What did Kepler notice about planetary motion that caused him to propose Kepler's first law of orbitation on motion

Johannes Kepler observed that the motion of planets around the Sun did not follow the circular orbits that were commonly assumed during his time. Instead, through meticulous analysis of Tycho Brahe's astronomical data, Kepler discovered that planets move in elliptical orbits, with the Sun positioned at one of the two foci of the ellipse. This observation led him to formalize what is now known as Kepler's First Law of Planetary Motion, which states that:

**"The orbit of a planet is an ellipse with the Sun at one focus."**

This was a significant departure from the prevailing belief in circular orbits and marked a key moment in the history of astronomy, laying the foundation for the later development of gravitational theory by Isaac Newton. Kepler's realization that planetary motion is governed by elliptical paths helped to explain the varying speeds of planets as they orbit the Sun, an aspect he also captured in his second law of planetary motion.