You may have heard the story that an apple hit Isaac Newton on the head and gave him the idea of gravity. No one knows for sure if an apple really fell the way the story says, and the part where it hits him on the head is called a myth. Newton did tell a story about watching an apple fall from a tree, but Newton did not make gravity. He said that everything pulls on everything else, and he called this pull gravity. In 1666 Newton imagined that the Earth's gravity reached the Moon, balancing the Moon's outward push. From this, together with Kepler's third law, he worked out the inverse-square law.

Kepler's third law links a planet's distance from the Sun to the time its orbit takes. Gravity does not give the Moon its speed. It stops the Moon from running away and keeps it in some orbit. Newton showed that if gravity gets weaker with the square of the distance, then gravity on Earth's surface correctly predicts how long the Moon takes to orbit. Newton's interest in mathematics began, according to de Moivre, in 1663 when he bought an astrology book at a fair in Cambridge and could not understand its mathematics. His method of fluxions rested on one insight: integrating a function is the inverse of differentiating it. His simple methods unified many techniques that had been used on separate problems, such as finding areas, tangents, the lengths of curves, and maxima and minima. Yet the proof Newton published for planetary orbits did not use calculus. It used the properties of ellipses and other conic sections.

A sickness called the plague closed Cambridge University, so Newton went home to Woolsthorpe. While away from Cambridge because of the plague, Newton began important work in mathematics, light, physics and astronomy in less than two years. He had early versions of his three laws of motion by 1666, working on them in a notebook at the family farm. In 1684 Edmond Halley asked Newton what orbit a body follows under an inverse-square force. Newton replied at once that it would be an ellipse. Halley persuaded Newton to write out his new physics in full. The result, the Principia, was published in 1687, and Halley paid to have it printed. A mathematician at the Open University wrote that the Principia made Newton famous even though few people read it and even fewer understood it.

Before Newton, thinkers assigned different causes to different types of motion, according to physicist Stephen Hawking. By unifying all motion, Newton shifted science toward a search for large, unifying patterns in nature. In the Principia, Newton applied his laws to orbiting bodies, projectiles, pendulums and free-fall near the Earth. It explained the eccentric orbits of comets, the tides and their variations, the wobble of the Earth's axis, and the Moon's motion as pulled by the Sun's gravity. His law of universal gravitation says all matter attracts all other matter. Yet scientists on the European continent did not accept Newton's idea of action at a distance. They kept to Descartes's vortex theory, in which forces work through contact.

In the Principia, Newton's first law says a moving object keeps its speed and direction, and a still object stays still, unless an outside force acts on it. The law is often called inertia. Newton wrote that a change of motion is proportional to the force pressing on an object and happens in the direction of that force. His third law says that when one object pushes on another, the second pushes back just as hard in the opposite direction. His second law is written F=ma: force equals mass times acceleration. A heavy object speeds up less than a light object when the same force pushes on each. Newton wrote that if you press a stone with your finger, the stone also presses on your finger.

He split white light into colors with one prism, then used a second prism to join the colors back into white light. In a key experiment, Newton passed one color from the spectrum through a hole and a second prism. The second prism bent it by the same amount without changing its color, showing that each color is a pure part of white light. White light is made of all the colors of the rainbow, and Newton showed this. Newton explained the color of an object as the colors it reflects after it absorbs all the others. He named seven colors in the spectrum and admitted that choosing seven was rather arbitrary.

Newton shone sunlight through a glass prism and saw a band of colors. Here is the problem Newton faced: telescopes with glass lenses showed blurry colors. A mirror reflects all colors of light by the same amount, so a mirror telescope fixes this. Others, such as James Gregory in 1663, had described ideas for a reflecting telescope, but Newton was the first to build one. His used a mirror to gather light and a second mirror to send it to an eyepiece. The first mirror telescope he built was only about six inches long. As of November 2026, reflecting telescopes, which use mirrors as Newton's did, are mainstays of astronomy, including the Hubble Space Telescope.

In 1669 Newton became the Lucasian Professor at Cambridge, a professor's chair that Isaac Barrow had held. Newton became Warden of the Royal Mint in 1696 and Master of the Mint in 1699, a post he kept until his death in 1727. There he helped take old coins out of use and replace them with more reliable ones. In 1703 he was elected President of the Royal Society, a group of scientists. Newton lived to be 84 years old. He is buried in Westminster Abbey in London. The carved scene on the monument shows figures with a telescope, a prism, and a balance weighing the Sun and planets. Newton's laws are still used to reach the Moon, Mars and other places beyond Earth. Near the end of his life Newton said he felt like a boy on the seashore, finding a smoother pebble or prettier shell while the great ocean of truth lay undiscovered.

Words to know

• fluxions: Newton's calculus
• ellipse: stretched circle
• inertia: keeps moving, stays
• spectrum: band of colors
• orbit: path around something