Jul 2, 2003

Harry Potter and the Moons of Jupiter

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Harry Potter and the
Moons of Jupiter


This week is your last good chance to see Jupiter's four largest moons before school starts next fall.




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July 2, 2003: Blistering-hot volcanoes that belch snow. Moons bigger than planets. Icy worlds with vast underground oceans. All of these things can be found in the latest Harry Potter novel. And according to NASA space probes, they're all real.


It was late one night at Hogwarts when Harry, Ron and Hermione were doing their homework: "a long and difficult essay about Jupiter's moons," but Harry and Ron didn't have their facts straight.

Right: a family portrait of Jupiter and its largest moons, from top to bottom: Io, Europa, Ganymede, Callisto. [more]

"Harry, you must have misheard Professor Sinistra," says Hermione on page 300 of Harry Potter and the Order of the Phoenix. "Europa's covered in ice, not mice!"

Correct. Jupiter's moon Europa is way too cold for mice: 260째 F below zero. Spacecraft have taken pictures of Europa's icy surface, and it looks totally lifeless.

Underground, however, might be a different matter. Some scientists think the ice on Europa hides the biggest ocean in the solar system--bigger than the Pacific, Atlantic and Indian oceans combined. Here on Earth water and life seem to go together. Could there be life in the waters of Europa? Microbes? Alien fish? Swimming mice? No one knows--not even Hermione.




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"And it's Io that's got the volcanoes," she says on page 295, correcting Ron's essay.

Right again. Io is even weirder than Europa. Some people say Io, dotted with volcanoes, looks like a pepperoni pizza, and that's about right. Io has more pepperoni-colored volcanoes than Ron Weasley has freckles. At any given moment, dozens of them might be active, spewing the hottest lava in the solar system. The plumes rise 300 miles into space where it's so cold that volcanic ash freezes before it falls back to the ground--sulfurous snow. NASA's Galileo spacecraft has actually flown through some of these plumes and survived.

"Jupiter's biggest moon is Ganymede, not Callisto," Hermione adds, pointing over Ron's shoulder at another mistake.

Indeed, Ganymede is the largest moon in the whole solar system. It's slightly wider than the planet Mercury and more than three-quarters the size of the planet Mars. If it orbited the sun instead of Jupiter, Ganymede would surely be considered a planet. Heavily-cratered Callisto is only a little smaller than Ganymede and, like Europa, might be hiding a subterranean ocean.


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These four wonderful moons, Io, Europa, Ganymede and Callisto are real. They were discovered by Galileo Galilei in 1610 when he looked at Jupiter through one of the first primitive telescopes. Galileo was amazed by the four little stars he saw near the giant planet, and even more amazed when they moved from night to night, orbiting Jupiter. Astronomers now call them the Galilean satellites.


Right: An excerpt from Galileo's 400-year-old manuscripts in which he sketched Jupiter's moons changing positions from night to night. [more]

Almost everything known about the Galilean satellites--other than their number, four, and the basic shapes and sizes of their orbits--comes from NASA spacecraft, especially the two Voyagers, which flew by Jupiter in 1979, and the Galileo space probe orbiting Jupiter now.

It's good to know that these missions have been closely followed at Hogwarts.

This week you can see the Galilean satellites for yourself. If you live at mid-northern latitudes, step outside around 9 p.m. and look west toward the setting sun. The first bright star emerging from the twilight is Jupiter. Point a telescope in that direction; a small one will do. (Remember, even the cheapest department-store telescope is better than Galileo's 400 year-old spyglass.) The four moons will appear in the eyepiece as a line of dim stars straddling the giant planet.

If you only see two or three moons, that's because one or two of them are probably behind Jupiter. Look again later or perhaps tomorrow. The missing moons will come out of hiding as they circle Jupiter.


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Above: Look low and west after sunset to see Jupiter this week. On July 2nd, our own planet's moon will glide by Jupiter--a magical sight. Click here to see what Jupiter and the crescent moon might look like.

Jupiter itself will look like a fat disk crossed by rust-colored cloud belts. First-time observers often note that the planet seems squashed--wider along the equator than between the poles. Is there something wrong with the telescope? No. Jupiter really is flattened. Although Jupiter is 11 times bigger around the middle than Earth, it spins more than twice as fast; a day on Jupiter lasts only 9 hours and 55 minutes. This rapid spin is what gives Jupiter its equatorial bulge.

Young wizards are advised not to wait too long to see these things because Jupiter and its moons will soon disapparate! Or as a muggle astronomer would say, they're about to disappear for a few months behind the sun. In fact, this week is your last chance to see the moons of Jupiter before school starts again in the fall.

So don't miss them. You do want to pass your O.W.L.s ... don't you?




Web Links


Harry Potter -- ( an overview of the five-book series. See also the Harry Potter home page from Warner Bros.


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Right: This is what Jupiter looks like through a telescope: a bright disk with four star-like moons lined up on either side of the planet. Jupiter's cloud belts are washed out in this over-exposure. [more]


Io: Dashing through the Snows of Io (Science@NASA); Alien Volcanoes (Science@NASA); Quick Facts (JPL)

Europa: Divining Water on Europa (Science@NASA); Quick Facts (JPL)

Ganymede: Great Ganymede (Science@NASA); Quick Facts (JPL);

Callisto: The Strange Spires of Callisto (Science@NASA); Jupiter's Moon Callisto May Hide a Salty Ocean (JPL); Callisto (JPL)

Galileo discovers the satellites of Jupiter -- (Rice University) an engaging account of Galileo's discovery.


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