict the positions of the planets. this is known as kepler’s first law of planetary motion.