Any telescope also reveals Jupiter’s four large moons. They typically appear in a straight line because all of them orbit in the planet’s equatorial plane. But this plane currently tilts enough from our point of view that the two outer moons, Ganymede and Callisto, can appear oddly out of line. Check out the scene the night of May 9/10, when Io, Ganymede, and Callisto appear in a straight line angled some 40° to Jupiter’s equator. The closest alignment occurs between roughly 1:00 and 1:30 a.m. EDT.
Yet there’s more to viewing Jupiter’s moons this night than one unusual configuration. All four perform an intricate dance with the planet and each other. A telescopic view in early evening reveals only three satellites because Jupiter’s shadow completely engulfs Ganymede. The solar system’s largest moon emerges from this eclipse at 10:55 p.m. EDT some 30" southeast of the planet’s limb. Also note Europa slightly west of Jupiter. It passes behind the giant world’s limb at 11:45 p.m.
Following the odd lineup of Callisto, Ganymede, and Io, Callisto coasts north of Jupiter while Io sets its sights on the planet’s midsection. The latter moon crosses in front of the gas giant’s eastern limb at 2:47 a.m. Like Jupiter, Io also casts a shadow, and it touches the planet’s cloud tops starting at 3:30 a.m. Six minutes later, Europa exits Jupiter’s shadow some 20" off the southeastern limb. The night’s final satellite events come when Io leaves Jupiter’s disk at 4:58 a.m. followed by its shadow at 5:41 a.m. Although the planet sets in the eastern half of North America before the conclusion plays out, those in the western half should enjoy excellent views.
Saturn rises shortly before 11:30 p.m. local daylight time at the beginning of May and some two hours earlier by month’s end. It climbs highest in the south during the wee hours of the morning and remains conspicuous until twilight is well underway.
The ringed planet will reach opposition and peak visibility in mid-June, but the view in May suffers little in comparison. You’ll be hard-pressed to notice the world growing brighter because its magnitude increases only from 0.3 to 0.1 during the month.
Once you soak in the view with your naked eye, target Saturn with binoculars. The ringed planet nestles among the rich star clouds of the Milky Way, not far from the galactic center. Saturn drifts slowly westward during May, starting in Sagittarius and crossing into Ophiuchus on May 18. You’ll see the misty glows of the Trifid Nebula (M20) and its neighbor, the open star cluster M21, some 5° east of the planet. The spectacular Lagoon Nebula (M8) lies less than 2° south of these splendid deep-sky objects. Also look for the fine open cluster M23 about 5° northeast of Saturn. Avoid the mornings of May 13 and 14 when a bright gibbous Moon passes a few degrees from Saturn.