Johannes Kepler, the renowned German mathematician and astronomer, initially discovered the third law of planetary motion on March 8, but quickly dismissed the idea after encountering difficulties with calculations. Kepler's remarkable framework for understanding planetary motion was rooted in meticulous observational data obtained primarily from Tycho Brahe, a prominent Danish astronomer. Despite initially rejecting his discovery, Kepler's intellectual pursuits continued unhindered as he refined his methods and sought clarity in his findings.

By May 1618, having reassessed his previous work and corrected his earlier misgivings, Kepler re-confirmed the third law. This law posits that the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. Essentially, this means that planets farther from the Sun take longer to complete their orbits than those closer in, a fundamental principle in the field of celestial mechanics. Kepler’s embrace of this law established a critical cornerstone for the later work of Isaac Newton and other scientists.

The significance of this confirmation by Kepler cannot be overstated. His laws of planetary motion not only provided a greater understanding of how celestial bodies interact but also laid the groundwork for the broader scientific revolution that followed. This validation of his third law aligned seamlessly with his other two laws, creating a cohesive model that described the motion of planets in the solar system with unprecedented accuracy.