Cobra Effect · Space

The moons that circled Jupiter

How a few moving dots beside Jupiter challenged the old picture of the heavens.

7 cards, read aloud in 3:18, with a test and sources.

A long telescope on a tripod stand, aimed up at a sky scattered with stars

In 1609 Galileo Galilei heard about a new Dutch invention, and built a telescope of his own.

Spyglasses that made distant things look closer had recently appeared in the Netherlands. Galileo, a professor at Padua in Italy, worked out how to make his own, and kept improving them. His first magnified about eight times. Before long he had one that magnified about twenty times. He turned it on the night sky.

In January 1610, he saw three small stars in a line beside Jupiter.

Two sat on one side of the planet, and one on the other. He took them for ordinary distant stars. But the next night, all three were on the same side of Jupiter. Night after night the little stars moved, yet stayed close to Jupiter as it crossed the sky.

Within a week he had seen a fourth, and realised they were moons circling Jupiter.

Their changing positions only made sense if they were going round the planet. Sometimes one vanished, hidden behind Jupiter or lost in its glare. Galileo sketched their positions carefully, night after night. Nobody had ever seen anything circling another planet before.

In March 1610 he published his discoveries in a short book, The Starry Messenger.

It also described mountains on the Moon, and countless stars too faint to see without a telescope. He named the four moons the Medicean Stars, after the powerful Medici family of Florence. The book made him famous, and won him a post at the Medici court. Today the moons are called Io, Europa, Ganymede and Callisto.

The moons were a problem for the old picture of the heavens.

Many people then held that everything in the sky went round the Earth. Some argued that if the Earth really moved round the Sun, it would leave its Moon behind. Jupiter plainly moved across the sky, and kept its four moons with it. Here were bodies circling something other than the Earth.

Galileo came to argue that the Earth moves round the Sun, and was put on trial for it.

He saw that Venus shows phases like the Moon, which fitted Venus circling the Sun. In 1632 he published a book comparing the two pictures, which plainly favoured the Sun at the centre. In 1633 the Roman Inquisition found him vehemently suspected of heresy, and made him take back his views. He spent the rest of his life under house arrest.

So when something does not fit the old picture, ask what new picture it points to.

Galileo was not looking for moons. He simply kept watching, and kept records. A few small dots that moved in an unexpected way changed how people saw the heavens. In 1995 a spacecraft named Galileo arrived at Jupiter, and studied the moons he had found. Jupiter is now known to have dozens of moons.

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