Observe Jupiter’s Galilean moons this month.
Observing Jupiter’s Galilean Moons is a great activity to undertake when the planet is approaching opposition.
Jupiter reached opposition on 3 November 2023, a time when it is opposite the Sun in the sky.
Opposition is when a planet appears largest and brightest to us viewing from Earth.
For more advice, read our guide on how to observe Jupiter through a telescope
Galilean Moon transits and shadows
December 2023 dates and times
Jupiter’s giant moon Ganymede will be involved in a number of interesting events in December 2023.
Being so large and casting such a huge shadow on Jupiter’s atmosphere, these should be relatively easy to see using a small scope.
We start with Ganymede transiting the planet from 03:08 to 04:56 UT on 2 December.
The start of this transit occurs with Jupiter at 17° altitude, but the end occurs as the planet is setting.
A better-timed event is on 12 December when Ganymede is occulted by Jupiter between 20:03 and 21:41 UT.
After reappearance, Ganymede has a brief respite before succumbing to being eclipsed by Jupiter’s shadow between 23:50 UT on 12 December and 01:49 UT on 13 December.
On 16 December, Ganymede’s shadow transits the planet during daylight from 14:03 until 15:58 UT.
Ganymede is once again occulted by Jupiter between 23:34 UT on the 19th and 01:37 UT on 20 December.
On 23 December, Ganymede’s shadow can be seen in transit from 18:04 to 20:00 UT.
Finally, on 30 December, Ganymede transits from 17:13 to 19:17 UT, its shadow appearing for a double shadow transit with Europa’s shadow between 22:08 and 00:02 UT.
November 2023 dates and times
On 4 November between 01:16 and 03:35 UT, Europa is chased by its shadow.
Just after opposition, moon and shadow appear to almost touch.
On 5 November between 03:55 and 06:07 UT and on 6/7 November between 22:21 and 00:36 UT it’s the turn of Io to be chased by an almost touching shadow.
On the morning of 7 November between 03:30 and 05:23 UT, Ganymede is occulted by Jupiter, followed by its reappearance from the planet’s shadow some distance away (an eclipse reappearance).
On 10 November, Ganymede and its shadow appear to pass in front of Jupiter between 17:30 and 19:45 UT.
Ganymede experiences another occultation disappearance on 14 November at 06:43 UT, but doesn’t reappear before Jupiter sets.
Later, Callisto can be seen south of Jupiter as darkness falls.
Also on that evening, Europa and its shadow transit at 16:35–19:28 UT, Io experiencing an occultation disappearance at 21:20 UT, reappearing from Jupiter’s shadow at 23:47 UT.
4 November Europa transit
Shadow: 01:13–03:36 UT
Moon: 01:13–03:30 UT
7 November
Ganymede occultation disappearance: 03:30 UT
Eclipse reappearance: 05:23 UT
10 November
Ganymede and shadow in transit: 17:30–19:45 UT
17 November
Ganymede and shadow in transit: 20:43–23:45 UT
24 November
Ganymede and shadow in transit: 23:58–03:50 UT
28 November
Ganymede occultation disappearance: 14:58 UT
Eclipse disappearance: 15:50 UT
Eclipse reappearance: 17:23 UT
Find out more about the Galilean moon events in our video below:
October 2023 dates and times
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 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
Shadow: 00:08–02:17 UT
Moon: 00:27–02:35 UT
27/28 October Europa transit
Shadow: 22:37–01:00 UT
Moon: 23:00–01:16 UT
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
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.
The Galilean Moons at opposition
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.
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:
- Io: 1.2 arcseconds
- Europa: 1.0 arcseconds
- Ganymede: 1.7 arcseconds
- Callisto: 1.5 arcseconds
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.