See Jupiter and its Galilean moons throughout November
Observe Jupiter's Galilean moons this month as the nights get longer and darker.
The planet Jupiter reached opposition on 26 September 2022, a term that describes when a planet is on the opposite side of the sky to the Sun.
Jupiter remains well-placed and is a great target to observe throughout November.
Even a modest refractor telescope will enable you to see Jupiter's bands and its 4 Galilean moons in orbit around the planet this month.
Larger telescopes will show a wealth of atmospheric detail on Jupiter.
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
Observing the Galilean moons in November 2022: dates and times
On 2 November, Europa and Ganymede can both be seen starting to transit the Jovian disc from 20:44 UT.
Io is very close to the planet’s western limb at this time, being occulted by Jupiter a few minutes later at 20:50 UT as the other two moons progress with their transit.
Europa and Ganymede then appear to race across Jupiter’s disc, Europa eventually catching Ganymede and taking the lead, courtesy of its closer, faster orbit.
At 22:30 UT, Europa’s shadow begins its transit, Europa, its shadow and Ganymede all appearing on Jupiter’s disc until the end of Europa’s transit at 23:15 UT.
Ganymede’s shadow also makes an appearance at 00:21 UT on 3 November, forming a double shadow transit with Europa’s shadow until 01:00 UT when the smaller shadow leaves the planet’s disc.
On 6 November, an early view of Jupiter through a telescope will show the giant moon Ganymede emerge from Jupiter’s shadow 1.1 arcminutes east of the planet at 16:49 UT.
For larger instruments, it may be possible to resolve the curving shadow across Ganymede’s tiny disc.
Europa and Ganymede are in transit again on the morning of 10 November. Between 01:06 UT and 01:38 UT, Europa, its shadow and Ganymede can be seen transiting at the same time.
Between 23:05 UT on 18 November and 00:15 UT on 19 November the outer moon Callisto will appear to almost touch Jupiter’s northern limb.
It will appear closest to Jupiter at 23:40 UT.
On the evening of 27 November from 21:00 UT, Ganymede will be occulted by Jupiter.
Starting at 21:08 UT, it will take 10 minutes for Ganymede to fully disappear.
Again, large telescopes set up for planetary imaging may get a shot of Ganymede’s resolved disc.
Short, high-frame-rate sequences may even be able to capture the event as a timelapse.
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.
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.
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.
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
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.
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.
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.
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.