The two-toned moon Iapetus shines at 11th magnitude due south of Saturn, making it easy to find this morning before dawn.
Iapetus stands due south of Saturn early on September 18. If you can spot the 11th-magnitude moon, try also for slightly fainter Enceladus, closer to the rings. Even if you can’t see it, you should be able to catch 10th-magnitude Tethys and Dione, as well as mid-8th-magnitude Titan. Credit: Stellarium/Oleg Pluton
- Saturn’s observation is optimal around 1 A.M. local daylight time, when it reaches its highest point (50° south) before morning twilight.
- Telescopic observation of Saturn reveals its ring system (nearly edge-on), along with prominent moons: Titan (magnitude 8.4, east of Saturn), Dione and Tethys (near the rings), and potentially Enceladus (12th magnitude, depending on equipment).
- Iapetus, an 11th magnitude moon, is currently positioned 1.3′ due south of Saturn. Its brightness varies due to its slow rotation and tidal locking.
- Iapetus will reach eastern elongation on October 7th, resulting in a decrease in brightness from magnitude 11 to 12.
Visit Saturn with your telescope this morning to spot the elusive moon Iapetus at superior conjunction, located due south of the ringed planet.
Saturn is highest in the sky around 1 A.M. local daylight time, when it is roughly 50° high in the south. However, it remains visible through morning twilight, slowly sinking toward the west. The earlier in the morning you can catch it, the better, as it’s higher and you’ll be observing it through less air. You can find it as the brightest point of light in southwestern Pisces, near the Circlet asterism and below the Great Square of Pegasus.
Zoom in on Saturn with a telescope to spot its stunning ring system, now seen nearly edge-on and tilted by only a few degrees. The brightest moon, magnitude 8.4 Titan, lies east of the planet and should be easy to spot. Closer to the rings on the eastern side is 10th-magnitude Dione, while similarly bright Tethys lies near the edge of the rings on the western side of Saturn. In between Dione and the rings on the eastern side, larger telescopes might show 12th-magnitude Enceladus, but don’t be surprised if you can’t spot it.
Iapetus is currently 11th magnitude and lies 1.3′ due south of the planet. This strange, two-toned moon orbits every 79 days, slowly turning as it moves, tidally locked and always showing the same face to Saturn. As seen from Earth, the slow rotation causes its brighter side to face us when it is at western elongation and its dimmer side to face us at eastern elongation. The moon is now heading toward eastern elongation, which it will reach October 7, fading from 11th to 12th magnitude as it heads to a point far east of Saturn.
Sunrise: 6:44 A.M.
Sunset: 7:03 P.M.
Moonrise: 3:16 A.M.
Moonset: 5:46 P.M.
Moon Phase: Waning crescent (10%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 12 P.M. local time from the same location.
For a look ahead at more upcoming sky events, check out our full Sky This Week column.