Modeling Jupiter's Moons

Sometimes you can’t get your hands on the authentic object in order to study it… When we study astronomy it’s rare that we can find evening hours to look at the sky and even then it can be cloudy or difficult due to light pollution. If we’re going to study celestial objects a model is often in order.

Here's a setup that will help you understand how Jupiter's four largest moons orbit the planet and how Galileo was able to make some historic observations. Gather a large ball (such as a basketball or volleyball) and four smallish balls. It will help if the smaller balls look different from each other. If you have a couple of friends you can tape strings to the small balls and proceed, but if you’re solo, here’s another option: find small bowls or rolls of tape on which to perch the balls so they won’t roll.

Line up the four small balls in order next to Jupiter. From the closest to the farthest away they will be representing: Io, Europa, Ganymede, and Callisto. These are the four Galilean moons of Jupiter. If you have three or four friends to hold the suspended balls (on strings) they will look more realistic (as in the above photo) but you can easily place them as described previously.

Once they’re in place, take a picture.

Envision each orbit of the moons. The farthest one, Callisto, will have the largest orbit, while the nearest, Io, will have the smallest. Next move each of the balls in their orbits around Jupiter as follows: Io, ½ an orbit, Europa ¼, Ganymede 1/8 and Callisto 1/16 of an orbit.

Take a picture.

Continue doing this a few more times; moving the moons in their orbits and taking pictures. The orbits don’t cross; you’ll have to imagine their locations as you reposition the moons. You’ll probably notice that Callisto has a much larger orbit than Io and seems to move very little from picture to picture.

Consider: What do the pictures represent?

When Galileo first looked thorugh a telescope at the night sky he found Jupiter and noticed that it seemed surrounded by four stars. As he watched it night after night he noticed that these stars seemed to change positions relative to each other. He understood almost immediatedly that what he was seeing was a BIG deal. Here’s what he wrote a few years later in the Sidereal Messenger:

But now we have not just one planet rotating about another [the Moon about Earth] while both run through a great orbit around the Sun; our own eyes show us four stars which wander around Jupiter as does the Moon around the Earth, while all together trace out a grand revolution about the Sun in the space of twelve years.

If you’d like a more formal treatment of this activity to use in your classroom you can purchase one here.

If you’d like a free copy of Jupiter’s moons as seen on subsequent nights, you can get one by signing up here.​