See Jupiter and its Galilean moons in October 2021

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

Jupiter moons double shadow transit

Jupiter’s largest moons are putting on something of a display in October 2021, making it a great time to observe them in the night and early morning sky.

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On Monday 4 October, there’s a rare opportunity to see the shadows of Galilean moons Ganymede and Callisto in transit simultaneously.

The event begins in daylight at 18:00 BST (17:00 UT) as Callisto’s shadow starts its passage.

Ganymede itself appears on disc at this time, but exits from view at 19:19 BST (18:19 UT) under darkening skies.

Ganymede Callisto shadow transit 4 October 2021
Catch the double shadow transit on
4 October at 20:53 BST (19:53 UT)

Ganymede’s shadow begins its transit at 19:50 BST (18:50 UT) under dark-sky conditions. Callisto’s shadow is conveniently positioned on Jupiter’s central meridian at this time.

Both shadows pose for an image either side of the central meridian at 20:53 BST (19:53 UT).

By the time Callisto’s shadow exits the disc at 22:25 BST (21:25 UT), Ganymede’s has largely caught up and will leave transit an hour later at 23:25 BST (22:25 UT).

The dark nature of both shadows makes this an ideal event to observe through smaller instruments.

Io and Callisto

io callisto transit 21 October
Io catches up with and passes Callisto during their dual transit on 20/21 October 2021. Credit: Pete Lawrence

Just after midnight BST on 20 October, it’s possible to see Io and Callisto in transit at the same time.

This event starts at 00:30 BST on 21 October (23:30 UT on 20 October) with both moons lined up as if about to start a race. If it were a race, the clear winner would be Io.

Being closer to Jupiter, Io orbits at a much faster pace and, given a flat southwest horizon, it should be possible to see the inner moon striving ahead of the outer moon Callisto.

This event occurs near to Jupiter setting and, sadly, concludes with the planet beneath the UK’s horizon.

You need a telescope to observe these moon events, but if the sky is clear on the evening of 15 October, there’s a nice conjunction of our 77%-lit waxing gibbous Moon and mag. –2.5 Jupiter that should be striking to the naked eye.

Both objects will appear at their closest in the early evening as the sky is darkening.

On a plus point, Jupiter is now increasing in altitude as seen from the UK. This year it’s in Capricornus and able to reach a height approaching 22° as seen from the centre of the UK. In 2022, that figure increases to around 36°.

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This guide originally appeared in the October 2021 issue of BBC Sky at Night Magazine.