Short answer
- Gravity was not discovered on a single date, because people have always lived with falling objects. What changed over time was the explanation. Ancient thinkers described falling in general terms. Later thinkers measured falling motion, described planetary orbits, and then joined those two subjects into one law. The date most often given, 1687, is the publication year of Isaac Newton's Principia, not the day someone first noticed that things fall. Newton published a mathematical law of universal gravitation in that book. That law said the same attraction governs an apple near Earth and the Moon in orbit. Later work tested, measured, and then reframed that idea, but it did not replace the simple fact that objects fall. So the honest answer has three parts: falling was always observed, a universal law was published in 1687, and a deeper description of gravity followed in 1915 with Albert Einstein's general relativity.
Why the question needs care
Search results often give a short answer such as Newton in 1687. That answer is useful as a landmark, but it can mislead if it sounds as if gravity began in that year or as if one person found it in a single moment. Gravity is a feature of the physical world, not an invention. A better way to read the question is to ask when people first described gravity well enough to predict motion. On that reading, the story is a chain, not a single event. Each link answered a different question. Why do heavy and light objects fall as they do? Why do planets follow regular paths? Can one rule cover both a stone and a planet? Can that rule be measured in a laboratory? And finally, is gravity best described as a pull, or as the shape of space and time? This article follows that chain in order, and it flags where a popular story is an anecdote rather than a settled fact.
Before Newton: falling as a natural tendency
Long before modern physics, falling needed no discovery. Farmers, builders, sailors, and children all knew that an unsupported object drops. The intellectual problem was to explain why it drops, and whether the same explanation applied everywhere. Ancient Greek thought treated falling as a natural tendency. In that picture, earthy material belonged below, and it moved toward its proper place. The University of Virginia history of gravity notes that Aristotle was the first writer to attempt a quantitative description of falling motion, and that he described an object as falling at a constant speed soon after release, with heavier things falling faster in proportion to their mass. That picture is understandable from everyday observation, especially in water, where resistance and buoyancy dominate. It is not, however, an accurate account of falling in air when resistance is small. It lasted for a long time because it matched common sense and because careful timing of fast falls was hard before accurate clocks and controlled experiments. The key shift came when thinkers stopped asking only where a thing belonged and started asking how its speed changed moment by moment.
Galileo and Kepler: measuring motion on Earth and in the sky
Galileo Galilei changed the study of falling by treating it as motion that could be measured. The University of Virginia account stresses his central result: a falling body picks up speed at a constant rate, which is constant acceleration, and, when air resistance and buoyancy can be neglected, bodies of different weights fall with the same acceleration and reach the ground together. Galileo also separated horizontal and vertical motion, which made projectile paths easier to understand. A cannonball fired sideways keeps moving sideways while it falls downward, and the two motions can be studied separately. That result matters for the later history because it gave Newton a clean description of motion near Earth. Galileo did not produce a universal law of gravitation, and he did not explain why masses attract. He gave later science a better description of how falling works.
Johannes Kepler worked on the other half of the puzzle: the sky. Using precise observations inherited from Tycho Brahe, Kepler spent years analyzing the motion of Mars before concluding that it moves in an elliptical path. He summarized planetary motion in three laws: planets move in ellipses with the Sun at one focus, a line from the Sun to a planet sweeps out equal areas in equal times, and the time for one orbit relates in a regular way to the size of the orbit. Those laws described planetary paths, but they did not by themselves state a universal force law. Together, Galileo and Kepler created the conditions for Newton’s step. Galileo supplied a measured account of falling and acceleration. Kepler supplied a measured account of orbits. Newton asked whether one cause could explain both.
Newton: from thinking in the 1660s to the Principia in 1687
Newton’s work did not arrive in one flash. The familiar plague years story places important thinking in 1665 to 1666, when Cambridge was disrupted and Newton spent time at Woolsthorpe. Historians treat that period as a time of intense work, not as a single day of discovery. The public, testable result came much later. In 1687, Newton published the first edition of Philosophiae Naturalis Principia Mathematica, often shortened to the Principia. The Oxford Newton Project introduction confirms that Newton, then Fellow of Trinity College and Lucasian Professor of Mathematics at Cambridge University, published the first edition in 1687. That date is the safest single date to attach to the phrase “discovery of gravity” in the scientific sense, and it should always carry that qualification.
The Principia contained Newton’s laws of motion and his law of universal gravitation. In plain terms, the law said every body attracts every other body, with a strength that grows with mass and weakens rapidly as distance grows. The crucial claim was universality. The same rule that describes a falling apple also describes the Moon, the planets, and comets. Britannica’s short history makes the same point: Newton proposed in 1687 that the same invisible force pulling an apple to the ground also keeps the Moon in orbit around Earth, and he built on Galileo’s work on acceleration and Kepler’s description of planetary motion. Newton also used the Moon test in a striking way. The Moon is about 60 times farther from Earth’s center than an object at Earth’s surface, and its fall toward Earth in each second is far smaller, in a ratio consistent with an inverse square weakening with distance. That comparison turned a general idea into a checkable pattern. It also explains why Newton, rather than an earlier thinker, is credited with the law. Earlier thinkers had described falling or orbits. Newton joined them with mathematics that could predict motion across both domains.
The apple story: anecdote, not a laboratory record
The apple story is the best known part of this history, and also the easiest to overstate. The careful version is this: Newton told friends late in life that watching an apple fall prompted him to think about gravitation. William Stukeley, who spoke with Newton in his later years, recorded the anecdote in a memoir worked up in 1752. The Oxford Newton Project introduction notes that Stukeley was the first person to record the anecdote, that Newton himself told it to Stukeley, and that Newton, by then an old and famous man, may well have been shaping his own myth. That caution matters. The story is not apocryphal in the sense of having no source, because it traces to Newton’s own telling. It is also not a contemporary laboratory record from the 1660s. It is a late recollection, written down decades later, and it should be read as a prompt for thought, not as proof of how the theory was formed.
Two popular additions should be set aside. The apple did not need to hit Newton on the head for the story to work, and no reliable version makes a head strike the point. The point in Stukeley’s telling is the question itself: why does the apple always descend straight down, rather than sideways or upward? That question leads naturally to the Moon comparison. If Earth draws the apple, does that draw extend to the Moon? Newton did not answer that question by watching one apple. He answered it by building a mathematical system, testing it against orbits, tides, and comets, and publishing it for others to challenge. The apple is a helpful image for the question. The Principia is the evidence for the answer.
Cavendish: measuring gravity in the laboratory
Newton’s law left an important gap. It described how gravity weakens with distance and grows with mass, but it did not by itself give the absolute strength of gravity between ordinary laboratory masses. That gap lasted for more than a century. Henry Cavendish closed much of it with a delicate experiment performed in 1797 to 1798 and reported in June 1798. The American Physical Society account explains that Cavendish used a torsion balance: a rod suspended from a wire, with small lead spheres at its ends, placed near much larger lead spheres. The faint attraction between the small and large spheres twisted the wire by a tiny amount. By measuring that twist, and by comparing it with the known weight of the small spheres, Cavendish could work out Earth’s density. He reported Earth’s density as 5.48 times the density of water, close to the modern accepted value of 5.52 given in the same account.
A common textbook shortcut says Cavendish measured the gravitational constant directly. That wording needs a qualification. Because of the unit conventions of his time, Cavendish expressed his result as Earth’s density, not as the modern constant. Later workers could derive a value for the constant from his result. The American Physical Society account makes the modern view clear: today the experiment is viewed as a way to measure the universal gravitational constant, rather than only as a measurement of Earth’s density. The experiment also deserves credit for its care. Cavendish worked in a closed room, observed through a telescope, moved weights from outside, and checked sources of error such as air currents, temperature differences, and magnetization. The result showed that gravity between laboratory masses was real and measurable, not only an astronomical inference. It also made it possible to weigh Earth in effect, and then to estimate the masses of the Sun and other bodies by comparison.
Einstein: gravity reframed in 1915
For more than two centuries, Newton’s law worked well for most practical purposes on Earth and in much of astronomy. It still works well in that range today. Albert Einstein did not discover a different falling effect. He offered a different description of what gravity is. After special relativity in 1905 showed space and time woven into a single structure, Einstein spent a decade bringing gravity into that picture. Britannica’s account of general relativity traces the key step to 1907, when Einstein realized that a person in a closed room could not distinguish uniform acceleration from a gravitational field by feeling alone. That equivalence idea led him to predict that light would bend in a gravitational field. Finding the mathematics took years, with help from tensor analysis introduced through his friend Marcel Grossmann in 1912. In November 1915, Einstein completed the work and published the field equations of gravity on November 25, 1915, according to NASA’s centenary account.
General relativity describes gravity as the curvature of space time caused by mass and energy. Objects follow paths through that curved geometry, which can look like a pull. NASA uses a stretched fabric image: a heavy ball dents the fabric, and a smaller ball rolled nearby follows the dent. The image is incomplete, as NASA notes, but it captures the shift. Gravity is no longer only a force acting at a distance. It is geometry. The theory also passed early tests that mattered for its acceptance. It explained the shifting perihelion of Mercury, the point of closest approach to the Sun, which Newtonian calculations had not fully explained. In 1919, eclipse expeditions observed starlight bending near the Sun in line with Einstein’s prediction, and the result made Einstein world famous. Later applications include precise satellite navigation, where both special and general relativistic effects must be accounted for to keep timing accurate. Newton remains useful because his law is simpler and accurate in many everyday settings. Einstein’s theory is needed where gravity is very strong, speeds are very high, or measurements are extremely precise.
What 9.8 meters per second squared means, and why it varies
Readers often meet gravity as a single number: about 9.8 meters per second squared near Earth’s surface. That number is a rounded average, not a universal constant stamped on every place on Earth. It means that, ignoring air resistance, a freely falling object near Earth’s surface gains about 9.8 meters per second of downward speed every second. After one second it moves at about that speed, after two seconds at about twice that speed, and so on, until air resistance or another force changes the picture. NASA’s Cosmicopia gravity answers give the same canonical numbers, 9.8 meters per second squared and 32 feet per second squared, and note that the actual value varies slightly from the poles to the equator.
Three reasons explain the variation. First, Earth is not a perfect sphere. It bulges at the equator, so a person at the equator stands farther from Earth’s center than a person near a pole, and gravity weakens with distance. Second, Earth’s rotation creates a centrifugal effect that slightly reduces the gravity felt at the equator compared with the poles. NASA’s Cosmicopia answers make this exact point: spinning does not cause gravity, and spinning actually reduces the gravity felt at the equator compared with a pole. Third, local mass matters. Mountains, dense rock below ground, ocean basins, and elevation all shift the local value by small amounts. Satellite missions that map Earth’s gravity field show those small deviations from a smooth model. The differences for everyday life are tiny, usually well under one percent across Earth’s surface, but they are real and measurable. They also explain why textbooks give a standard or average value rather than pretending one number fits every beach, mountain, and city exactly. The surface value is a local result of Earth’s mass, shape, rotation, and nearby geology. The gravitational constant behind Newton’s law is a different quantity: it sets the strength of attraction between any two masses, anywhere, while the surface value combines that constant with Earth’s mass and radius at a given place.
How the understanding changed, rather than a single discovery
A timeline makes the honest answer easier to remember. Ancient thinkers described falling as natural motion. Galileo measured acceleration and the independence of falling from weight when air resistance is negligible. Kepler described elliptical orbits and regular orbital patterns. Newton published a universal law in 1687 that joined falling and orbiting. Cavendish measured gravity between laboratory masses in 1797 to 1798 and gave a firm basis for Earth’s density and, by derivation, the gravitational constant. Einstein published general relativity in 1915 and reframed gravity as curved space time. Each step kept something from the step before and corrected something else. Galileo’s falling measurements still describe everyday falls. Newton’s law still guides spacecraft in many settings. Einstein’s theory extends the account to strong gravity and precise measurement. None of these steps means that earlier observers were foolish for noticing that things fall. They were answering different questions with the tools they had. The discovery, if the word must be used, is the whole chain: from noticing a fall, to measuring it, to joining Earth and sky, to weighing Earth, to rethinking space and time itself.
For a reader who needs one line for notes, use this form: Newton published the universal law in 1687, Cavendish measured its laboratory strength in 1798, and Einstein reframed it in 1915. That line is accurate only with its labels attached. Remove the labels and it becomes misleading again, because it suggests three discoveries of the same thing. Keep the labels, and the dates become a useful map: a law, a measurement, and a new geometry. The map also leaves room for later work and ever more precise tests, without forcing every new result into a single discovery date.
Frequently asked questions
When was gravity discovered?
Gravity was not discovered on one date. Falling was always observed. The scientific landmark most often cited is 1687, when Newton published his law of universal gravitation. Einstein reframed gravity in 1915.
Did Isaac Newton discover gravity?
Newton is credited with the law of universal gravitation, not with creating gravity or being the first person to see something fall. He joined falling motion and planetary motion into one mathematical law that others could test.
Did an apple really fall on Newton’s head?
No reliable version makes a head strike the point. Newton told friends late in life that a falling apple prompted his thinking. Stukeley recorded that anecdote in 1752, decades after the events it recalls, so it is best treated as a remembered prompt, not a laboratory record.
When was the gravitational constant measured?
Cavendish performed his torsion balance experiment in 1797 to 1798 and reported Earth’s density in June 1798. Later workers derived a value for the gravitational constant from his result, so textbooks that say he measured the constant directly need that qualification.
Why is gravity 9.8 meters per second squared on Earth?
That rounded value is the average acceleration of a freely falling object near Earth’s surface, ignoring air resistance. It varies slightly with latitude, elevation, and local mass, because Earth bulges at the equator, rotates, and is not uniform inside.
Did Einstein prove Newton wrong?
Einstein did not make everyday falling stop working. Newton’s law remains accurate for many ordinary purposes. General relativity gives a deeper and more accurate account in strong gravity, at very high speeds, and for extremely precise measurements.
Keep reading: How Does Gravity Work?
Sources & references
- Britannica, How Was Gravity Discovered? Opened October 30, 2026. Used for the 1687 publication claim, the apple and Moon comparison, and Newton's build on Galileo and Kepler.
- Britannica, How Albert Einstein Developed the Theory of General Relativity. Opened October 30, 2026. Used for the 1907 equivalence idea, the 1912 tensor analysis step, the November 1915 completion, Mercury's perihelion, and the 1919 eclipse test.
- NASA Blueshift, 100 Years of General Relativity, November 25, 2015. Opened October 30, 2026. Used for the November 25, 1915 field equations, the space time curvature description, and the satellite navigation point.
- NASA Cosmicopia, Ask Us: General Physics, Gravity. Opened October 30, 2026. Used for the canonical 9.8 meters per second squared surface value, its variation from poles to equator, and the point that rotation reduces felt gravity at the equator.
- American Physical Society, June 1798: Cavendish weighs the world, June 2008. Opened October 30, 2026. Used for the 1797 start, June 1798 report, torsion balance method, Earth's density of 5.48 times water, and the modern view of the experiment as a constant measurement.
- University of Virginia, Discovering Gravity, by Michael Fowler. Opened October 30, 2026. Used for Aristotle's falling account, Galileo's constant acceleration and equal falling, Kepler's elliptical Mars analysis, Newton's Moon comparison, and Cavendish's 1798 measurement context.
- Newton Project, University of Oxford, Introduction to the Stukeley memoir material. Opened October 30, 2026. Used for the 1687 first edition of the Principia and for Stukeley first recording the apple anecdote in his 1752 memoir, told to him by Newton himself.
