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.
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. October 2023 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 November. 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.
In October 2023, Jupiter rises in the east shortly after evening twilight subsides and is visible until dawn. It shines near the pretty Pleiades star cluster in the constellation Taurus the Bull. It’s racing toward opposition on the evening of November 1-2, 2023, when we fly between it and the sun. By then, Jupiter will be visible all night. Jupiter lies in the dim constellation of Aries the Ram. It brightens from -2.7 to -2.8 magnitude in October. The moon will float by Jupiter on October 29, 2023. Chart via EarthSky.In the late evening of October 27, 2023, the bright waxing gibbous – an almost full moon – will glow near bright Jupiter. The following evening, on October 28, the full Hunter’s Moon will rise next to Jupiter shortly after sunset. The pair will travel across the sky together all night. Chart via EarthSky.View at EarthSky Community Photos. | Animation of Jupiter and 2 of its Galilean satellites by Steven Bellavia of Surry, Virginia. The images were captured on October 19, 2023, over the course of 100 minutes. Europa is on the left and Io moves in to view on the right. Thank you, Steven!
Jupiter in November
Jupiter will rise in the east at sunset and will be visible until dawn. It will shine near the pretty Pleiades star cluster in the constellation of Taurus the Bull. Overnight on November 1-2, 2023, it will reach perihelion – or closest point to the sun – when the distance between the sun and Jupiter will be 462 million miles (744 million km). And the distance between the Earth and Jupiter will be 370 million miles (595 million km). Then, it will reach opposition overnight on November 2-3, 2023, when we fly between it and the sun. Jupiter lies in the dim constellation of Aries the Ram. It will shine at -2.9 magnitude in November. The waxing gibbous moon will float by Jupiter on November 25, 2023. Chart via EarthSky.In the late evenings of November 23 and 24, 2023, the waxing gibbous moon will glow near the bright planet Jupiter. The moon and Jupiter will set several hours after midnight. Chart via EarthSky.The bright waxing gibbous moon will pass between Jupiter and the Pleiades star cluster on the evenings of November 25 and 26, 2023. The Pleiades is also known as the Seven Sisters or Messier 45 and appears as a glittering, bluish cluster of stars in the constellation of Taurus the Bull. The pair will be especially close on the evening of November 25, when the moon will pass about 1 degree – or 2 full moon widths – from the Pleiades. It’ll be a great photo opportunity for astrophotographers. The moon and Pleiades will cross the sky together until around sunrise. Chart via EarthSky.
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″ (current opposition). 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
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 show Jupiter’s moons.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!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.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.
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 November 2023
On November 2-3, 2023, 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!
Bottom line: October and November 2023 are great months for seeing four of Jupiter’s moons – Io, Europa, Ganymede and Callisto – with binoculars or a small telescope.
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