Jupiter: Transit of Io

Jupiter’s four major moons are regularly seen to transit the face of the giant planet. Depending on the Sun-Jupiter-Earth geometry, the moon itself, its shadow or both may be seen in transit.

In this sequence, Io is about to ingress onto Jupiter’s disk, and the Great Red Spot (GRS) is clearly seen near the lower left limb.

Thirteen minutes later Io has fully moved on to the disk. This illustrates the speed at which Io orbits Jupiter, taking just 42 hours for a revolution – hence it moves noticeably within just a few minutes.

In the third image, taken 2 hours after the first, the rotation of Jupiter (whose ‘day’ is just 9hrs 55 mins long) has carried the GRS about a fifth of the way around the planet, to the other side of the visible disk. We now see Io’s shadow crossing the disk, with the moon itself just visible at the far edge of the GRS.

Imaged through a Meade 14″ LX200ACF, using a Point Grey Research Flea-3 camera and Baader RGB filters. Stack of approx 200 frames taken through each filter.