Saturn’s two-toned moon Iapetus is now passing south of the ringed planet, making this satellite easier than usual to find.
On October 22, Iapetus is just past superior conjunction, located close to Saturn. Only moons brighter than 11th magnitude are shown. Credit: Stellarium/Oleg Pluton
Looking for a sky event this week? Check out our full Sky This Week column.
October 21: Catch early-morning Orionids
Saturn’s strange, two-faced moon Iapetus orbits on a wide path that it takes nearly 80 days to complete. During that time, it slowly rotates in synchrony with its orbit, brightening and fading by up to two magnitudes, depending on which hemisphere is facing Earth.
Tonight, Iapetus is just past superior conjunction, sitting south of Saturn and glowing at 11th magnitude. It is visible with a telescope. Saturn is already well-positioned soon after darkness falls, reaching a height of roughly 30° above the eastern horizon by 8 P.M. local daylight time. Through a telescope, you’ll easily view its lovely rings, showing off their southern side. Bright Titan, its largest moon, is just 1.6’ east of the planet, shining at mid-8th magnitude. Tethys, Rhea, and Dione shine around 10th magnitude and are closer to the ringed world. Around 8 P.M. CDT, Dione is east of Saturn and Tethys and Rhea are west of the planet.
Iapetus lies 1.7’ south-southeast of Saturn, fainter than the other four moons. Because its orbit takes it so far from the planet and because it is faint, Iapetus is easiest to find at superior and inferior conjunction, so tonight is a great time to track it down.
Sunrise: 7:18 A.M.
Sunset: 6:10 P.M.
Moonrise: 4:18 P.M.
Moonset: 3:20 A.M.
Moon Phase: Waxing gibbous (89%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
Alison Klesman is senior editor of Astronomy magazine. She holds a Ph.D. in astronomy and has studied a variety of topics, from minor planets to supermassive black holes.