Satellite Imagination: The Galilean Moons of Jupiter

Like many astronomers, amateur and otherwise, I would love to go visiting planets and moons. For me, it’s especially the moons. Unfortunately, that’s not likely to happen. My wife Anita would never let me strap in for a space mission, present or future. Probably born a century or two too early to boot. Darn.

This past year, I’ve written a series of articles for the local astronomy club‘s newsletter under the heading of “Satellite Imagination.” The above paragraph is how I opened my first essay. I aimed at blending three aspects in my writing: the amateur observation of the satellites of other planets, the stories of their discovery, and my own experiences of viewing them.

For this web page, I can supplement my original pieces with pretty pictures, like the Galilean moons above. (Io, Europa, Ganymede, and Callisto, left to right.) I can also adjust the writing style for those among you who aren’t active astronomers, and maybe inspire a few of you to have a look yourself. Pull out a pair of binoculars or dust off that telescope in your den.

Four planets are easily visible to the unaided human eye from Earth. Add Mercury if you have a clear horizon. Two other planets, Uranus and Neptune, can be seen in small telescopes or really good binoculars if you know where to look.

But ever since I was a boy, I was fascinated not so much by the planets as their moons. I wrote lists of moons and learned the names of the thirty-one that were known in the 1960’s. Much dimmer than planets, fifteen to twenty of these are possible to view with a basic knowledge and a good telescope.

When I pull out the binoculars and aim at Jupiter, the planet doesn’t always hold my interest. True, Jupiter is handsome, and even limited magnification can resolve a disk with bands and a red spot. A day lasts ten hours there, so even in a few hours of viewing, you can see that Great Red Spot as it experiences a two-and-a-half-hour morning, then an equally short afternoon.

With that simple pair of binoculars, I can imagine myself back at the dawn of the seventeenth century, sitting alongside Galileo and his simple hand-made telescope. The great scientist had no sooner aimed his optics at Jupiter than he discerned four moving “stars.” To curry favor with the Grand Duke of Tuscany, Cosimo de Medici, he later named these points of light the Medicean Stars.

What was remarkable is that they stayed close to Jupiter, moving from side to side. After a few days, Galileo realized they all orbited that small disk of a planet. The Earth-centered universe was shattered by a few nights of observation that began not with the intent to be heretical, but to satisfy a curiosity.

In the late 1500’s, the Polish astronomer Copernicus quietly suggested the sun, not the Earth, was the center of the universe. With these four wanderers centered on Jupiter, it seemed that the sun wasn’t holding a monopoly on centrality either.

Along with Galileo, I ponder the systems within systems of the Copernican model. Not only does the Earth travel around the sun, but these star-like specks circle the orb of Jupiter. If I stay out a few hours, the movement is noticeable. If I come back the next night, the inner three will scramble themselves.

Reflecting on these mobile points of light I’m soon dreaming about other things: the volcanoes of Io, the subsurface oceans of Europa, the strange icescapes of Ganymede and Callisto. This is what modern astronomy brings us: astronomy opens the door to geology as we come to view extraterrestrial landscapes. Some scientists dream of biology coming to the fore as we probe the ice crusts of these strange new little world. We may have to wait on biology, but pondering life off Earth is a sign of a grand imagination.

As I’m looking up with Galileo, he with his handmade telescope and I with my binoculars, I can imagine dark skies over an Italian night and the unknown frontier out there.

Look over there on the right. That’s the view we see. Bright Jupiter is a glare in the night sky. Its moons are actually bright enough to be seen from Earth, but only if they weren’t so close to the planet. They are as bright as the dimmer stars in the Little Dipper.

Now, if I were on Mars, I’d be a little closer. Maybe I’d see them through the visor of my pressure suit. Wouldn’t that be an adventure!

…

Then Anita calls me from the kitchen door. Back from outer space. Back to the present day. Goodbye, Galileo.