In 1618, the same year Isaac Beeckman was imparting the principle of inertia to René Descartes, Johannes Kepler achieved one of the most crucial breakthroughs in the annals of astronomy and physics: his third law of planetary motion. This law is among the groundbreaking discoveries that radically altered humanity’s understanding of the universe, yet it was largely overlooked at first. Kepler’s third law, which indicates that the square of a planet’s orbital period is proportional to the cube of the semi-major axis of its orbit, uncovers a deep connection between the duration a planet takes to revolve around the Sun and the dimensions of that orbit. Despite its importance, Kepler’s third law went unrecognized upon its unveiling.
During that era, the concept of force was not well grasped, with notions of motion deeply entrenched in Aristotelian doctrine and subsequent medieval ideas like impetus. Scholars posited that projectiles necessitated an impetus to maintain their movement after the application of force ceased, a concept that was later contested with the emergence of inertia. In the celestial sphere, it was believed that heavenly bodies moved in circular paths inherently or were driven by angels or impetus.
Kepler’s research diverged from these beliefs. Drawing inspiration from William Gilbert’s magnetism theories, he theorized that a force emanated from the Sun, propelling planets along their orbits, with this force diminishing as distance increased. His third law offered a mathematical connection between a planet’s orbital period and its proximity to the Sun, yet its presentation in Kepler’s “Harmonices Mundi,” a treatise rooted in Pythagorean cosmic balance, obscured its recognition as a scientific milestone. Kepler’s work, heavily laced with metaphysical ideas, did not accentuate the significance of the third law, and “Harmonices Mundi” failed to attract a wide audience.
Nonetheless, Kepler’s “Epitome Astronomiae Copernicanae,” where he laid out the third law alongside his other laws between 1618-1621, gradually garnered attention after the release of the “Tabulae Rudolphinae” in 1627. These tables, informed by Tycho Brahe’s data, illustrated the practical applications of Kepler’s theories, including enhanced accuracy in planetary positions. In spite of initial pushback from some leading astronomers and competing theories—especially regarding the second law—Kepler’s concepts began to circulate.
By the mid-17th century, Kepler’s elliptical orbits found acceptance among European astronomers, partly due to the precision of the Rudolphine Tables. English astronomers such as Jeremiah Horrocks and his network, alongside others in Europe like Boulliau, were instrumental in promoting Kepler’s elliptical orbits despite disputes surrounding the second law. In Italy, scholars like Riccioli and Cassini also engaged with Kepler’s theories.
Kepler’s third law remained underrated until Isaac Newton, in his “Principia Mathematica” (1687), illustrated its crucial role in the laws of motion and universal gravitation. More than seventy years after its inception, Newton demonstrated that Kepler’s harmonic law was essential for comprehending the fundamental forces that govern planetary motion, cementing its status as a significant scientific revelation.