For nearly two thousand years, most astronomers in Europe and the Islamic world placed the Earth motionless at the center of the universe. It seemed obvious: we feel no motion, and the Sun, Moon and stars all appear to circle us. Yet between 1543 and 1687 a handful of thinkers overturned that picture and replaced it with one in which the Earth is a planet, the planets follow precise mathematical paths, and a single force governs both a falling apple and the Moon. This level tells that story as astronomy.

The Earth-centered model

The most successful geocentric model was set out by Claudius Ptolemy in Alexandria in the second century CE. To explain why planets sometimes seem to move backward, an effect called retrograde motion, he had them travel on small circles riding on larger circles. The system was complicated, but it predicted planetary positions well enough to be used for well over a thousand years. Some ancient thinkers disagreed. Around 270 BCE, Aristarchus of Samos proposed that the Earth goes around the Sun, but his idea found few followers.

Copernicus puts the Sun in the middle

The Polish astronomer Nicolaus Copernicus revived the Sun-centered idea in his book On the Revolutions of the Heavenly Spheres, published in 1543, the year he died. In his model, retrograde motion is simply an effect of perspective: Earth, on a faster inner track, overtakes outer planets like Mars, which then seem to slip backward against the stars, just as a slower car seems to move backward when you pass it on a highway. Copernicus still used perfect circles, so his predictions were not much better than Ptolemy's. His achievement was a simpler and more natural explanation.

Tycho's data and Kepler's laws

The Danish nobleman Tycho Brahe built the finest observatory of his age on the island of Hven and, without a telescope, measured planetary positions more accurately than anyone before him. When Tycho died in 1601, his assistant Johannes Kepler inherited the records. Kepler spent years trying to fit the orbit of Mars with circles and failed by a mere eight minutes of arc, an error Tycho's data was too good to allow. So he abandoned circles, and the result was three laws:

• First law: planets move in ellipses, slightly flattened circles, with the Sun at one focus.
• Second law: a planet moves faster when it is closer to the Sun and slower when it is farther away, sweeping out equal areas in equal times.
• Third law: the square of a planet's orbital period is proportional to the cube of its average distance from the Sun.

Kepler published the first two in 1609 and the third in 1619. For the first time, the planets followed rules simple enough to state in a sentence and accurate enough to match the best observations.

Galileo's telescope

In 1609 and 1610, Galileo Galilei turned a telescope to the sky and found moons orbiting Jupiter, proof that not everything circles the Earth, and saw Venus go through a full set of phases, which only makes sense if Venus orbits the Sun. His defense of Copernicus led to his trial by the Roman Inquisition in 1633 and house arrest for the rest of his life.

Newton explains why

Kepler described how planets move but not why. In 1687 Isaac Newton published the Principia, which showed that one law of universal gravitation explains it all: every mass attracts every other mass with a force that grows with their masses and weakens with the square of the distance between them. From that single idea, together with his laws of motion, Newton could derive all three of Kepler's laws, the tides, and the path of a cannonball. The heavens and the Earth obeyed the same physics.

The decisive test came from a comet. Newton's friend Edmond Halley, who paid to publish the Principia, noticed that comets seen in 1531, 1607 and 1682 followed similar paths and predicted that the same comet would return around 1758. Halley died in 1742. On Christmas night in 1758 an amateur astronomer in Germany spotted it, and it has carried his name ever since. Direct proof that Earth moves came later, in 1838, when Friedrich Bessel measured stellar parallax, the tiny yearly shift in a nearby star's position as Earth travels around its orbit.

Worked example: checking Kepler's third law

Astronomers measure distances in the solar system in astronomical units (AU), where 1 AU is Earth's average distance from the Sun, about 150 million kilometers. Measured in years and AU, Kepler's third law becomes simply: period squared equals distance cubed. Try it on Jupiter.

• Distance: Jupiter orbits about 5.2 AU from the Sun.
• Cube it: 5.2 × 5.2 × 5.2 is about 140.6.
• Take the square root: the square root of 140.6 is about 11.9.
• Compare with observation: Jupiter takes 11.86 years to go around the Sun.

A rule found by fitting the orbit of Mars predicts the year of a planet more than three times farther out. When a pattern holds far beyond the data that produced it, that is a strong sign it reflects something real, and Newton's gravity turned out to be the reason why.