See Jupiter and its Galilean moons this week

The planet Jupiter reaches opposition on 26 September 2022, a term that describes when a planet is on the opposite side of the sky to the Sun.

A good challenge this month 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.

Observing Galilean moons around Jupiter's opposition

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).

On 24 September, moon 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.

Observing Jupiter's Galilean moons: tips and tricks

Not only is Jupiter the largest of the planets – it would take 1,321 Earths to fill the volume of Jupiter – it’s also more than likely that it keeps the largest entourage of moons.

It’s the massive gravitational effect of Jupiter that does the trick, attracting more than 100 moons into orbit around the planet at the latest estimate.

Many of these satellites are fairly small and can’t be observed from Earth, but the biggest four are easy to spot with just a small pair of binoculars.

Binoculars

A minimum size pair of binoculars for spotting the four Galilean moons of Jupiter would be 7x50s, which magnify what your eyes see seven times and have front lenses that are 50mm in diameter.

You can certainly catch glimpses of these Galilean moons (named after Galileo, who first observed them) with hand-held binoculars.

However, your view will be much improved by resting the binoculars on a wall or fence, or even attaching them to a binocular tripod or mount with an inexpensive bracket.

With binoculars though, Jupiter itself will not appear as anything more than a large, slightly oval-shaped disc.

Telescope

The next step in viewing Jupiter Moons is to use a small telescope – one with a front lens 3 to 6 inches in diameter. As this gathers more light, it can magnify the view more, so the Moons will appear brighter and fill more of the field of view.

Don’t necessarily expect to see all four, however: as the moons travel around the planet they may be behind or in front of Jupiter when you’re looking.

It’s by using a larger scope with a front lens over 6 inches in diameter that you really start to see detail on Jupiter itself: not only the darker belts and lighter zones, but features within the gaseous atmosphere as well.

At this level of detail, observers can also see the occasional dark spot caused by the moons casting their shadows onto Jupiter’s atmosphere.

The joy of Jupiter is that whatever your level of equipment, there’s always something to see.

Facts about the Galilean Moons

Io

Diameter: 3,650km

The tremendous gravitational pull of Jupiter on this innermost of the four Galilean moons, together with its closeness to the planet, means Io whizzes round Jupiter in just 1.75 Earth days.

This fast orbital speed is easily seen in a small telescope: it visibly shifts position in just a few hours.

Physically, Io is the most volcanic place in the entire Solar System.

The whole world is covered in sulphurous lava flows and volcanoes erupting in plumes more than 500km high.

Europa

Diameter: 3,140km

The second Galilean moon out from Jupiter, Europa, should theoretically be visible with the naked eye since it shines at magnitude +5.3.

But Jupiter’s overwhelming brightness makes it difficult to separate the moon from the planet.

Europa’s brightness is due to its surface being smooth and icy.

Scientists suspect that underneath is a liquid water ocean, leaving open the possibility that life may lurk in the depths.

Ganymede

Diameter: 5,260km

The third major moon out from the planet is not only Jupiter’s biggest, but it is also the largest moon in the entire Solar System.

This is a world with a cold ice surface, a large warm ice (possibly water) mantle, a rocky interior and a liquid iron core. It measures a tremendous 5,260km across, which is bigger than Mercury.

Indeed, if Ganymede was released into space, it would be classed as a planet.

Callisto

Diameter: 4,820km

The last of the four giant Galilean satellites is Callisto.

It is the third largest of the Solar System, after Titan, the biggest of Saturn’s moons.

Callisto ranks as one of the most cratered worlds known – its entire icy, ancient surface is covered with impact craters that date right back to the time of the early Solar System, when the moon formed.

Like Europa, it is thought that beneath the surface may lie a watery ocean.

This guide originally appeared in the September 2022 and September 2010 issues of BBC Sky at Night Magazine.