The month’s most dramatic scene occurs the following night, however. Both Io and Ganymede start to cross Jupiter’s disk the evening of June 11. But with the gas giant so close to opposition, the shadow that each satellite casts falls onto the jovian cloud tops almost directly beneath the moon.
Io’s transit begins at 10:22 p.m. EDT. Within five minutes, you should notice its shadow immediately to the moon’s east, with the two overlapping. Ganymede lies north of Io and begins to transit at 11:28 p.m. Its shadow falls on the cloud tops just southeast of the moon. Because Ganymede lies farther from Jupiter than Io, its shadow appears slightly more separated from the moon. Io completes its transit at 12:33 a.m., when Ganymede is about halfway across the giant planet’s disk. The outer moon completes its trek around 1:40 a.m.
Saturn lies 30° east of Jupiter, which means it trails about two hours behind its bigger brother. The ringed planet rises around 11 p.m. local daylight time June 1 and some two hours earlier by month’s end.
Saturn lies in northern Sagittarius, just south of that constellation’s Teaspoon asterism. It shines at magnitude 0.2 in mid-June and appears four times brighter than any of the Archer’s stars.
The best time to view Saturn through a telescope occurs during the early morning hours when it climbs highest in the south. Even the smallest telescope delivers stunning views. The planet’s disk measures 18" across while the rings span 41" and tilt 24° to our line of sight. Saturn’s disk shows little detail, though you might spot an equatorial belt and a dark polar hood.
Although Saturn’s moons don’t glow as brightly as Jupiter’s Galilean satellites, small scopes reveal at least four of them. Titan is the easiest. It shines at 8th magnitude and shows up through any instrument. This large moon orbits Saturn in 16 days, passing south of the ringed world the mornings of June 5 and 21 and north of the planet on the 13th and 29th. Look for three 10th-magnitude moons — Tethys, Dione, and Rhea — closer to Saturn.