The Jovian Moons

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The images, captured by the Galileo probe, show Jupiter's four largest satellites to the same scale. Click on each satellite image for more information. Images and captions courtesy of NASA.

Jupiter's Moons are also known as the Jovian Moons because Jove is another name for Jupiter. Technically, we should not use the term "moons" here because there really is only one "Moon," Earth's only natural satellite. So, we really should be talking about Jupiter's satellites. By last count, Jupiter had 16 satellites, but it is the four largest that were discovered by Galileo, hence they are called the "Galilean satellites." The rest are just too small to be seen with small telescopes. (Interestingly, Galileo himself wanted to name the satellites the "Medicean Planets" in honor of his patron, Cosmo de Medici, Grand Duke of Tuscany.)

There are several important things you should note about these satellites. One is that they are all except Europa are larger than our Moon. Another is that three of them have lower densities than our Moon, a result of forming farther from the Sun. Moreover, their densities decrease the farther the satellite is from Jupiter, just like the planets orbiting the Sun. Io, the closest of the Galilean satellites, has the highest density, and Callisto, the farthest, has the lowest density.

This selection from Galileo's own sketches shows the arrangement of the Galilean satellites (as stars either side of the planet) around Jupiter (the large circles) over a period of nine days.

The Galilean satellites are easy to see with minimal equipment. Viewed from Earth in a small telescope such as the one Galileo used, or even in an ordinary pair of binoculars, the Galilean satellites appear as tiny starlike objects strung out on either side of the planet. Their positions change each night, and can even be seen to move over a period of hours. It didn't take long for astronomers to realize that these objects orbited around Jupiter a bit like a miniature solar system. While the Galilean satellites are technically bright enough to sometimes be seen with the unaided eye, this cannot be done under ordinary circumstances. The chances of doing this are extremely small because the glare of Jupiter is too great. Nevertheless, observers have reported being able to see the satellites under rare conditions without a telescope.

If you tried the rather difficult activity that accompanies our installment on density, you know that by using Newton's refinement of Kepler's third law, we can calculate the mass of Jupiter simply by observing these satellites orbit the giant planet. Given the distance to Jupiter, and good amateur astronomer can make the observations necessary to calculate the planet's mass.

What you may not know is that the satellites of Jupiter were key players in a scientific mystery whose solution led to the first reasonably accurate determination of the speed of light. In this case the detective was not the famous Sherlock Holmes, but a Dane by the name of Ole Roemer. But let's hold off the rest of the story for the activity!

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