Four Lights Beside Jupiter

For two thousand years the sky was a finished thing: perfect spheres turning around a motionless Earth.

Then in the winter of 1609 a forty-five-year-old professor in Padua, short of money, ground his own lenses and pointed a tube at it.

In three months he found mountains on the Moon, four moons circling Jupiter, and more stars than anyone could count. He also found the end of his freedom.

Aristotle taught that everything above the Moon is perfect: smooth spheres of unchanging substance, carrying the lights around us.

Ptolemy turned that into working mathematics, good enough to keep calendars and ships in order for fourteen centuries.

Copernicus suggested in 1543 that the Sun might be at the centre. It was treated as a calculating trick. There was no evidence either way.

Galileo was born in Pisa in 1564, the son of a musician who argued that the rules of music should answer to experiment, not to authority.

He left medical school without a degree, taught himself geometry, and talked his way into mathematics chairs at Pisa and then Padua in 1592.

Professors were paid badly. He had three children and a workshop in the house. Then, in 1608, news came from Holland of a tube that made distant things look near.

The instrument was not his. A spectacle maker in the Netherlands applied for a patent in 1608, and the rumour reached Venice the next summer.

Galileo worked out what the arrangement had to be and started grinding glass. Within weeks he had better than the Dutch toys sold in the street.

On the twenty-fifth of August 1609 he showed the Venetian Senate ships two hours before the eye could see them. They doubled his salary.

By that winter he had reached twenty times. Press the buttons and see what each glass shows you.

From the thirtieth of November to the eighteenth of December 1609 he drew the Moon, night after night.

The line between day and night was not the clean curve a smooth ball would give. It was ragged, bitten into.

Beyond it, single points of light appeared in the darkness and joined the lit side as the Sun rose on them. On Earth, that is what mountain tops do at dawn.

From how far those points stood from the line he worked out a height: at least four miles. The heavens were supposed to be perfect. He had just drawn their flaws.

On the seventh of January 1610 he turned the glass on Jupiter and saw three small bright points in a line beside it.

Fixed stars, he assumed. He wrote down where they were, which is the whole of the discovery.

The next night all three were on the same side. Stars do not do that. Press the buttons and run the nights yourself.

He watched every clear night. The points swung from one side of Jupiter to the other and never strayed far.

On the thirteenth of January a fourth appeared. His drawings are rows: a circle for Jupiter, little stars for the lights, one row per night.

What makes the pattern is four orbits of different lengths, from a day and three quarters for the innermost to nearly seventeen days for the outermost.

The conclusion is short. The lights never leave Jupiter and they swing from side to side. Therefore they go around Jupiter.

So the Earth is not the one centre that everything turns about. There is at least one more.

That did not prove the Earth moves. It removed the strongest objection, that a moving Earth would leave its own Moon behind. Here was a moving planet keeping four.

Sidereus Nuncius, the starry messenger, went on sale in Venice in March 1610: twenty-four pages, about five hundred and fifty copies, gone almost at once.

Wherever he pointed the glass there were more stars. Six in the Pleiades became thirty-five. Nine around the belt of Orion became eighty.

The Milky Way came apart into separate stars.

Then, from September 1610, Venus showed phases like the Moon, from a small full disc to a large crescent. In the old Earth-centred arrangement that is impossible.

Twenty years before the telescope he had been working on how things fall.

Falling is too fast to time with a pulse, so he diluted gravity: a straight groove in a long plank, one end raised a little, a bronze ball rolling down.

For a clock he used water. A vessel with a thin spout poured while the ball rolled, and he weighed the water afterwards. Equal weights meant equal times.

The distances went one, four, nine, sixteen: the squares. The gaps between them went one, three, five, seven: the odd numbers. Set the slope and release the ball.

Everyone knows the story: two balls dropped from the Leaning Tower of Pisa, landing together.

It comes from a biography by his pupil Viviani, written decades later. Galileo never describes doing it. The careful work is the ramp, repeated.

And he was honest about the air. In a hundred-cubit fall, he wrote, a gold ball would beat a copper one by perhaps four fingers, not by the lengths Aristotle required.

The trouble was not the moons. It was what he said they meant.

In February 1616 Cardinal Bellarmine ordered him to abandon the opinion that the Earth moves: not to hold it, not to defend it, not to teach it.

In 1632, with a licence from the censors, he published a dialogue on the two systems of the world. The defender of the old order gets the weakest arguments and the name Simplicio.

In June 1633, at sixty-nine, he was tried, found vehemently suspect of heresy, and knelt to abjure. Prison became house arrest the next day.

The line everyone quotes, and yet it moves, first appears about a century after his death.

He spent the rest of his life in a villa at Arcetri, forbidden to publish and watched.

And there he wrote the book physicists still read. Two New Sciences is the work of a lifetime on materials and motion: the ramps, the squares of the time, the beginnings of inertia.

It could not be printed in Italy. The manuscript went north and was published in Leiden in 1638. By then he was blind. He died at Arcetri on the eighth of January 1642, still a prisoner.

The moons turned out to be useful. Their eclipses behind Jupiter are a clock in the sky, and Galileo spent years trying to sell it as a way of finding longitude at sea.

In 1676 Ole Rømer noticed that the clock ran late when Earth was far from Jupiter, and realised the delay was the travel time of light. It was the first sign that light has a speed at all.

Newton took the law of fall and built the law of gravity on top of it. In 1992 the Vatican closed its review of the case and acknowledged that the theologians had been wrong.

Two spacecraft are on their way to those four lights now. The chapter stills in this film are AI-generated; the drawings, printed pages, paintings and photographs are real. Sources are listed below.