Jupiter’s extra ‘moon’
WHEN: Nights of 25/26 and 26/27 April
The term planet comes from the Greek ‘planetes’ or ‘wandering stars’, so given because these bodies appear to move against the ‘fixed’ stars. It’s not unreasonable to think that, as they move, they can pass close to background stars, but this is less common than you might think. Jupiter’s four largest and brightest moons, Io, Europa, Ganymede and Callisto, orbit close to the planet’s equatorial plane. If Jupiter does wander near to a star, it’s usually obvious that the star is an interloper because it doesn’t line up with Jupiter’s equatorial plane.
On the night of 25/26 April, a ninth-magnitude star, HIP 74235 does line up to the left of the planet, in line with Jupiter’s equatorial plane. The other Galileans will be notably brighter, so there shouldn’t be any real confusion as to which is which. On the following night, the same star can be seen much closer to Jupiter’s disc, just to the north of Callisto, with Io below it. Just after rising, this ‘imposter moon’ lies 1.5 arcminutes from the centre of Jupiter.
If you’re up for a challenge, on 30 April, Jupiter will occult a mag. +10.8 star, TYC 6169372-1 between 01:24 and 02:21 BST (00:24 and 01:21 UT). Jupiter’s brightness will make this really hard to observe.
Jupiter gained an extra ‘moon’ on 12 April 2016 in the form of HIP 54057, third from the left