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May 21: Check in on Saturn and Mars
Tonight we’re focused on Jupiter, as two of its Galilean moons cross the planet’ disk this evening. East Coast observers can zoom in on the gas giant shortly after sunset to see Io alone to the planet’s west, with Ganymede (closest), Europa, and Callisto all to the east. Ganymede is closing in on the eastern limb and begins transiting first, starting at 9:57 P.M. EDT, as darkness is falling across the Central time zone. Then it’s a bit of a wait for more — Europa finally reaches the eastern limb and begins transiting 2 hours later, with Ganymede slightly more than halfway across.
The two moons continue across the disk together, and the planet sets with the pair visible against the cloud tops for observers in the eastern half of the U.S. In the meantime, Io is approaching the western limb. At 11:02 P.M. MDT (note the time change, as Jupiter has now set farther east), with Ganymede very close to the western limb, Io winks out of view, passing behind the planet in an occultation. Ganymede finishes its transit 20 minutes later, with Europa now in the center of Jupiter’s disk.
Pacific time zone observers are now in for a real treat: About 10:55 P.M. PDT (again, note the time zone), both Europa’s and Ganymede’s shadows appear at the eastern limb, nearly at the same time. Europa’s arrives just 2 minutes before than Ganymede’s, but the latter shadow is larger and likely easier to see. Europa’s shadow may be smaller but it is moving faster, so pulls ahead quickly and the two become easier to view separately as Jupiter sets along the West Coast. Any observers in Alaska and Hawaii will get the best view of the two shadows crossing the gas giant’s disk.
Sunrise: 5:39 A.M.
Sunset: 8:15 P.M.
Moonrise: 11:46 A.M.
Moonset: 1:17 A.M.
Moon Phase: Waxing crescent (46%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
