Equally captivating are Jupiter’s four big moons: Io, Europa, Ganymede, and Callisto in order of distance from the gas giant. The satellites spend most of their time either east or west of the planet, but each of the three inner ones passes in front of the jovian disk once an orbit. During such a transit, the moon also casts its shadow onto the planet’s bright cloud tops, where it appears as a distinct black dot. Half an orbit later, the moon passes behind Jupiter (an occultation) and disappears in the planet’s shadow (an eclipse).
Although satellite events occur every day, some appeal more than others. Here we highlight several of the best for North American observers. Io transits Jupiter on the evening of April 2. The Moon’s shadow touches the planet’s disk at 11:31 p.m. EDT followed eight minutes later by Io itself. The moon seems to hover just east of the shadow throughout the transits. This is a sign that the Sun lies almost directly behind Earth, as it will at opposition on the 7th.
Compare these transits to the ones the night of April 9/10. In the latter case, Io first appears against Jupiter’s cloud tops at 1:22 a.m. EDT followed by the shadow three minutes later. The moon now leads its shadow as they cross the planet’s disk.
On the night of opposition, April 7/8, Jupiter’s shadow falls directly behind the planet. Although all four moons show up in the evening, Europa passes behind the planet from 1:36 to 4:04 a.m. EDT. In a rare circumstance that can happen only at opposition, Jupiter simultaneously occults and eclipses the moon. A day later, on April 9, Io emerges from behind the planet’s eastern limb at 6:15 a.m. EDT, but you won’t see it until it emerges from Jupiter’s shadow three minutes later.
On the evening of April 14, watch Europa and Callisto approach Jupiter almost in lockstep. After midnight, Europa passes behind Jupiter’s limb at 3:52 a.m. EDT and emerges from the planet’s shadow at 6:38 a.m. (after the planet sets from eastern North America). Meanwhile, outermost Callisto passes above the planet’s south pole.
Saturn rises near 1:30 a.m. local daylight time at the start of April and some 30 minutes earlier with each passing week. The magnitude 0.3 ringed planet lies against the Milky Way star fields of northwestern Sagittarius. It moves slowly eastward during the month’s first few days, then reaches a stationary point on the 6th before starting a westward trek.