Jupiter’s four Galilean moons reign supreme


In my dotage, my mind often turns to my quarter century at Perkins Observatory, where I reveled in showing people what their universe looks like. I remember my thousands of public programs with joy — mostly. However, sometimes the experience can be downright humbling.

An experience of the latter variety happened many years ago during late autumn. I was giving the usual talk before the typical telescope observing session. I paused dramatically to begin my inspirational “big finish.”

A 6-month-old baby in arms named Gabriella began to speak. “Blah, blah, blah, blah, blah, blah, blah,” she babbled cheerfully. The audience erupted with laughter, and so did I.

You can elaborate on the universe’s wonders to the exhaustion of yourself and your audience, but nothing beats the direct experience of its beauty. Conversely, looking at a collection of specks in the sky can be deadly dull if you don’t have a little scientific and cultural context.

Jupiter, which is currently high in the southern sky just after dark, is a case in point. Besides the moon and Venus, it is the brightest object in the nighttime sky, so it’s hard to miss.

In binoculars, the most apparent features are its four bright satellites, which usually appear in a straight line near the disk of the planet.

I have shown folks those moons many times, so I am familiar with the responses telescope operators get from the public. They range from breathless excitement to bored complaisance.

A practiced eye can easily see Jupiter’s cloud bands stretched horizontally around the planet. They are caused by a combination of Jupiter’s rapid rotation and rising and falling heat. To most audiences, any further explanation falls into the “too much information” classification.

However, most folks have seen spectacular images of Jupiter from space telescopes. That’s a tough act to follow.

Thus, the speck-like moons are often all you have to work with, and they look like specks in a telescope. So I chatter away, as follows, about the moons at the long line of people waiting to look through the telescope.

Jupiter has 80 or so satellites, most of which are so small that they had to be discovered by spacecraft.

The number sounds impressive, but the moons don’t amount to much. If you combined all the other moons besides the big four and Jupiter’s nearly invisible dust rings, you’d get only 0.003 percent of all the stuff orbiting Jupiter.

No wonder only those four brightest, called the Galilean satellites, are easily visible in Earth-based amateur telescopes.

They were among the first things Galileo saw in 1609 and 1610 when he trained his first crude telescope on the heavens.

They proved that another planet besides Earth has 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.

Galileo accused Marius of plagiarism. The accusation stuck, perhaps unjustly, because of Galileo’s reputation in the scientific community.

Thus, we still call the four satellites the “Galilean satellites” and not the “Marius moons.”

However, Marius one-upped Galileo in one respect. At the suggestion of Johannes Kepler, he named the moons after paramours of the Roman god Jupiter.

As Marius wrote, “Io, Europa, the boy Ganymede, and Callisto greatly pleased lustful Jupiter.”

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 19th century, the mythological names had started to come back into favor. Consequently, the fifth-discovered moon was named Amalthea after its discovery in 1892.

It pays to know which moon is which before you start to observe it at a public program. For one thing, some inquisitive kid will ask. For another thing, it gives you a chance to “blah, blah, blah” about specific moons.

From the perspective of a public program, Io is the most fascinating of the Jovian moons. Its proximity to Jupiter bends and stretches Io because of the giant planet’s gravitational oomph.

Those tidal forces cause Io to spew out volcanic plumes of sulfur. Some of that sulfur contributes to Jupiter’s dusty ring system. However, amateur telescopes can’t see any of those spectacular details.

Of course, you can always fall back on mythological stories. Fortunately, Io’s tale incorporates Jupiter and Io and two prominent constellations. Unfortunately, like most Greco-Roman myths, the story is unsuitable for many audiences without heavy expurgation, as you shall see.

Jupiter’s, the king of the Roman gods, engaged in frequent marital infidelities, despite the watchful eye of Juno, his justly jealous wife. One ill-omened day he fell in lust with Io, the stunningly beautiful daughter of the river god Inachos.

Io couldn’t resist the charms of Jupiter. Power, as Henry Kissinger once said, is a powerful aphrodisiac.

Still, Io had to be cautious. Juno could make Io’s life extremely unpleasant if she discovered the illicit relationship.

Jupiter was not so squeamish. Io was a priestess in Juno’s temple, which added to the allure as far as Jupiter was concerned.

When Juno inevitably found out, she turned poor Io into a white heifer, a relationship killer if there ever was one. Because she didn’t trust her philandering husband even under those weird circumstances, she instructed Argus, a giant with 100 eyes, to watch the imprisoned heifer.

Argus made an excellent prison guard. Because a few of his eyes were always open, he didn’t get much sleep.

Jupiter figured the least he could do was to free Io from prison. Because he never did his own dirty work, he sent Mercury, the fleet-footed messenger of the gods, to kill the huge, hairy insomniac.

It was no mean task. Argus had eyes in the back of his head (literally!). However, lightning speed triumphed over 20-20 vigilance. Mercury lopped off the giant’s head before he knew what hit him.

Juno was furious, but she couldn’t do much to punish her more-powerful husband. In one version of the myth, she contents herself by placing the slain giant’s 100 eyes in the sky in the tail of the constellation Pavo, the Peacock.

In another rendition of the story, Juno exacts a certain measure of revenge. Just as Io is freed from imprisonment, Juno sends a gadfly to bedevil the heifer.

Io jumps into the sea to escape the annoyance. There she remains, immersed in the ocean up to her shoulders as the constellation Taurus, the Bull.

Despite his many flaws, Jupiter is, if nothing else, loyal to his fallen lovers. As he moves among the stars of the Zodiac, he occasionally revisits the beauty of the constellation.

Jupiter will revisit his bovine buddy starting in May 2023, at which point I will try to figure out how to tell the story and leave out the naughty bits.

Ganymede, the third moon out, is almost as interesting as Io because it is the largest planetary satellite.

At 3,270 miles wide, Ganymede is the ninth-largest object in the solar system. Only the sun and seven of the planets are bigger. It surpasses even Mercury in size.

Ganymede is so big and bright that it may be visible to the unaided eye as a very faint star-like point close to the planet. When Jupiter is closest to Earth, Ganymede shines at magnitude 4.5, 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.

According to David Seargent in his delightful book Weird Astronomy, the Russian explorer Ferdinand Wrangel reported an encounter with a Siberian hunter. The hunter pointed to the planet and claimed that he had just seen Jupiter “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 or behind it, and then reappearing on the other side.

Most stargazers scoff at such reports. The problem is glare. Jupiter’s brightness tends to wash out the moon’s fainter glow.

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

I am not one of the skeptics. Children’s eyes gather more light and generally resolve images better than adults’ eyes. Kids stand a better chance of seeing Ganymede than adult scoffers with their shrunken pupillary openings.

As a kid, I saw it myself — or at least persuaded myself I did — by blocking out Jupiter’s brightness with a note card.

Did I see Ganymede? Ganymede’s observers, myself included, may have seen Jupiter passing in front of a star.

However, such nerdish memories are the foundation of my six-decade love of stargazing. I saw Ganymede. Let’s just leave it at that.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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