Hate to be the AI dump guy, but there's a good overview: Big picture first The classical understanding of gravity wasn’t a single leap—it was a three-stage convergence: 1. Kinematics: how things move (Kepler, Galileo) 2. Dynamics: why they move that way (Newton) 3. Empirical grounding: what the constants actually are (Cavendish) By ~1800, the formula F = G \frac{m_1 m_2}{r^2} was no longer speculative—it was overdetermined by astronomy, mechanics, and laboratory experiment. 1. Galileo Galilei (c. 1600–1638) — Gravity as uniform acceleration Key ideas • Falling bodies accelerate at a constant rate (ignoring air resistance) • Horizontal and vertical motion are independent • Motion can be described mathematically, without invoking purposes or essences What gravity was not yet • No force law • No dependence on mass of Earth or object • No connection to planetary motion Why Galileo mattered He reframed gravity as a quantitative physical phenomenon, not a philosophical tendency. This made later force laws even possible. ⸻ 2. Johannes Kepler (1609–1619) — The planetary clues Kepler’s Laws 1. Planets move in ellipses with the Sun at one focus 2. Equal areas in equal times 3. T^2 \propto a^3 Why this was explosive Kepler gave exact, data-driven regularities that screamed: “There is a central force pulling planets toward the Sun, and it weakens with distance.” But Kepler did not know the force law. He suspected something like magnetism and thought the force weakened with distance—but not exactly how. ⸻ 3. Isaac Newton (1665–1687) — The synthesis This is the true birth of classical gravity. Newton’s key insights • If a force causes circular or elliptical motion and Kepler’s 2nd law holds, the force must be central • If Kepler’s 3rd law holds, the force must fall off as 1/r² • The same force explains: • Falling apples • The Moon’s orbit • Planetary motion The inverse-square law emerges From orbital mechanics alone, Newton showed: F \propto \frac{1}{r^2} Then he generalized it to: F = G \frac{m_1 m_2}{r^2} Important nuance Newton did not measure G. He only needed the product GM_\oplus, which could be inferred from orbital motion. What cemented Newton’s law (1687) • Exact recovery of Kepler’s laws • Quantitative prediction of tides • Correct explanation of projectile motion and pendulums • Predictive power across terrestrial and celestial physics After the Principia, competing gravity laws were basically dead. ⸻ 4. Henry Cavendish (1797–1798) — Weighing the Earth Cavendish performed the first laboratory measurement of gravitational attraction between masses using a torsion balance. What he actually measured • A tiny torque between lead spheres • From this: the density (mass) of Earth • Implicitly: the gravitational constant G This was the final missing piece. Why this mattered • Gravity was no longer inferred only from astronomy • The same inverse-square law held at human scales • G became a universal constant, not a fitting parameter After Cavendish, Newtonian gravity was: • Universal • Quantitative • Experimentally verified in the lab ⸻ When was F = G m_1 m_2 / r^2 truly “cemented”? Short answer: 1687 conceptually, ~1800 empirically