Star Watch

When Jupiter sparked a revolution

Jupiter outshines everything else high in the sky these evenings, noticed only glancingly by most (if noticed at all). Four hundred and two years ago this month, it was a different story.

In the city of Padua in Italy during the Renaissance, the brash and controversial scholar Galileo Galilei was just begin­ning to publicize a record-­breaking new claim. Using the magic tubes with which he was already building a reputation — but which were notoriously hard for anyone else to aim and use — he declared to have seen, at Jupiter, proof that the Earth is not creation’s only center of motion.

This was a world-shaking claim for the time and place, and if Galileo wanted attention, he got it.


Just a few months before, his telescopes had shown that the moon was not a perfect crystalline sphere, as orthodox philosophy had held since ­Aristotle, but an imperfect, broken world of mountains, clefts, and plains like our poor Earth itself. The heavens, ­Galileo brazenly showed, were not heavenly. He happily ridiculed “the great crowd of philosophers” as dullards besotted with Aristotle. One of them announced in reply that Galileo’s moon mountains might exist, but were nevertheless encased in invisible crystal that formed Aristotle’s perfect sphere. Galileo responded, to the laughter of his allies, that he would accept the layer of heavenly crystal and declare that it had its own invis­ible mountains 10 times higher still.

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Now he was announcing another heresy: that the planet Jupiter was attended by four satellite stars circling closely around it.

Conservative scholars had argued that Copernicus, with his new theory that Earth moves around the sun, had to be wrong because Earth could be the only center of motion. Galileo declared that anyone with a spyglass could see other­wise.

You still can. It doesn’t take much of a telescope to show Jupiter’s four big moons. A pair of binoculars usually shows at least two or three of them lined up on opposite sides of the planet, especially if you can brace the binoculars firmly against something to hold them still.

Try it this evening. Face southeast and aim at Jupiter shining high. Barely to the planet’s upper right, almost lost in its glare, will be a tiny row of two or three little points: The outer is Callisto, the next in is Ganymede, and if your binocular’s optics are sharp and your hold is steady, you may see Io closer in.


Cloudy this evening? Try ­tomorrow. Now Callisto will be somewhat closer to Jupiter, Io and Ganymede will be invisible next to the bright planet, and the fourth moon, Europa, will be in sight on Jupiter’s other side. Every night the pattern will change.

It took Galileo only a few days to figure out what was going on. He drew the positions of Jupiter’s attendants every clear night, aligned the drawings one below the other in ­sequence, and looked for patterns. He found them.

All four “satellite stars” were shuttling back and forth in the same plane, in a way that meant each was moving in a circle ­lying edge-on to our viewpoint. Io goes around ­Jupiter every 1.8 days, Europa every 3.5, Ganymede every 7, Callisto every 17.

These objects (named a few years later) evidently regarded Jupiter as the center of their universe, just as the moon does Earth. And if a center of motion like Jupiter could travel through space, it implied that Earth could, too.

This was an outrage to values and traditions that the ­educated Christian world, just emerging from the Middle ­Ages, held dear indeed. Galileo and others turned up yet more evidence of Earth’s motion and the sun’s centrality.


History records the denialist arguments, the threats and imprisonments, book burnings, demonizations, the increas­ingly extreme intellectual retrenchments, and other known bugs of human nature that come to the fore in the presence of unsettling new evidence. Yet within a generation or two it all gave way, and the medieval view of the cosmos started yielding to the modern one as confirming evidence spread.

As John Adams would say much later (during the Boston Massacre trial), “Facts are stubborn things; and whatever may be our wishes, our inclinations, or the dictates of our passions, they cannot alter the state of facts and evidence.” That is the foundational faith of science, and perhaps our best hope in present times.

Whole-sky maps

Easy-to-use maps of stars and constellations across the entire evening sky are available at

Alan M. MacRobert is a senior editor of Sky & Telescope magazine ( The Star Watch column ­appears the first Saturday of every month.