See Jupiter’s moons cast their shadow on the gas giant

This month, Io and Europa can be seen as part of a daylight double shadow Jovian transit.

View a double transit of Jupiter with moons Io and Europa on 24 October. Credit: Pete Lawrence.

On 24 October 2019 the Galilean moons Io and Europa cast their shadows onto Jupiter’s atmosphere at the same time. The shadows are on the disc together between 15:43 BST (14:43 UT) and 17:22 BST (16:22 UT), during daylight conditions.

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Although Venus can be spotted against a bright blue sky with unaided eyes, Jupiter and Saturn require help. Mars could also be a potential daylight object, but for this to happen it needs to be at its brightest.

This occurs around opposition, which places the planet firmly in a dark nighttime sky.

The biggest hurdle with daylight viewing is finding your target. This can be achieved with a Go-To system as long as it can be operated during the day.

Obtaining a reference position using the Sun works well, but you need to make sure no unprotected optical apertures are pointing at the Sun when you do so.

This includes the main scope and the finder. If you’re not sure what you are doing, don’t attempt it.

Look at Jupiter on 24 October, from 15:43 BST, to track the shadows of its moons Io and Europa (shown here as a south-up telescopic view). Credit: Pete Lawrence
Look at Jupiter on 24 October, from 15:43 BST, to track the shadows of its moons Io and Europa (shown here as a south-up telescopic view). Credit: Pete Lawrence

Offsetting from the Sun using setting circles is another way to locate a planet.

If you are struggling, it’s worth noting that Jupiter will be due south at 16:26 BST (15:26 UT). From latitude 53° north, this places it at an altitude of 14°.

Jupiter by day is not like Jupiter by night. Although bright at mag. –1.8, it appears with extremely low contrast against a bright blue daylight sky.

The moon shadows should fare quite well but aren’t the largest cast by the Galilean four – Ganymede’s and Callisto’s are much larger.

The best way to observe the transit is to use a scope with an infrared-sensitive camera fitted.

Using an infrared pass filter will then provide higher contrast, causing the blue of the surrounding sky to appear very dark.

Never observe or image the Sun with the naked eye or any unfiltered optical equipment.

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This article originally appeared in the October 2019 issue of BBC Sky at Night Magazine. Pete Lawrence is an experienced astronomer and a co-host on The Sky at Night.