Learn how to see Jupiter’s moons
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.
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.
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.
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.
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.