Which did Kepler notice about planetary motion that caused him to propose Kepler’s first law of orbital motion?

Kepler noticed that the planets do not move in perfect circular orbits, as was commonly believed before his time, but rather in elliptical orbits with the Sun at one of the foci of the ellipse. This observation was based on meticulous astronomical data collected by Tycho Brahe, particularly the very accurate measurements of Mars's position. Kepler's analysis of this data led him to formulate his first law of orbital motion, which states that the orbits of planets are ellipses with the Sun at one focus. This was a significant departure from the prevailing circular orbit model and laid the groundwork for a better understanding of planetary motion.