The falling apple and the Moon
Aristotle thought heavier objects fall faster, but Galileo Galilei used experiments such as inclined planes to show that without air resistance all bodies fall with the same acceleration, setting this out in Two New Sciences (1638); the Leaning Tower of Pisa story may not be true. In 1609 Johannes Kepler published the law that planets move in ellipses. In the Principia (1687) Isaac Newton argued that the force pulling an apple down is the same one holding the Moon in orbit, and stated the law of universal gravitation: attraction proportional to the product of the masses and inversely proportional to the square of the distance. With Newton's laws of motion this explained the planets of the Solar System, the tides and the return of comets.
Weighing the Earth and finding a planet
In 1798 Henry Cavendish used a torsion balance to measure the tiny attraction between lead balls in a laboratory and calculated Earth's mean density — in effect the first measurement of the gravitational constant. In 1846 Urbain Le Verrier and John Couch Adams used Newtonian gravity to explain irregularities in Uranus's orbit and predict an unseen planet; Neptune was found near the predicted spot. But in 1859 Le Verrier also reported that Mercury's perihelion advances slightly more than Newton's theory allows, a small discrepancy that stayed a puzzle for over 50 years.
Curved spacetime and open puzzles
In 1915 Albert Einstein completed general relativity, describing gravity not as a force but as mass and energy curving spacetime, with objects following curved paths. It matched Mercury's perihelion precisely, and the 1919 eclipse expeditions were announced as confirming that starlight bends near the Sun (the accuracy of those measurements was later debated, but later observations confirmed the effect). In 1960 Pound and Rebka measured gravity shifting light's frequency in the lab, and gravitational time dilation is corrected for in GPS satellite clocks today. In 2015 LIGO first detected gravitational waves from merging black holes, and in 2019 the Event Horizon Telescope imaged the shadow of the black hole in galaxy M87. Yet gravity has not been unified with quantum mechanics, and galaxy rotation speeds cannot be explained by visible matter alone, leaving dark matter unsolved.
FAQ
- Why are astronauts weightless on the space station?
- Gravity there is still about 90% as strong as at the surface. The station and crew are constantly falling toward Earth while moving sideways fast — free fall — so they feel weightless.
- How much would I weigh on the Moon?
- The Moon's gravity is about one-sixth of Earth's, so a scale would read about one-sixth as much. Your mass itself does not change.
Sources reviewed: 2026-10-26
References
- Kepler, J. (1609). Astronomia nova. Heidelberg: Vögelin.
- Galilei, G. (1638). Discorsi e dimostrazioni matematiche intorno a due nuove scienze. Leiden: Elzevir.
- Newton, I. (1687). Philosophiæ Naturalis Principia Mathematica. London: Royal Society.
- Cavendish, H. (1798). Experiments to determine the density of the Earth. Philosophical Transactions of the Royal Society of London, 88, 469–526.
- Le Verrier, U. J. J. (1859). Lettre de M. Le Verrier à M. Faye sur la théorie de Mercure et sur le mouvement du périhélie de cette planète. Comptes rendus de l'Académie des Sciences, 49, 379–383.
- Einstein, A. (1915). Die Feldgleichungen der Gravitation. Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften, 844–847.
- Dyson, F. W., Eddington, A. S., & Davidson, C. (1920). A determination of the deflection of light by the Sun's gravitational field. Philosophical Transactions of the Royal Society A, 220, 291–333.
- Pound, R. V., & Rebka, G. A. (1960). Apparent weight of photons. Physical Review Letters, 4(7), 337–341.
- Abbott, B. P., et al. (LIGO Scientific Collaboration and Virgo Collaboration). (2016). Observation of gravitational waves from a binary black hole merger. Physical Review Letters, 116(6), 061102.
- Event Horizon Telescope Collaboration. (2019). First M87 Event Horizon Telescope results. I. The shadow of the supermassive black hole. Astrophysical Journal Letters, 875(1), L1.