Where great rings of light stop, nobody knows

When Galileo first laid eyes on Jupiter’s four largest moons in 1610, he did not know what he was seeing.

He had built a telescope for himself that was blurrier and less powerful than the $30 binoculars you can buy at Kmart. He already suspected that the planets were traveling around the sun, not the Earth (and later got in trouble with the church for this), but was stumped when he saw tiny pinpricks of light changing location near Jupiter every night. After two months of carefully observing and recording their shifting positions, and trying to visualize what might be happening, the truth broke like daylight on his mind’s eye. These pinpricks were not “stars,” he wrote in his pamphlet “The Starry Messenger” in March 1610, but “four PLANETS never seen from the creation of the world up to our own time” circling around Jupiter.

You can still see them up there, of course, on clear, dark nights with your Kmart spyglasses. Every time Jupiter rises, they’re in different places, exactly as Galileo sketched – sometimes two tiny white dots on one side of the planet and one on the other, or four all on the same side, or one and three, and so on.

The reason they change place every night is that they’re all orbiting around Jupiter at titanic speeds. The most distant is Callisto, which revolves around the giant planet in 16.7 Earth-days. Next is Ganymede, the largest of the four, whose orbit takes just over a week. Closer in, Europa takes about 31/2 days, and closest is Io, which whirls around in less than two days.

You can’t see them moving, naturally. When you look, they seem frozen for that moment in a little line straddling Jupiter. In fact, the whole sky at any particular moment seems as fixed as a painting to us, trapped here in the shadow of time.

But if you set it all in motion in your mind’s eye, as Galileo did to Jupiter’s moons, you can see it as it actually is.

Watch the horizon for half an hour or so, and you notice the stars aren’t really stationary, but are slipping behind treetops at about the same undetectable rate as a minute hand on a clock. This first motion is an illusion, though, because it’s not the stars who plod across the sky, but the Earth’s rotation creating moon, sun and star-set.

But Venus over weeks is actually moving, creeping up and down the sky in its 224-day orbit of the sun. Jupiter’s circuit takes almost 12 years, and round and round it on that path, Callisto, Ganymede, Europa and Io make endless epicircles. Saturn’s moons and rings revolve around it as it rounds the sun; the moon circles Earth on the Earth’s circle of the sun; and countless asteroids, and Mars, Uranus and Neptune, also with moons, are circling the sun. When thawed from the freeze of time, the solar system is a set of turning wheels. And beyond the planets, shells of comets and icy rocks like Pluto loop continuously around, and this and the sun and other stars are swirling around the center of the Milky Way.

The sky is all awhirl in great rings of endless light. If Galileo had known all this, he would have fallen on his face and prayed. He probably did just for Jupiter’s moons.

-DWILDE@BANGORDAILYNEWS.NET