The Big Three
DON’T MISS Jupiter moon events The three top sights to observe or image this month
This month’s top sights to observe.
WHEN: 2, 5, 6, 8, 9, 13, 14 & 16 May
Among Jupiter’s extended family of nearly 70 moons, only four can be easily seen through amateur telescopes. These are the Galilean moons, Io, Europa, Ganymede and Callisto, so called because they were first identified by Galileo in 1610.
Over time their star-like points flit back and forth either side of Jupiter’s disc. When they approach the disc from the west they are on the far side of their orbit relative to Earth and will pass behind Jupiter’s giant globe or into the planet’s shadow. When they approach from the east they pass in front of Jupiter, casting dark shadows on the planet’s atmosphere below. One exception to this is Callisto which has a large enough orbit to be able to pass above or below Jupiter when the planet’s small axial tilt is inclined enough.
For much of the time the Galilean moons and their shadows appear well separated from one another. This changes near opposition when Jupiter is on the opposite side of the sky to the Sun. Before opposition a moon’s shadow appears to the west of it, preceding the moon across the planet’s disc. After opposition the shadow follows the moon to the east of it. At opposition, the moon and shadow line up, crossing Jupiter’s disc in unison.
Typically this alignment isn’t perfect because a line from the shadow through the moon doesn’t directly point at Earth. Normally this line points either above or below our planet resulting in the moon’s opposition shadow appearing above or below the moon. Catching a moon and shadow transit at opposition is a matter of luck as a small offset in time either side of opposition makes a big difference to the appearance of the pairing.
There are a number of good examples of shadow transits this month occurring before and after 9 May, which is the date of Jupiter’s opposition. On 2 May Io can be seen chasing its shadow from 20:31 BST (19:31 UT). On 5 May it’s Europa’s turn to do the same thing from 23:03 BST (22:03 UT).
A more impressive moon and shadow transit occurs on 6 May when Ganymede can be seen chasing its shadow from 22:09 BST (21:09 UT). On 8 May, just before opposition, Io’s transit at 03:56 BST (02:56 UT) sees the moon virtually on top of its shadow. The end of this event occurs in daylight with Jupiter close to setting. A similar transit, again involving Io, occurs on 9 May from 22:24 BST (21:24 UT).
On 13 May Europa’s transit from 01:29 BST (00:29 UT) will see the moon preceding its shadow and another nice symmetry of events occurs after this with Ganymede also preceding its shadow from 01:53 BST (00:53 UT) on 14 May. Io can once again be seen preceding its shadow on 16 May at 23:08 UT.
5/6 May 9/10 May Io transit 22:24-00:32 BST Io shadow transit 22:25-00:35 BST Europa’s shadow Europa Europa transit 23:15-01:24 BST Europa shadow transit 23:03-01:19 BST Io’s shadow Io 6 May 14 May Callisto Ganymede’s shadow Ganymede Ganymede...
Io in particular can be tricky to spot in transit visually. It stands out well if using an IR-pass filter fitted to a monochrome camera (inset)