Astronomy Essentials

Jupiter’s moons: How to see and enjoy them

Closeup of part of Jupiter, with a large, black, oval shadow on its colorful bands.
The shadow of Io, one of Jupiter’s moons, is cast on the giant planet’s cloud tops. This image was captured by the JunoCam camera aboard NASA’s Juno spacecraft, currently orbiting Jupiter. The image was acquired on September 19, 2019. Image via NASA/ JPL-Caltech/ SwRI/ MSSS/ Kevin M. Gill (public domain).

How to see Jupiter’s moons

All you need is a good pair of binoculars (or a telescope) to see the four largest moons of the biggest planet in our solar system, Jupiter.

Three of the four moons are bigger than Earth’s moon. And one – Ganymede – is the largest moon in the solar system. These four satellites are collectively called the Galilean moons to honor the Italian astronomer Galileo, who discovered them in 1610. November 2024 is a great month to look for Jupiter’s four large moons. That’s because the king of planets is nearing opposition – when Earth will sweep between it and the sun – in early December. So the distance between Earth and Jupiter is now less than usual. And Jupiter is bright!

From Earth, through a small telescope or strong binoculars, the moons of Jupiter look like tiny starlike pinpricks of light. But you’ll know they’re not stars because you’ll see them stretched out in a line that bisects the giant planet.

Depending on what sort of optical aid you use, you might glimpse just one moon or see all four. If you see fewer than four moons, that might be because a moon is behind – or in front of – Jupiter. If a moon is in front of the planet, you probably can’t see it. The moon is too tiny and gets lost from our view. But observers do sometimes see a moon shadow crossing Jupiter’s cloud-tops. That event is called a transit.

Going from the moon closest to Jupiter to the outermost, their order going outward from Jupiter is Io, Europa, Ganymede and Callisto.

What you’ll see

Writing at SkyandTelescope.com, Bob King has said:

Etched in my brain cells is an image of a sharp, gleaming disk striped with two dark belts and accompanied by four starlike moons through my 2.4-inch [6 cm] refractor in the winter of 1966. A 6-inch [15 cm] reflector will make you privy to nearly all of the planet’s secrets …

When magnified at 150x or higher [Jupiter’s 4 largest moons] lose their starlike appearance and show disks that range in size from 1.0″ to 1.7″ (arcseconds). Europa is the smallest and Ganymede the largest.

Ganymede also casts the largest shadow on the planet’s cloud tops when it transits in front of Jupiter. Shadow transits are visible at least once a week with ‘double transits’ – two moons casting shadows simultaneously – occurring once or twice a month. Ganymede’s shadow looks like a bullet hole, while little Europa’s more resembles a pinprick. Moons also fade away and then reappear over several minutes when they enter and exit Jupiter’s shadow during eclipse. Or a moon may be occulted by the Jovian disk and hover at the planet’s edge like a pearl before fading from sight.

Images of Jupiter’s moons from the EarthSky community

Five white dots in a line on black background, with one of them very much bigger and brighter.
View at EarthSky Community Photos. | Nanci McCraine at Finger Lakes, New York, took this photo on September 30, 2023, and wrote: “I noticed craggy edges around Jupiter. Zooming in, I spotted this line of 4 straight lights across the planet that I assume are satellites.” That is correct! Binoculars or a small telescope will show Jupiter’s moons.
Jupiter's moons: Large banded planet with two labeled dots of light, one on each side.
View at EarthSky Community Photos. | Cathy Adams in St. Stephen, New Brunswick, Canada, captured 2 of Jupiter’s moons and giant Jupiter itself on September 3, 2022. Cathy wrote: “After so many cloudy nights I was fortunate to get a beautiful clear one! And it was absolutely wonderful to enjoy a night out observing, and imaging our neighboring planets!!” Thank you, Cathy!
Jupiter with detailed bands and red spot, with 2 little dots of light (its moons) nearby.
View at EarthSky Community Photos. | Around the time of its yearly opposition, Jupiter is brightest in our sky, best through a telescope, and visible all night. Michael Terhune in Lunenburg, Massachusetts, captured Jupiter in August 2021. He wrote: “My sharpest image of Jupiter! Showing 2 of its Galilean satellites, Io and Europa. The Great Red Spot is also visible.” Thank you, Michael.

More images

3 images with large dot for Jupiter, and 4 small labeled dots in line, for its moons in different positions.
View at EarthSky Community Photos. | Meiying Lee in Taipei, Taiwan, took these images of Jupiter’s 4 largest moons over the course of a single night. She wrote: “I always thought that to see obvious changes in the 4 major satellites of Jupiter would take several nights of continuous observation. Later, I discovered that the Galilean satellites move very fast around Jupiter.” See the volcanic moon Io move behind Jupiter and emerge on the other side just a few hours later? Amazing! Thanks, Meiying.
14 views of Jupiter with little dots for its moons in different positions, with time of night noted.
Meiying Lee in Taipei, Taiwan, shared this chart with us on October 6, 2023, and wrote: “From the evening of August 15 to the early morning of August 16, 2021, the Galilean satellites experienced very exciting changes. Callisto, Ganymede, and Europa passed through the surface of Jupiter one after another, while Io was occulted by Jupiter. This resulted in the rare phenomenon that there were no Galilean satellites around Jupiter for 20 minutes late at night on August 16th. Finally, before dawn, the 4 satellites appeared around Jupiter one after another. I watched the Galilean satellites show all night, it was really exciting!” Thank you, Meiying.
Left: A full Jupiter with a black spot. Right: close-up of the moon and its shadow over swirly bands.
View at EarthSky Community Photos. | Sona Shahani Shukla in New Delhi, India, caught a transit of the innermost Galilean moon, Io, across the face of Jupiter on July 7, 2021, and wrote: “Io appears to be skimming Jupiter’s cloud tops, but it’s actually 310,000 miles (500,000 km) from Jupiter. Io zips around Jupiter in 1.8 days, whereas our moon circles Earth every 28 days. The conspicuous black spot on Jupiter is Io’s shadow and is about the size of the moon itself (2,262 miles or 3,640 km across). This shadow sails across the face of Jupiter at 38,000 mph (17 km per second).” Thank you, Sona!

Special viewings of Jupiter’s moons

As with most moons and planets, the Galilean moons orbit Jupiter around its equator. We do see their orbits almost exactly edge-on, but, as with so much in astronomy, there’s a cycle for viewing the edge-on-ness of Jupiter’s moons. This particular cycle is six years long. So every six years we view Jupiter’s equator – and the moons orbiting above its equator – at the most edge-on. During these special times, we can see the moons eclipse and cast shadows on not just giant Jupiter but on each other.

In 2021 we were able to view a number of mutual events (eclipses and shadow transits) involving Jupiter’s moons. The next cycle of mutual events will be in 2027.

Another special event, a rare triple transit, occurs on October 18, 2025, when three of Jupiter’s moons will pass in front of the giant planet at once. The last time Earth could witness a triple transit was in 2021. Triple transits are not visible from all parts of the globe, however.

You can find information here for dates and times to observe the Galilean moons

Part of Jupiter with Great Red Spot and photos of 4 largest moons, enlarged and colorful, on black background.
Composite image of Jupiter and its 4 Galilean moons. From left to right the moons are Io, Europa, Ganymede and Callisto. The Galileo spacecraft obtained the images to make this composite in 1996. Image via NASA Photojournal.

Jupiter at opposition in December 2024

On December 7, 2024, Jupiter is at opposition, when the planet is opposite the sun in the sky as seen from Earth. When Earth passes directly between Jupiter and the sun, we’ll see Jupiter rise at sunset and set at sunrise. Opposition is the middle of the best time of the year to see a planet, since that’s when the planet is up and viewable all night and is generally closest for the year. But any time Jupiter is visible in your sky, you can view Jupiter’s four major moons.

So if you get a chance, grab some binoculars or a small telescope and go see Jupiter’s Galilean moons with your own eyes!

Click here for recommended sky almanacs; they can tell you Jupiter’s rising time in your sky.

Diagram: sun and Jupiter with Earth exactly lined up between them.
Opposition – when Earth is directly between Jupiter and the sun – is the best time to observe the largest planet and its 4 Galilean moons. In 2024, Jupiter’s opposition is December 7. Image via EarthSky.

Bottom line: November and December 2024 are great months for seeing Jupiter’s moons Io, Europa, Ganymede and Callisto with binoculars or a small telescope.

Check here for dates and times to observe the Great Red Spot

Posted 
November 5, 2024
 in 
Astronomy Essentials

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