Jupiter has 79 moons but only four of them are bright enough to see in a small telescope. Even a 2.4-inch (60 mm) scope will show them as bright "stars" that change position nightly around the planet. Some nights, all four moons are strung out on either side of Jupiter. On others, one or two are temporarily missing, either eclipsed by the planet's shadow or passing in front or behind the globe.
Jupiter is often compared to a miniature solar system because it's a massive body orbited by so many smaller ones. Like the planets around the sun, each moon travels at a different speed depending upon its distance from the gas giant. Io is closest and circles fastest, in just 1.8 days, while the most distant of the four, Callisto, takes 16.7 days.
On Sunday night, Sept. 12, you'll have the opportunity to see these dynamics at play. First, you'll need to find the planet. It looks like a really bright star low in the southeastern sky as soon as dusk gets underway. You can't miss it. For observers in the eastern half of the U.S., only Ganymede and Callisto (see diagram above) are visible during the early evening. Io is in eclipse, hidden in Jupiter's shadow, while Europa parades in front of the bright disk and blends in with Jupiter's clouds, making it difficult to see..
Then at 8:36 p.m. Central Daylight Time (9:36 p.m. EDT; 7:36 p.m. MDT), Io emerges from eclipse close to the eastern edge of the planet. To fully appreciate the sight, start looking a few minutes before that so you can watch Io gradually brighten as it exits Jupiter's shadow.
Only minutes after Io fully emerges into the light, Europa's shadow begins to cross Jupiter starting at 8:38 p.m. It will look like a black pinpoint and slowly travel from east to west across the disk. As Europa is the smallest of the four major moons it casts the smallest shadow, so be sure to use a magnification of 100x or more. The event, called a shadow transit, ends at 10:18 p.m. when the shadow departs Jupiter's disk.
Meanwhile, Io continues to move eastward in its orbit after eclipse and approaches Ganymede, the brightest and largest of the quartet. At 9:16-17 p.m. it passes just 4 arc-seconds above Ganymede, so close that at low magnification the two will appear to merge into a single object. You'll need at least 150x to split them apart in a small scope. Io moves pretty quickly and soon separates from Ganymede. You should be able to detect its movement in 10 minutes or less. Pretty amazing!
At 10:18 p.m., Europa, which has been transiting in front of Jupiter, reappears at its western edge. At this time, all four or Jupiter's bright moons will now in view, up two from when we started in twilight. But wait, there's more!
If you're a night owl you can watch Ganymede start to cross the planet beginning at 11:22 p.m., followed by its shadow starting at 1:48 a.m. Monday morning. As the largest moon, it casts the largest shadow, which is much easier to see than Europa's.
Anytime you'd like to know what's happening with Jupiter's moons go to Sky & Telescope's Jupiter's Moons. It defaults to the current UT or Universal Time. Subtract 4 hours from that time for Eastern Daylight Time; 5 hours for CDT; 6 hours for MDT and 7 hours for PDT. For example, an event occurring at 1:38 UT on September 13th translates to 8:38 p.m. September 12th CDT or 9:38 p.m. EDT.
There's one more thing for you to check out tonight that doesn't require a telescope. The thick crescent moon will be in conjunction with Scorpius's brightest star Antares at dusk in the southwestern sky. They'll appear 3° apart although their true separation is 555 light-years! Antares is a red supergiant star about 700 times larger than the sun with a distinctive red hue. Sometime in the future it will likely explode as a supernova and light up the sky with a brilliance equal if not surpassing tonight's moon.
Good luck and clear skies!
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.