Problem
During Kepler's lifetime in the early 17th-century, only Mercury, Venus, Earth, Mars, Jupiter and Saturn were known planets. Tycho Brahe told Kepler that the data for the position of Mars were the most difficult to match using a circular orbit. Based on this information Kepler selected the orbit of Mars as the starting point to developing his first law of planetary motion.
Briefly discuss why the orbit of Mars was so difficult to reconcile with a circular orbit?