Outta sight: once in a while, Jupiter's moons take a walk

Natural History, Nov, 2001 by Richard Panek

If Galileo had first looked at Jupiter through a spyglass powerful enough to discern the planet s moons a few evenings earlier or later than January 7, 1610, what he might have seen was this: Jupiter. Just Jupiter.

Instead, what he famously observed on the night of the 7th was Jupiter accompanied by a distinctive arrangement of "stars"--three pinpoints of light in a curiously straight line, two on one side of the planet, one on the other. Galileo looked at Jupiter again the following evening; this time he found the three stars all to one side of Jupiter. At first he suspected that the planet had changed position relative to the stars, even though such repositioning would have contradicted standard predictions for Jupiter's trajectory. Over the following week, however, Galileo not only located a fourth star accompanying Jupiter but also began to suspect that what was jumping around wasn't Jupiter; it was the mysterious "stars" themselves--the four bodies that today we know as the Galilean moons.

Of all the revolutionary discoveries Galileo made in his first months of using a telescope to study the night sky--including mountains on our own Moon and multitudes of stars invisible to the naked eye--it was this observation that prompted him to rush these findings into print. The presence of moons orbiting Jupiter didn't decisively validate Copernicus's 1543 hypothesis that the planets go around the Sun (Galileo's observation of the phases of Venus would soon do that; see "Celestial Events," June 1999). But it did demonstrate that Earth, dragging along its own Moon, isn't the only body in the universe that serves as a center of motion.

The Galilean moons aren't always visible to us. Each disappears from view when it passes behind (is occulted by) the planet. And when one of them passes in front of (transits) Jupiter, the two bodies can't be distinguished from each other with even quite sophisticated amateur instruments. Certainly Galileo's primitive telescope would have been inadequate to the task.

The occultations and transits of Io, the Galilean moon closest to Jupiter, begin about every twenty-one hours and last just over two hours. Callisto, the moon farthest from Jupiter, disappears, or nearly so, every eight and a third days for about four hours--"nearly so" because from our perspective, its orbit often takes it just above or below Jupiter rather than across it. Between these two extremes falls Europa, which vanishes every forty-two hours, and Ganymede, every three and a half days. The simultaneous disappearance of two Galilean moons isn't all that unusual. The disappearance of three at the same time, however, is.

Unusual but not irregular. Every six months or so, the three innermost moons vanish from sight simultaneously for a few minutes at a time. These disappearances can occur four, five, or more times at three-and-a-half-day intervals (the length of time between an occultation and a transit of Ganymede, the outermost of these three moons).

By a quirk of fate, Galileo began his historic observations of Jupiter on January 7, 1610, smack in the middle of one such spate of disappearances. Five evenings earlier, for an observer who happened to be where Galileo was (in Padua) at the very time that Galileo would soon begin his daily studies of the planet through his optic tube ("the first hour of night"), three of Jupiter's moons would have been out of sight. The fourth and farthest, Callisto, would have been distant enough in its orbit that Galileo might very well have overlooked it or, at any rate, failed to associate it with the planet. This precise celestial arrangement happened to repeat itself on January 6, though not at a time of day when Jupiter would have been above the horizon for an observer in Padua. On January 9, when the same circumstances recurred, Galileo in fact attempted to observe Jupiter--but cloud cover obscured his view.

Another such spate of disappearances is currently coming to an end. For an observer in New York, the moons Europa, Ganymede, and Io disappeared at about 5:00 A.M. on October 18 and remained out of sight right through sunrise. The same three moons disappeared again on October 21, but Jupiter was below the U.S. horizon at the time. On November 8, three Galilean moons will vanish once more, with Callisto--usually the distant straggler--among the missing. Ganymede will be the odd moon out, though by a distance from Jupiter of less than fifty arc seconds, or about 1/37 the width of our full Moon. So for virtually any amateur observer, the four Galilean moons will pass temporarily out of sight that day, beginning about 16:30 Universal time.

Not, alas, in New York, where the disappearance will occur at 11:30 A.M. And not in Padua, where Jupiter will be below the horizon. But if you happen to find yourself in Hawaii in the predawn hours of November 8, you'll be able to witness a sight that even Galileo never got to see, although it was one he thoroughly expected to find when he looked through his primitive telescope on January 7, 1610: just Jupiter.