Stellarium, with additions from the author
On January 10th, Jupiter’s moon Callisto will cross in front of the giant planet. In this rare event, the moon itself will cover the shadow it casts on the planet’s cloudtops. The event will be visible in its entirety in all of North and Central America, and in part in South America, Australia, and East Asia. Callisto and its shadow will touch Jupiter’s disk at 1:55 a.m. EST / 6:55 Universal Time on January 10th. Mid-transit occurs at 3:55 EST / 8:55 UT and egress commences at 5:49 EST / 10:49 UT.
Not your average lunar transit
Unlike Io, Europa, and Ganymede, Callisto transits Jupiter only occasionally. That’s because the moon circles its planet on a much wider orbit than the others (1.9 million kilometers compared to 1.1 million for the 2nd outer moon, Ganymede). Combined with its slightly tilted orbit (0.3° relative to Jupiter’s equator, the second largest value after Europa’s), Callisto usually passes north or south of Jupiter’s disk.
Only when Earth and the Sun move near Jupiter’s equatorial plane (and thus the orbital planes of its large moons) can Callisto move in front of and behind Jupiter when viewed from Earth. Its transits and shadow events are therefore seasonal and centered around plane crossings (when Earth crosses Jupiter’s equatorial plane) and equinoxes (when the Sun does the same). These seasons happen twice during Jupiter’s 11.9-year-long orbit around the Sun, approximately every six years. Around these dates, transits and shadow events of Callisto are possible for up to three years. The next plane crossing will be on October 13th, and the next equinox is on December 16th. This means we’re currently within a Callisto transit and shadow season.
Curiously, the January 10th event coincides almost to the minute with Jupiter’s opposition (which happens at 4 a.m. EST / 9:00 UT). At that time, the Sun, Earth, Callisto, and Jupiter will be perfectly arranged along a line more than 780 million kilometers long. That’s an extremely rare coincidence!
What to expect?
Consequently, Callisto will cover most of its shadow during this transit. As the moon is far from its planet, Callisto’s shadow has a large penumbra, appearing fuzzier than the shadows of the inner moons. Callisto also features the lowest geometric albedo of all four large Jovian moons (0.22), making it look darker than Io, Europa, and Ganymede.
It’s not entirely clear how this event will appear: Callisto will probably look like a curiously dark spot against Jupiter’s bright southern equatorial zone, maybe surrounded by a strange fuzzy edge if the faint outer penumbra of its shadow is perceptible.
The Association of Lunar and Planetary Observers (ALPO) is encouraging observers and photographers to submit observations: “Your reports along with imaging and sketches should be sent to [email protected] for review by the Section Coordinator and posting in our gallery.”
If you’re in the right time zone, you might also want to try two transits of Io, which partially occulting its own shadow on January 9th, 15:55 UT and January 11th, 10:25 UT (times are for mid-transit). Clear skies!