Naturalist rings of light

Where the great rings of endless 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 3½ 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.

© Dana Wilde 2007

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