If you have binoculars or a telescope, it’s fairly easy whenever Jupiter is visible to see the giant planet’s four largest moons. They look like pinpricks of light – like tiny “stars” – all on or near the same plane crossing the planet. They’re often called the Galilean moons to honor Galileo, who discovered them in 1610.
In their order from Jupiter, these moons are Io, Europa, Ganymede and Callisto.
Writing at SkyandTelescope.com this year, 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 star-like 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 star-like 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.
You’ll find a complete list of all eclipses, transits, and occultations for 2019 by downloading Sky & Telescope’s Phenomena of Jupiter’s Moons pdf. You can also gets daily predictions for the moons and a diagram showing their relative positions by consulting the online Jupiter’s Moons Observing Tool.
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 – most edge-on.
Starting in late 2016, Jupiter’s axis began tilting enough toward the sun and Earth so that the outermost of the four moons, Callisto, had not been passing in front of Jupiter or behind Jupiter, as seen from our vantage point. This will continue for a period of about three years, during which time Callisto is perpetually visible to those with telescopes, alternately swinging above and below Jupiter as seen from Earth.
The next eclipse series of Callisto, whereby this moon actually passes behind Jupiter, starts 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.
Click here for recommended sky almanacs; they can tell you Jupiter’s rising time in your sky.
Bottom line: How to see Jupiter’s 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.