See Jupiter and its Galilean moons in July 2021

Jupiter's moons are a great target for astronomers. With Jupiter becoming a better nighttime target, take time to observe its Galilean moons.

A view of Jupiter and its moons Io and Europa, captured by NASA's Juno spacecraft. Can you spot its fainter moons? Image Credit: NASA/JPL-Caltech/SwRI/MSSS/Roman Tkachenko

There are more opportunities to observe Jupiter interacting with its four major moons in July 2021. The four moons are Europa, Io, Ganymede and Callisto, and are known as the Galilean moons because of the fact that they were discovered by Galilleo.

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On 9 June, moons Io and Europa appear separated by 3 arcseconds – the apparent size of Uranus. Closest separation is at 02:43 BST (01:43 UT).

On 12 June, Callisto’s shadow transits Jupiter between 22:48 BST (21:48 UT) and 03:32 BST (02:32 UT). This begins before Jupiter rises and progresses as the gas giant gains altitude.

Ganymede’s shadow transits on 17 and 25 July. The event on 17 July occurs between 23:40 BST (22:40 UT) and 02:17 BST (01:17 UT) on 18 July.

View wonderful displays by Jupiter’s Galilean moons Io, Europa, Ganymede and Callisto in July 2021. Credit: Pete Lawrence
View wonderful displays by Jupiter’s Galilean moons Io, Europa, Ganymede and Callisto in July 2021. Credit: Pete Lawrence

Ganymede itself transits on 18 July from 02:53 BST (01:53 UT). The shadow transit on 25 July starts at 03:44 BST (02:44 UT).

On 29 July, a telescopic view of Jupiter just after rising will show two moons and a moon shadow in transit. Callisto and Io will be in transit along with Io’s shadow.

Io’s shadow exits transit at 23:23 BST (22:23 UT), Io at 23:53 BST (22:53 UT). Callisto leaves Jupiter’s disc at 02:36 BST (01:36 UT) on 30 July.

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Pete Lawrence is an experienced observer and a presenter on The Sky at Night. This guide originally appeared in the July 2021 issue of BBC Sky at Night Magazine.