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

Kepler's first law of planetary motion, also known as the Law of Ellipses, was proposed after Johannes Kepler noticed that the paths of planets around the Sun were not circular, as was commonly believed at the time, but elliptical. This conclusion was drawn from his meticulous analysis of astronomical data collected by Tycho Brahe.

Kepler observed that the distances of planets from the Sun varied over time and that their speeds changed depending on their position in their orbits. He concluded that a planet moves faster when it is closer to the Sun and slower when it is farther away. This led him to formulate his first law, which states: "The orbit of a planet around the Sun is an ellipse, with the Sun at one of the foci."

In summary, it was the recognition of the elliptical shape of planetary orbits, along with the variable speeds of planets, that prompted Kepler to propose this foundational law of celestial mechanics.