The history
From falling stones and a famous apple to weighing the Earth, curved spacetime and a lander on the Moon's south pole.
Everyone has always known that things fall. The hard part was seeing that the same pull that drops an apple also holds the Moon in its orbit. Newton wrote the rule in 1687, Cavendish measured its tiny strength in 1798, and Einstein explained it again in 1915 as curved spacetime. Today we steer satellites, correct GPS clocks and land on the Moon with it.
628
The Earth described as attracting things
Brahmagupta, India (as quoted by al-Biruni)
1687
Law of universal gravitation published
Isaac Newton, England
1798
Earth's density measured in a lab
Henry Cavendish, England
1846
Planet found by calculation
Urbain Le Verrier and Johann Galle
1915
Gravity explained as curved spacetime
Albert Einstein, Germany
1957
First artificial satellite
Sputnik 1, USSR
2015
Gravitational waves detected
LIGO, USA
2023
Soft landing near the Moon's south pole
ISRO, Chandrayaan-3
350 BCE – 1000
Greek and Indian thinkers argue about why heavy things fall and what holds a round Earth together.
350 BCE
c. 350 BCE
Aristotle taught that heavy things fall towards the centre of the universe because that is their natural place, and that a heavier body falls faster than a lighter one. It matched what people saw with feathers and stones in air.
Why it mattered. It was the accepted answer for almost two thousand years, until careful experiments showed that only air makes the difference.
628
499 and 628 CE
In 499 Aryabhata described a round Earth that spins, hanging in space. In 628 Brahmagupta wrote, as later quoted by al-Biruni, that all heavy things fall to the Earth because it is the nature of the Earth to attract and keep them, just as it is the nature of water to flow. It was an idea in words, not yet a law with numbers.
Why it mattered. It is one of the earliest clear statements that falling is caused by the Earth pulling on things.
1589 – 1700
Galileo times falling balls, Kepler finds the rules of planets, and Hooke and Newton tie them together with an inverse-square pull.
1590
c. 1589–1592 (story)
Galileo's student Vincenzo Viviani later wrote that Galileo dropped balls of different weights from the Leaning Tower of Pisa to show they land together. Historians are not sure it really happened, but Galileo did argue it in his writing at Pisa.
Why it mattered. It became the most famous test of the idea that mass doesn't change how fast things fall.
1609
1609–1619
Working from Tycho Brahe's careful observations, Kepler found that planets move on ellipses with the Sun at one focus, sweep out equal areas in equal times, and take longer to go round the further out they are. His first two laws came out in 1609 and the third in 1619.
Why it mattered. Newton later showed that an inverse-square pull explains all three laws at once.
1638
In his last book, Two New Sciences, Galileo showed with balls rolling down slopes that a falling body gains speed at a steady rate, so the distance grows with the square of the time. He also studied pendulums and argued that without air, all bodies would fall together.
Why it mattered. It turned falling into mathematics, the first step towards the idea of g.
1666
c. 1666 (told in 1726)
Sent home from Cambridge by the plague, the young Newton wondered whether the pull that brings an apple down might reach as far as the Moon. He told the story himself, much later, to his friend William Stukeley, who published it. There is no evidence the apple hit his head.
Why it mattered. It captures Newton's great leap: the same gravity works on Earth and in the sky.
1679
1674 and 1679
In 1674 Hooke suggested that all heavenly bodies attract each other, and that an orbit is a straight motion bent by that pull. In letters to Newton in 1679–80 he proposed that the pull falls off with the square of the distance. Hooke could not do the mathematics to prove it; the two men argued about credit for the rest of their lives.
Why it mattered. Hooke's letters pushed Newton to work out orbits under an inverse-square force.
1687
5 July 1687
Newton's Principia showed that every mass pulls every other mass with a force proportional to both masses and inversely proportional to the square of the distance. With it he explained falling, Kepler's laws, the tides and the paths of comets. Edmond Halley paid for the printing.
Why it mattered. It was the first law of nature that worked the same everywhere in the universe.
1770 – 1850
A mountain and a twisting rod weigh the Earth, and pencil-and-paper gravity finds a new planet.
1774
Maskelyne measured how much the mountain Schiehallion pulled his plumb lines sideways, using the stars as a reference. Hutton then worked out the mountain's mass and, from that, estimated that the Earth is about 4.5 times as dense as water.
Why it mattered. It was the first real measurement of the Earth's density, and so of its mass.
1798
21 June 1798 (published)
Using a torsion balance designed by John Michell, Cavendish hung a 1.8 m rod with small lead balls on a thin wire and swung 158 kg lead balls close. The rod twisted by a tiny angle, which he watched with telescopes from outside the room. He found the Earth is 5.48 times as dense as water (5.45 after a slip was corrected); today's value is 5.51.
Why it mattered. His result gave the mass of the Earth and, later, the value of G.
1846
23 September 1846
Uranus kept drifting from where Newton's law said it should be. Le Verrier calculated that an unseen planet must be pulling it, and where to look. Galle pointed the Berlin telescope there and found Neptune within about 1 degree of the prediction on his first night. John Couch Adams in England had made similar calculations.
Why it mattered. It was a triumph for Newton's gravity: a new world discovered by mathematics.
1915 – 1920
Einstein rebuilds gravity as the shape of spacetime, and an eclipse shows starlight bending round the Sun.
1915
25 November 1915
Einstein presented the final equations of his general theory of relativity to the Prussian Academy of Sciences. Mass and energy curve spacetime, and things move along the straightest paths through it. The theory explained a small wobble in Mercury's orbit that Newton's law could not.
Why it mattered. It replaced Newton's force at a distance with the shape of spacetime, and predicted black holes and gravitational waves.
1916
January 1916
While serving in the German army in the First World War, Schwarzschild found an exact solution of Einstein's equations around a single round mass. It contains a special radius, 2GM ÷ c², now called the event horizon. He died a few months later.
Why it mattered. It was the mathematical seed of the black hole.
1919
29 May 1919
During a total solar eclipse, expeditions photographed stars near the darkened Sun. The stars appeared shifted by about the amount Einstein predicted, twice what a Newtonian estimate gave. The news made Einstein world-famous overnight.
Why it mattered. It was the first big test that general relativity passed.
1957 – 2023
Satellites fall round the Earth, GPS clocks obey relativity, detectors hear black holes merge, and India lands near the Moon's south pole.
1957
4 October 1957
Sputnik 1, a polished 58 cm ball with four antennas, was launched into orbit. It went round the Earth about every 96 minutes, beeping radio signals that anyone could hear. It was Newton's cannonball made real.
Why it mattered. It began the space age: every satellite since has used the same balance of falling and sideways speed.
1971
2 August 1971
Standing on the airless Moon, astronaut David Scott dropped a geological hammer and a falcon feather at the same moment. They hit the ground together, live on television.
Why it mattered. It settled Aristotle's question in front of millions: without air, mass doesn't change how things fall.
1977
23 June 1977
NTS-2, the first satellite for the new GPS system, carried the first caesium atomic clock into orbit. Before its correction was switched on, it ran fast by 442.5 parts in 10¹², close to the 446.5 relativity predicted. GPS clocks have been tuned to cancel this ever since: about 38 microseconds a day.
Why it mattered. Without relativity, GPS positions would drift by kilometres every day.
1981
19 June 1981
APPLE, the Ariane Passenger Payload Experiment, was India's first experimental communication satellite in geostationary orbit, parked at 102° E. It relayed TV and radio for two years and led to the INSAT and GSAT satellites that serve India today.
Why it mattered. It showed India could place and run a satellite that hangs over one spot of the Earth.
2015
14 September 2015
Both LIGO detectors caught a brief chirp: gravitational waves from two black holes, about 35 and 30 times the Sun's mass, merging about 1.4 billion light years away. Their 4 km arms changed length by about a thousandth of a proton's width. The 2017 Nobel Prize in Physics went to Rainer Weiss, Barry Barish and Kip Thorne.
Why it mattered. It confirmed the last big prediction of Einstein's gravity and opened a new way to observe the universe.
2023
23 August 2023
After climbing out of Earth's gravity in ever-bigger orbits and being captured by the Moon's, the Vikram lander touched down at 18:04 IST near the lunar south pole. India became the fourth country to soft-land on the Moon, and the first near its south pole.
Why it mattered. Every step of the journey, from parking orbits to the final braking burn, was planned with the law of gravity.
Did you know?
Two 1-tonne lead balls 1 m apart pull on each other with 0.000 067 N, about the weight of 7 mg.
At the ISS, 400 km up, gravity is still 89% as strong as on the ground. The crew float because they are falling.
GPS clocks are set to tick at 10.229 999 995 43 MHz on the ground so that, in orbit, relativity speeds them to exactly 10.23 MHz.
If the Earth were squeezed into a black hole, it would be under 2 cm across.
A pendulum 99.4 cm long ticks once a second on Earth, but only once every 2.5 s on the Moon.
The people
c. 598 – c. 668 · Mathematician and astronomer · India
Described the Earth as attracting heavy things, and wrote rules for zero.
1564 – 1642 · Physicist and astronomer · Italy
Showed that falling bodies speed up steadily and that, without air, all fall alike.
1571 – 1630 · Astronomer · Germany
Found the three laws of planetary motion from Tycho Brahe's data.
1635 – 1703 · Natural philosopher · England
Suggested the inverse-square pull and that orbits are bent straight lines.
1643 – 1727 · Mathematician and physicist · England
Wrote the law of universal gravitation in Principia, 1687.
1731 – 1810 · Chemist and physicist · England
Measured the tiny pull between lead balls and so weighed the Earth.
1811 – 1877 · Mathematician and astronomer · France
Predicted where Neptune would be from the pull it had on Uranus.
1879 – 1955 · Physicist · Germany
Explained gravity as the curving of spacetime in 1915.
1882 – 1944 · Astrophysicist · England
Led the 1919 eclipse expedition that measured starlight bending round the Sun.
Where it happened
Sources
Dates marked “c.” are approximate, and historians sometimes disagree about who was first. If you spot a mistake, tell us.
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