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 the planets were traveling around the sun, not the Earth (and later had trouble with the church for saying so), but was stumped when he saw tiny pinpricks of light changing location near Jupiter every night.

After two months of carefully observing and recording the pinpricks’ 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, with your Kmart spyglasses on clear, dark nights, when Jupiter’s in the sky. (Jupiter is now rising at about 6 a.m. and setting at about 5 p.m., too close to the sun to catch more than a glimpse with dawn and dusk light flooding the sky at both ends.)

Every time Jupiter rises, its moons appear 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.

Jupiter and its three moons, from left, Callisto, Ganymede and Europa, are seen in this image taken May 8, 2003, by the Mars Global Surveyor spacecraft from Mars orbit. At the time, Io was behind Jupiter, as seen from Mars. Photo courtesy of NASA/JPL/Malin Space Science Systems

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 3½ days, and closest is Io which whirls around in less than two days. (Our moon takes about 27.3 days to orbit the Earth.)


You cannot 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 appears 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 a half-hour or so and you will notice the stars are not 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 is not the stars moving, but the Earth rotating, making the sun, moon, planets and stars seem to be rising, traveling and setting.

The stars, for all practical purposes in a human lifetime, do not move, but the planets do – around the sun. From Earth we see Venus over weeks creep 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 circles too. Saturn’s moons and rings revolve around it as it rounds the sun. Countless asteroids, Mars, Uranus and Neptune, each with circling 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 in the Kuiper belt, shells of comets and icy rocks, such as Pluto, loop continuously around, and all of this, and the sun and other stars, are swirling around the center of the galaxy.

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.

Dana Wilde lives in Troy. Contact him at [email protected] His book, “Winter: Notes and Numina from the Maine Woods,” is available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.

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