Where the action is: if you don't like the weather on Jupiter, just wait

Natural History, Dec, 2001 by Richard Panek

Let's face it, the night sky doesn't do much. Sure, the stars appear to execute one rotation on the celestial sphere every twenty-three hours and fifty-six minutes. And yes, the planets, as the Greek derivation of the word suggests, wander slightly against the starry backdrop from one night to the next. And then there's the Moon, forever going through its fractional phases, over and over, unto eternity. But action? Put all these incremental changes together, and what you get is a drama of Noh-like minimalism, at most.

This helps explain why a favorite target of many amateur observers is the one telescopic object they know will actually change right before their eyes: Jupiter. Last month, this column described Galileo's discovery of the moons of Jupiter in January 1610, when the telescope was still in its infancy. Even that primitive telescope revealed the constant dance of Jupiter's four innermost moons, a piece of celestial choreography that visibly varies not only from night to night but over the course of a single evening. For amateur observers, Jupiter's moons offer the astronomical equivalent of an old saw, "If you don't like the weather, just wait." So does Jupiter itself--especially because part of what's always changing on the planet is, in fact, the weather.

Take the Great RKed Spot. It's a cyclonic storm similar to a hurricane (though its 15,400-mile diameter is large enough to swallow two Earths). The spot churns counterclockwise at speeds of up to 270 miles per hour. A constant presence--at least during the four centuries since the invention of the telescope--the Great Red Spot undergoes visible changes in shape, size, and even color. In recent years, it has appeared not so much red as bleached orange.

And the spot is only the most famous manifestation of Jupiter's volatile meteorology. Other storms arise frequently, sometimes drifting across the planet's atmosphere, sometimes growing from one day to the next, and sometimes simply popping out of existence. Cloud formations, too, are constantly shifting. Dark bands such as the north and south equatorial belts reveal where gases are descending, while white zones show where gases are rising. But even if Jupiter's atmospheric conditions were as calm as Colin Powell, the planet would still reward observers looking for action.

Jupiter, with a diameter of 88,789 miles at its equator, is the largest planet in the solar system, but despite its size, its rate of rotation is the fastest: it completes one revolution every nine hours and fifty-five minutes. Under ideal circumstances, an observer could watch an entire day pass on Jupiter in the course of one night on Earth.

Such circumstances arise during December and January. At 1:00 A.M. Eastern Standard Time (EST) on New Year's Day, Jupiter reaches opposition--the point in a planet's orbit at which Earth lies between it and the Sun. When that happens, observers on Earth can see a planet "in front of" them only when the Sun is "behind" them. Because Jupiter will be rising at sunset and setting at sunrise--a length of time that on some of the longest nights of winter happens to pretty much coincide with the planet's orbital period--observers will be able to see Jupiter's full rotation. What's more, this particular opposition of Jupiter finds the planet passing just about as close to Earth as it ever does, providing amateur observers with a generous target, forty-seven arc seconds wide.

The best opportunities to monitor the passage of a Jovian day in its entirety arise during the week before and the week after opposition--in this case, the last week of December and the first week of January. During that period, the planet will be high enough above the horizon throughout the crucial dusk-to-dawn time frame that watchers shouldn't be hindered by the significant blurring created by Earth's atmosphere when objects are low in the sky. Select a feature to monitor (a band, a belt, a storm), then follow it as it moves across the planet's visible disk, slips out of view, eventually reappears, and returns to its initial position. On New Year's Eve, for instance, the Great Red Spot will be crossing the planet's central meridian (the imaginary vertical line running from pole to pole down the center of the disk) at 7:06 P.M. EST and returning to the meridian at 5:01 A.M. EST on New Year's Day.

As always, the only obstacle standing between you and a good round of observations is the weather. Not there; here. But, hey, if you don't like the weather--well, you know what to do.

Richard Panek's next book, The Invisible Century: Einstein, Freud, and Our Search for Hidden Universes, will be published in 2003 by Viking.