Jupiter’s ex­tra ‘moon’

WHEN: Nights of 25/26 and 26/27 April

Sky at Night Magazine - - THE SKY GUIDE -

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

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