Even though Jupiter is way out in the outer solar system, you can still see its four largest moons. They’re often called the Galilean moons to honor the Italian astronomer Galileo, who discovered them in 1610. You can glimpse them with binoculars – and see them with a telescope – whenever Jupiter is visible
From Earth, through a small telescope or strong binoculars, they 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.
Going from closest moon to Jupiter to the outermost, their order is Io, Europa, Ganymede and Callisto.
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 150× or higher [the four Galilean 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.
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. That is, every six years, we view Jupiter’s equator – and the moons orbiting above its equator – at the most edge-on.
The next eclipse series of Callisto, whereby this moon actually passes behind Jupiter, started on November 9, 2019, and ends on August 22, 2022, to present a total of 61 eclipses. After that, the next eclipse series will occur from May 29, 2025, to June 7, 2028, to feature 67 eclipses.
On August 19-20, 2021, Jupiter is at opposition, when Earth passes directly between Jupiter and the sun. 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 anytime Jupiter is visible in your sky featues a good time for viewing 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.
Bottom line: How to see Jupiter’s four largest moons.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.