Astro Bob: How to use binoculars to see Jupiter's bright moons
A calendar guide to help you pinpoint the four Galilean moons the next time the sky is clear.
A moon is a natural satellite that revolves around a planet or an asteroid and shines by reflected sunlight. There are more than 200 known planetary moons in the solar system, but most of us have seen just one — Earth's moon. With an 8-to-10-inch telescope and dark skies you can add about 14 more. Not many people own an instrument that size, but many of us have a pair of binoculars lying around. Fortunately, that's all you'll need to see at least two and possibly four additional moons.
The next brightest after our own moon are the four Galilean satellites that orbit Jupiter. They range from magnitude 4.6 for Ganymede to 5.6 for Callisto. If they stood apart from the giant planet, all would be visible with the naked eye from a dark sky, but they orbit close enough to suffer from Jupiter's bright glare.
Galileo discovered the foursome in early 1610 using with a 1.5-inch (37mm) homemade telescope that magnified 20 times. They're called the Galilean moons in honor of his achievement. Jupiter has 80 known satellites, but these are the only ones bright enough to see in even larger amateur telescopes.
Each of them has its own period of revolution just like the planets do around the sun: Io is closest and revolves once in just 1.8 days; Europa in 3.6 days; Ganymede in 7.2 days and Callisto in a leisurely 16.7. That's why you'll never see them in quite the same positions night to night. They're always mixing it up. Observers have likened their movements to a ballet. Once you've had a look yourself I think you'll concur.
I got my first good look at Jupiter and its satellites when I was 11 years old through an inexpensive Japanese refracting telescope my parents bought me. The planet was bright and crisp, with the moons neatly aligned in a row on either side — a perfect family. How could seeing something this remarkable be so easy? That first impression remains fresh in my memory to this day.
Even if you don't own a telescope you can spot the moons in a pair of 7x to 10x binoculars. To aid your quest, use the included nightly guide showing their locations. I've deliberately shrunk the simulations down to resemble what you might see at very low magnification. At times, two moons will appear too close together to tell apart, so I've labeled them "E+G" or Europa plus Ganymede, for example. The easiest ones to see are Ganymede and Callisto because they orbit farther from Jupiter and better escape its glare.
Io and Europa are trickier because they orbit closer in and are frequently swamped by the planet's bright glow. Persistence will pay off in the long run, especially for observers with binoculars magnifying 10x or more.
Two things to remember to increase your chance of success: hold the binoculars steady and focus sharply. Hand-holding is fine for general sweeping and pointing, but to pick out the moons you'll want to buttress the instrument against the roof or hood of a car or attach it to a tripod for best results. Tripod adapters are inexpensive — Amazon has a large selection .
Good focus is essential. Before aiming at Jupiter, point the binoculars at a bright star and focus it down to the sharpest point possible. Then you'll be ready to see what brilliant Jupiter has been hiding from your eyes all these years. Oh, how to find the planet? Just face southeast as soon as it gets dark. The big, bright thing you see is Jupiter. Can't miss it!
The guide included here shows the moons' positions for Oct. 25 - Nov. 2. To know exactly where they are any night of the year from any location, go to Jupiter's Moons on skyandtelescope.org , or download the free program from stellarium.org , set the time and zoom in on Jupiter with your mouse scroll wheel.