April Sky Talk | Seeing Jupiter’s Moons

With the giant planet currently perfectly placed high in the southern sky for viewing these April evenings, an opportunity presents itself to answer a fascinating and often-asked question: Can Jupiter’s four bright satellites be seen without a telescope? Most skywatchers will respond by saying “That’s impossible.” But it actually is possible!

The discovery of the satellites of Jupiter is traditionally attributed to Galileo using his primitive little telescope. But there is some evidence that others may have seen them before him following the invention of the telescope. His real claim to fame is that he was the first to publish his discovery (along with those of the craters on the Moon, the phases of Venus, sunspots, etc.). In any case, even the smallest, lowest-power, and poorest- quality telescope will indeed reveal the four Galilean moons (those discovered by him).

But what about seeing them in binoculars? There’s no question that these glasses will show them—especially the outer ones, Callisto and Ganymede, when near the extreme elongations of their orbits about Jupiter. The two inner moons, Europa and Io, are more difficult being closer to the planet’s glare. In any case, on some nights one or more of the moons may be hidden in Jupiter’s shadow, or be behind the planet, or even transiting across the front of it. So if the satellites aren’t seen when you first look, try again later that same night or at another time.

It’s fascinating to see just how small a glass and low a magnification will show the moons. Steadily-held (or better yet, mounted) 7×35 and 7×50 binoculars will definitely allow glimpsing one or more of them, while a 10×50 glass makes them definite. Image-stabilized binoculars of all sizes and magnifications provide the best views, with 12x to 15x being ideal. It’s also fun to observe the planet with zoom binoculars, beginning with the highest power and then slowly working your way down in magnification to see at what point they are no longer visible.

We’ve saved the ultimate challenge (and experience!) here for last—glimpsing the moons of Jupiter with the unaided eye! Yes, this is actually possible. In fact, there are many reports going back long before the invention of the telescope by keen-eyed skywatchers seeing “lights” next to Jupiter without knowing what they were. On several occasions as a teenager when my eyesight was nearly perfect I did, in fact, see one or more of the moons on nights of excellent seeing conditions. But this was knowing that they were supposed to be there—proving the great astronomer Sir William Herschel’s dictum that when a superior (higher) power reveals something and inferior (lower) one will show it thereafter. By the time I reached middle age, however, I lost this ability even wearing corrective glasses. In any case, with Jupiter now in prime viewing position, give it a try yourself. You might just be surprised at what you see!


— James Mullaney

Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing.   His latest, Celebrating the Universe!, is available from HayHouse.com.