Ganymede occults Europa
BEST TIME TO SEE: As specified
Jupiter reaches opposition on 19 August and there are a number of fascinating events visible involving its four largest ‘Galilean’ moons. On 1 August, Ganymede partially occults Europa from 23:00 BST (22:00 UT). Then on the 7th, the outer moon Callisto is eclipsed by Jupiter’s shadow from 03:22 BST (02:22 UT).
On the 9th, Europa virtually disappears as it’s eclipsed by Ganymede’s shadow between 04:37–05:44 BST (03:37–04:44 UT). Smaller scopes will show Europa dimming out of view for a short time.
At opposition on 19 August, Io and Ganymede will appear close as Jupiter approaches the west-southwest horizon in the morning sky; at 05:15 BST (04:15 UT) they appear just 2 arcseconds apart.
On 20 August, starting at 02:50 BST (01:50 UT), Io and its shadow begin to transit Jupiter’s disc. Being near opposition and as we’re just past a Jovian equinox (2 May), Io and its shadow almost line up on Jupiter’s disc. Both appear central on the disc at 04:00 BST (03:00 UT), the transit concluding at 05:10 BST (04:10 UT).
The opposition moon–shadow alignment doesn’t last long and on the evening of 21 August between 21:15-23:35 BST (20:15-22:35 UT) Io transits with its shadow now notably out of sync.
On the 22nd, an evening view of Jupiter through a telescope will show Europa and Ganymede in transit at the same time as their shadows. The event is underway as Jupiter rises, ending at 23:20 BST (22:20 UT) as Ganymede’s shadow transit ends.
On the 29th, Europa and Ganymede transit Jupiter again accompanied by their shadows. The fit isn’t quite as good as the event on the 22nd as it’s not possible to see all four entities on the disc at the same time. It starts at 21:00 BST (20:00 UT) and ends at 03:20 BST (02:20 UT) on the 30th.