Right now Jupiter utterly deserves its ancient moniker as king of the gods, at least during the hours of the night. Until Venus proclaims the dawn, the gaseous Goliath is the brightest star in the heavens, its creamy dazzle a lighthouse beacon in the southern sky. Unfortunately, though, this is its final season of prominence for a while. Jupiter won’t be this high again until 1998. Until then the solar system’s largest planet will hug the southern horizon, in the low part of its 12-year orbital path across our sky.
Jupiter’s so conspicuous now because it’s relatively close to us, a situation that recurs every 13 months when our faster-moving Earth catches up with the lethargic giant. As our two worlds pass, Jupiter seems to slip backward like a truck overtaken on the highway. Such retrograde motion was as bewildering to the ancients as programming a VCR is to us. They’d be amazed at the simple explanation.
Now, when Jupiter is shining brightest and appearing biggest, is a good time to own a small telescope. Galileo, using a truly pathetic low- power instrument, peered at Jupiter in 1610 and instantly revised the universe. He discovered four moons circling Jupiter, proof that Earth is not the center of all celestial motion.
Today, of course, you can still see those four dots lined up like track lights, even through binoculars. Impress your friends; announce them by name. The inner one, Io, an orange world of violently erupting volcanoes, zips around Jupiter in just 1.77 days, changing position while you wait. The next nearest moon, Europa, with a smooth, ice-covered surface, takes twice as long to orbit--3.55 days. That’s still impressively fast when you remember our own moon requires nearly a month. Next comes Ganymede. With a diameter of 3,279 miles, Ganymede is the solar system’s largest satellite--bigger even than Mercury or Pluto. You can quickly spot it because it’s the brightest of Jupiter’s moons. Watch it make one circuit per week, a period twice Europa’s and almost four times Io’s. Finally, farthest from Jupiter there’s Callisto, nearly as large as Ganymede. It takes 17 days to make its way around its parent planet. A dozen other moons have been found, but they’re faint chunks of debris not in the same league as the Galilean heavyweights.
You should check out Jupiter’s bands--really the tops of clouds-- running parallel to its equator. The bright bands are hot, rising gas clouds of red phosphorus or yellow sulfur; the darker bands show where these cooled gases sink down again. If conditions are steady, you might also see white ovals, swirls, festoons, and the Red Spot, which is as often as not a grayish pink. Jovian markings rush by like frenzied New York taxis, thanks to the planet’s snappy ten-hour rotation. Powerful stuff, that the largest planet also boasts the solar system’s fastest spin. Jupiter’s equator races along at an amazing 28,000 miles an hour (more than 25 times faster than ours). The centrifugal force of this planetary amusement-park ride makes the whole planet bulge outward at the equator. See it for yourself; any department store telescope will do the trick.
With naked eye, binoculars, or telescope, Jupiter is the award winner for planetary observers. Try it out. If you’re not satisfied with the view, return the instrument. Galileo had to deny what he saw for fear of death. Let’s hope the refund policy at your store is less intimidating.