Copernicus, Kepler and Galileo: how the sun-centred system was argued and proved

Who were Copernicus, Kepler and Galileo?

Nicolaus Copernicus (1473 to 1543) was a canon of Frombork Cathedral in Royal Prussia, trained in law, medicine and astronomy in Kraków and Italy. His book On the Revolutions of the Heavenly Spheres (De revolutionibus) came off the press in 1543, the year he died.

Johannes Kepler (1571 to 1630) was a German mathematician from Württemberg who became Imperial Mathematician in Prague. He found the three laws of planetary motion and explained how lenses and the eye work.

Galileo Galilei (1564 to 1642) was a Tuscan mathematician and physicist who turned the telescope on the sky in 1609 and 1610, defended Copernicus in print, and was tried by the Roman Inquisition in 1633.

Copernicus died before the other two were born. Kepler and Galileo were contemporaries who wrote to each other but never met.

The starting point: an earth-centred universe

For 1,400 years, educated Europeans and Muslims used the system of Claudius Ptolemy, whose Almagest was written around 150 CE. The earth sat still at the centre. Planets moved on small circles (epicycles) carried around larger circles, which reproduced their looping paths across the sky reasonably well. Aristotle's physics backed it up: heavy things fall toward the centre of the universe, and if the earth moved, we would feel it and objects would be left behind.

A sun-centred idea was not new. Aristarchus of Samos proposed it in the 3rd century BCE, as Archimedes reports in The Sand Reckoner. But it found few followers, because it seemed to contradict common sense and physics.

Step by step: how the argument was won

Step 1: Copernicus makes the case (1543)

Copernicus first sketched his idea in a short manuscript, the Commentariolus, around 1514, and shared it privately. His only student, Georg Joachim Rheticus, persuaded him to publish; Rheticus's own summary, the Narratio prima, appeared in 1540. The full book came out in Nuremberg in 1543 with an unsigned preface by the Lutheran theologian Andreas Osiander saying the theory was only a convenient hypothesis for calculation, which Copernicus had not written.

His strongest argument was order. With the sun at the centre, the backward loops of Mars, Jupiter and Saturn become a natural effect of the faster earth overtaking them, and the planets line up by orbital period, from Mercury (fastest) to Saturn (slowest). In his dedication to Pope Paul III he called the existing models a monster made of mismatched limbs.

The weakness: he kept perfect circles, so he still needed epicycles, and his tables were not much more accurate. Critics also asked why, if the earth moved around the sun, the stars did not seem to shift position over the year. Copernicus answered that the stars are immensely far away. He was right, but it could not be shown for three centuries.

Steps 2 and 3: measurements, then Kepler's laws (1572 to 1619)

The Danish nobleman Tycho Brahe built observatories on the island of Hven from 1576 and measured planetary positions to about two minutes of arc with naked-eye instruments, far better than anyone before. He proposed a compromise: the planets circle the sun, and the sun circles a still earth.

Kepler, a convinced Copernican since his student days at Tübingen, joined Tycho in Prague in 1600. When Tycho died in 1601, Kepler inherited his data and spent years on the orbit of Mars. A circular orbit missed Tycho's observations by eight minutes of arc, a tiny amount, but Kepler would not ignore it: in Astronomia nova (1609) he wrote that this discrepancy, given so accurate an observer, pointed the way to reforming all astronomy. The answer was an ellipse. His third law followed in Harmonices Mundi (1619), and his Rudolphine Tables (1627) were dramatically more accurate than any before.

Step 4: Galileo's telescope (1610 to 1613)

In The Starry Messenger (March 1610) Galileo reported four moons orbiting Jupiter. This answered an old objection: if the earth moved, how could it keep its Moon? Jupiter plainly moved and kept four. He also showed the Moon has mountains, so heavenly bodies are not perfect spheres unlike the earth.

Later in 1610 he saw Venus go through a full set of phases, like the Moon. In Ptolemy's arrangement Venus always sits between the earth and the sun, so it could never appear full. The Ptolemaic system was finished. But Tycho's hybrid system also predicted full phases, so the telescope alone could not decide between Tycho and Copernicus.

Galileo's physics mattered as much. In the Dialogue (1632) he argued that inside a smoothly moving ship's cabin, no experiment shows that the ship moves, so we would not feel the earth's motion either. His proof from the tides, which he thought the strongest, was wrong.

The Church's response came in stages. In 1616 the Congregation of the Index suspended De revolutionibus until corrected, and Galileo was warned not to hold or defend the earth's motion. After the Dialogue, the 1633 trial condemned him. Books teaching the earth's motion were removed from the general prohibition in 1758, and the Dialogue left the Index in 1835. In 1992 Pope John Paul II formally acknowledged the errors of Galileo's judges. Read the myth check did Galileo say and yet it moves.

Did Kepler and Galileo help each other?

Partly. In 1597 Kepler sent Galileo his first book. Galileo replied on 4 August 1597 that he had accepted Copernicus's view for years but had not dared publish. In 1610, when other scholars doubted the Starry Messenger, Kepler published his Conversation with the Starry Messenger in support, and later confirmed the moons of Jupiter with a borrowed telescope. Galileo rarely answered Kepler's letters and never accepted elliptical orbits. The two men who together made the case convincing were never really allies.

Questions to ask each of them

1. Copernicus: what first made you think the earth moves?
2. Copernicus: why did you wait until the end of your life to publish?
3. Copernicus: what did you think of Osiander's preface?
4. Kepler: why did eight minutes of arc matter so much to you?
5. Kepler: how did you work out that orbits are ellipses?
6. Galileo: why did the phases of Venus matter?
7. Galileo: why did you never accept Kepler's ellipses?
8. All three: what evidence would have convinced your critics?

On Yuga you can ask each by voice: Copernicus, Kepler and Galileo, and follow up with Newton. They are AI recreations built from their books and letters and can be wrong. Cite De revolutionibus, Astronomia nova and the Starry Messenger by name in your work.

Which one for your project?

Choose Copernicus for the moment the idea was published and the debate about how to present a dangerous theory. Choose Kepler for how careful data overturns a beautiful assumption, and for his extraordinary life, including defending his mother at a witchcraft trial from 1615 to 1621. Choose Galileo for observation, the telescope and the trial. Choose all three, with this timeline, for any project on the scientific revolution. For the next step, see Newton vs Galileo.