Astronomy Essentials

Learn how to see Jupiter’s moons

How to see Jupiter's moons: Bright yellowish planet Jupiter in center, with 4 bright dots in a diagonal line, all labeled.
View at EarthSky Community Photos. | Kannan A in Singapore captured this photo of Jupiter and its 4 Galilean moons on April 22, 2021. He wrote: “The position of Jupiter’s Galilean moons this morning in the skies here in Singapore. The Galilean moons (or Galilean satellites) are the 4 largest moons of Jupiter and were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610. It is amazing to see it with my point-and-shoot camera without the aid of any additional equipment. How technology has evolved over the years to make this possible!” It is indeed quite remarkable. Thank you, Kannan!

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 Jupiter. Three of the four moons are larger than Earth’s moon. And one – Ganymede – is the largest moon in the solar system! These four satellites are often called the Galilean moons to honor the Italian astronomer Galileo, who discovered them in 1610. Now is a great time to look for Jupiter’s four largest moons because the king of planets is near opposition and thus closest to Earth in its orbit.

From Earth, through a small telescope or strong binoculars, the moons 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 Jupiter. Depending on what sort of optical aid you use, you might glimpse just one moon or see all four. If you’re using a telescope, and 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 can sometimes see the moon’s shadow on Jupiter’s cloudtops in what is called a transit.

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

Banded planet with four small labeled moons near it, on black background.
Fernando Roquel Torres in Caguas, Puerto Rico, captured Jupiter, its Great Red Spot and all 4 of its largest moons – the Galilean satellites – at Jupiter’s 2017 opposition.

What you’ll see

Writing at SkyandTelescope.com in June 2019, Bob King 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 refractor in the winter of 1966. A 6-inch 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″ (current opposition). Europa’s the smallest and Ganymede 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.

Bright full moon, bright dot of Jupiter nearby, 4 pinpricks of light aligned with Jupiter's equator.
View at EarthSky Community Photos. | Beautiful shot of Earth’s moon – plus Jupiter and its 4 largest moons – on May 20, 2019, via Asthadi Setyawan in Malang, East Java, Indonesia. Thank you, Asthadi!

Special viewing opportunities

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 2015 we were able to view a number of mutual events (eclipses and shadow transits) involving Jupiter’s moons.

Read more about how you can see Jupiter’s moons eclipse each other in 2021.

Another special event, a rare triple transit, occurs on August 15, 2021, when three of Jupiter’s moons pass in front of the giant planet at once. The last time Earth could witness a triple transit was in 2015, and the next time will be 2025. Triple transits are not visible from all parts of the globe, however. Only lucky individuals in parts of Asia and Australia will get to witness the 2021 event.

Composite view: part of Jupiter with Great Red Spot with its 4 largest moons, variously colored, 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.
Diagram of sun, Earth, and Jupiter lined up with orbits and line of sight shown.
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 2021, Jupiter’s opposition is August 19-20. Image via EarthSky.

Jupiter reaches opposition in August 2021

On August 19-20, 2021, 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.

A composite image, with a full Jupiter on the left and a close-up of the moon and its shadow on Jupiter's clouds on the right.
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 cloudtops, 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!

Bottom line: You can see Jupiter’s four largest moons – Io, Europa, Ganymede and Callisto, known as the Galilean satellites – with your own eyes with the help of binoculars or a small telescope.

Posted 
July 12, 2020
 in 
Astronomy Essentials

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