See Jupiter and its Galilean moons throughout October

Observe Jupiter’s Galilean moons this month as the planet approaches opposition.

By Pete Lawrence

Published: Tuesday, 03 October 2023 at 09:16 AM

Observing Jupiter’s Galilean Moons is a great activity to undertake when the planet is approaching opposition.

Jupiter reaches opposition on 3 November 2023, a time when it will be opposite the Sun in the sky.

Opposition is the time when a planet appears largest and brightest to us viewing from Earth.

Although you’ll only be able to appreciate its size through the eyepiece of a telescope, Jupiter has an apparent diameter of 49.5 arcseconds on 3 November.

The Galilean Moons at opposition

Jupiter’s four bright Galilean moons appear in a line, shining like stars near to the planet. Credit: Pete Lawrence

At opposition an interesting thing happens to the Galilean Moons when they transit the planet.

At this time their shadows appear to fall directly in line with the moons.

The alignment would be perfect if the declination of the Sun from Jupiter were 0° and the Sun–Earth–Jupiter alignment a perfect straight line. 

In practice the declination won’t be 0°, but a little off at 3.1°.

Catching a moon passing at the exact point of opposition is down to luck and an offset of just a few hours either side is enough to show a misalignment.

However, it is interesting to see how a moon’s shadow precedes the moon that’s casting it before opposition and follows it after opposition.

Jupiter's moon Callisto casting its shadow on the gas giant. Credit: Pete Lawrence
Jupiter’s moon Callisto casting its shadow on the gas giant. Credit: Pete Lawrence

In our graphics below we’ve shown some interesting events to observe in the run-up to 3 November.

We’ve included several so that if the weather is poor, you will hopefully stand a chance of seeing some of them.

Owners of smaller scopes will even be able to enjoy some amazing interactions between the planet and its 4 largest moons: Io, Europa, Ganymede and Callisto.

These moons are bright and easily visible through smaller instruments as points of light.

For those with larger-aperture scopes, it may be possible to perceive the moons as tiny discs, the apparent diameters being:

Observing Galilean Moon transits and shadows, October 2023 dates and times

5 6 october 2023 ganymede transit

5/6 October Ganymede transit

Shadow: 21:44–23:50 UT
Moon: 00:56–02:22 UT

6 October Io transit

Shadow: 01:51–04:00 UT
Moon: 02:33–04:41 UT

13 october ganymede io transit

13 October Ganymede transit

Shadow: 01:47–03:50 UT
Moon: 04:17–05:41 UT (daylight)

13 October Io transit

Shadow: 03:45–05:54* UT
Moon: 04:17–06:25* UT

22 october io transit

22 October Io transit

Shadow: 00:08–02:17 UT
Moon: 00:27–02:35 UT

27 october europa transit

27/28 October Europa transit

Shadow: 22:37–01:00 UT
Moon: 23:00–01:16 UT

30 october io transit

30 October Io transit

Shadow: 20:31–22:41 UT
Moon: 20:36–22:45 UT

30/31 October Ganymede eclipse occultation

Disappearance: 23:41 UT
Reappearance: 01:50 UT

4 november europa transit

4 November Europa transit

Shadow: 01:13–03:36 UT
Moon: 01:13–03:30 UT

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

Jupiter’s moon Io is the most active volcanic body in the Solar System. Credit: NASA/JPL/USGS

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

Two views of moon Europa captured by NASA’s Galileo spacecraft in June 1997. Credit: NASA, NASA-JPL, University of Arizona

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

Images of Jupiter’s moon Ganymede captured on 26 December 2019 showing infrared mapping of its North Pole. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

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

A view of Jupiter’s moon Callisto captured on May 2001. Could this heavily cratered moon host a slaty ocean? Credit: NASA/JPL/DLR

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.