Jupiter’s moon show continues
WHEN: 2, 3, 23/24 and 27 February, times as specified
JUPITER REACHES OPPOSITION on 6 February and experiences an equinox, the time that the Sun is on its equatorial plane, on the 5th. This orientation is interesting because from Earth, although the alignment isn’t precisely the same, it’s similar enough to allow us to see some interesting interactions between the planet and its four largest moons as well as some rather special mutual events that can occur between the moons themselves.
The four moons in question are Io, Europa, Ganymede and Callisto, collectively known as the Galilean Moons. To start, on 2 February innermost moon Io will pass in front of the second moon out, Europa. This is a large partial occultation – Io almost, but not quite, covering Europa.
Though a small telescope it will be possible to see the moons approach one another in the lead up to the occultation; at 03:25 UT, it will be hard to separate the pair. The occultation begins at 03:29:39 UT, with maximum coverage at 03:31:24 UT. At this point, both moons will appear as one even through a large scope. The event ends almost as quickly as it begins at 03:33:08 UT.
On 3 February, there’s a rather lovely regular interaction between Io and Jupiter. Look at the planet at 19:00 UT and you’ll see the dot of Io very close to Jupiter’s eastern limb. The Great Red Spot should also be visible at this time, closer to the west limb. At 19:25 UT, as Io appears to be virtually touching the limb, the moon’s shadow will make its presence known on Jupiter’s disc, close to Io itself. Io officially begins to transit Jupiter at 19:30 UT and – being so close to opposition – the moon and its shadow appear as a tight pair as they cross Jupiter’s globe. The shadow leaves the disc at 21:43 UT, closely followed by Io at 21:47 UT.
By the 23rd, Jupiter will have moved slightly away from opposition and this is shown by a lovely dual transit of giant Ganymede and its shadow. After opposition Ganymede leads the way, passing onto the disc at 19:26 UT. The increased Sun angle means that its shadow takes longer to appear, passing onto Jupiter’s globe at 21:07 UT. Ganymede exits off the west limb at 23:03 UT, with its shadow following suit at 00:45 UT on 24 February. Finally, on the 27th, Io and Ganymede put on another show, the smaller moon attempting to occult its larger stablemate. The alignment is almost perfect but, with a smaller disc, Io is unable to fully cover Ganymede and ends up passing inside the larger moon’s boundary in an annular occultation or transit. This event starts at 02:17:11 UT and ends at 02:23:55 UT, with maximum annularity occurring at 02:20:33 UT.