Observe how the shadows of the Jovian moons shift around at opposition

The planet Jupiter reaches opposition on 26 September, a term which describes when a planet is on the opposite side of the sky to the Sun. This month’s challenge is to see some effects visible around the time of Jovian opposition. We would recommend using a 75mm or larger telescope, with a minimum magnification of 100x. The larger the telescope, the clearer the view.

Jupiter reaches perihelion on 25 January 2023, when it will be closest to the Sun in its orbit. This means that Jupiter’s 2022 and 2023 oppositions will be particularly good as seen from Earth, the planet appearing at its brightest and largest for some time. Opposition also has an effect on the appearance of its four largest Galilean moons because as they transit across the Jovian disc, they are closely accompanied by their shadows.

Before opposition, a moon’s shadow will appear to precede the moon. After opposition, a shadow will follow its moon.

At the time of opposition itself, the shadow and moon appear in sync, moving across Jupiter together.

Catching this moment, when a moon and its shadow are exactly at opposition, is a matter of luck with timing and often doesn’t work out. However, there are normally a few events visible in the days before and after opposition when the sync is still pretty close. On 21 September, the giant moon Ganymede is preceded by its shadow. This can be seen from 01:04 BST (00:04 UT) through to 05:31 BST (04:31 UT). Then on 24 September, Io transits with its shadow fractionally ahead of its disc between 02:55 BST (01:55 UT) and 05:10 BST (04:10 UT).

Also on 24 September, there’s a chance to see Europa and its shadow passing across the Jovian disc between 21:19 BST (20:19 UT) and 23:49 BST (22:49 UT). Here Europa and its shadow will appear close but separated, the shadow fractionally ahead of the moon.

An excellent transit of Io and its shadow can be seen on the evening of 25 September starting at 21:25 BST (20:25 UT) and concluding at 23:37 BST (22:37 UT). Being very close to opposition, in this instance the moon and its shadow will be overlapped as they transit together. Then on the morning of 28 September, Ganymede crosses Jupiter’s disc, this time followed by its shadow. This event runs from 05:10 BST (04:10 UT) until Jupiter sets around 07:00 BST (06:00 UT) as dawn breaks.