Astronomy: Moons of Jupiter, Part 1


Jupiter is high in the south just after dark right now. Start looking in evening twilight just after sunset. As the sky darkens, it will be the first “star” that pops out. Besides the moon and Venus, Jupiter is the brightest object in the night sky, so it’s hard to miss.

In even the simplest binoculars, the most obvious features are the four moons that usually appear in a straight line just outside the disk of the planet.

Jupiter actually has 67 or so satellites, some of which are so small that they had to be discovered by spacecraft as they flew by or orbited the planet. In fact, if you combined all of the other moons besides the big four and Jupiter’s nearly invisible dust ring, you’d get only 0.003 percent of all the stuff orbiting Jupiter.

It is no wonder that only those four brightest, called the Galilean moons, are easily visible in Earth-based amateur telescopes.

In fact, they were the first astronomical objects that I observed besides the craters on the moon with anything besides my unaided eye.

I did so in my old man’s $3 pair of plastic-lens opera glasses. He had bought them to go to the opera. My father had only a ninth-grade education, but he loved the opera. However, I’ll wager that those opera glasses got used more to observe real stars than opera stars.

Galileo’s experience matched my own. The Galilean moons were among the first things that Galileo saw in 1609 and 1610 when he trained his first crude telescope on the heavens. They proved that other planets besides Earth have moons and helped to doom the notion that Earth was the center of the universe.

Around the same time, German astronomer Simon Marius claimed to have discovered the moons independently a few days before Galileo. However, Marius appears to have replicated Galileo’s drawing of the moons to report his discovery, and Marius published after Galileo.

Thus, we still call the four satellites the “Galilean satellites” and not the “Mariusian moons” to this very day.

But Marius got the naming rights. At the suggestion of Johannes Kepler, Marius decided to name the moons after close personal “friends” of the Roman god Jupiter. Here is how he justified the names: “Io, Europa, the boy Ganymede, and Callisto greatly pleased lustful Jupiter.” (Ganymede was male, but Jupiter didn’t seem to have any prejudices in that regard.)

The names almost immediately fell out of use. For the next three centuries, astronomers referred to the Galilean moons as (from closest to farthest) Jupiter I, Jupiter II, etc.

By the end of the nineteenth century, the mythological names had started to come back into favor. Thus, he fifth-discovered moon was named Amalthea after its discovery in 1892.

Three of the four — Io, Ganymede, and Callisto — are larger even than Earth’s moon, and Europa is almost as big at about 2,000 miles in diameter.

Ganymede, the third moon out, is the largest planetary satellite. In fact, Ganymede is the ninth largest object in the solar system. Only the sun and seven of the planets are larger. It surpasses Mercury in size.

In fact, Ganymede is so large at about 3,200 miles in diameter that it may be visible to the unaided eye as a very faint star-like point very close to the planet. When Jupiter is closest to Earth, it shines at magnitude 4.5, which is well within the range of human visibility.

As early as 364 BCE, Chinese astronomer Gan De claimed to see one of the moons, most likely Ganymede.

The Russian explorer Ferdinand Wrangel is reported to have encountered a Siberian hunter who pointed to Jupiter and claimed that he had just seen the planet “swallow a small star next to it and vomit it up shortly afterwards.”

That’s a perfect description of a moon approaching a planet, passing in front of it or behind it, and then reappearing on the other side.

Many experienced stargazers scoff at such reports. The problem is glare. Ganymede may be the second most distant from Jupiter of its visible moons, but at half a billion miles from Earth, the two objects are going to look awfully close to each other. Jupiter’s brightness would tend to wash out the moon’s fainter glow.

Also, you’d have to catch Ganymede at just the right time. Jupiter must be at or near its closest point to Earth. Ganymede must be at a point in its orbit far enough away from Jupiter to escape part of its glare.

Despite the difficulties, I am not one of the scoffers, and here’s why. Children’s eyes gather more light that adults, and they stand a better chance of seeing Ganymede. When I was a kid, I saw it myself or at least persuaded myself I did.

Several times over the course of an Earth year, I used a note card to block out the light from Jupiter at every possible angle. After many tries, I saw a tiny point of light huddled up against the planet.

Had I seen Ganymede? Of course, the observers, myself included, may have been reporting Jupiter passing in front of a star. If I were trying it now, I would check a star map for nearby stars and a table of Ganymede’s orbit to verify its apparent distance from the planet to make sure.

I’m glad I didn’t do it then. It is out of such memories that my current love of stargazing persists after half a century. I saw Ganymede. Let’s just leave it at that.

Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.

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