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Iapetus: East Is Least and West Is Best
What's Up: Brought to you by Jane Houston Jones

Viewing Saturn's Moon Iapetus in the Night Sky in 2013

Saturn's third largest moon Iapetus is a bright magnitude 10.1 at western elongation and a faint 11.9 magnitude at eastern elongation. This makes Iapetus a challenging but fun observing project when the Saturn system is in our evening skies.

Sky chart featuring Saturn and Iapetus
Saturn and Iapetus on July 28. The times when Iapetus is north or south of the planet are the easiest times to see it.
Its 79 day orbit takes Iapetus far outside the usual planetary eyepiece view. Iapetus is 3 times further from Saturn than Titan, or 12 ring diameters from Saturn when it shines the brightest. Iapetus is easier to locate near Saturn at both inferior and superior conjunction, when it is closest to the planet and visible to the north and south of the planet, respectively.

Half of Iapetus appears as dark as asphalt, the other half, as bright as snow. The dark half faces forward as Iapetus moves in its nearly circular, inclined orbit around Saturn. Scientists generally believe that it has swept up the orbiting dark material (perhaps originally from Phoebe) that covers its forward-facing surface. Iapetus' light side is more than five times brighter than its dark side.

We are looking at the light trailing side of tidally locked Iapetus when it is at western elongation, and we are looking at the dark leading or forward side of the moon at eastern elongation. When Cassini discovered Iapetus in 1671 he noted that he could only see Iapetus on one side of Saturn and not on the other side. On January 1, 2005, Cassini flew by Iapetus at a distance of 76,000 miles or 123,000 kilometers. The next Iapetus flyby was on September 10, 2007 when Cassini flew by Iapetus from distance of only 763 miles or 1,227 kilometers.

To find Iapetus at either conjunction or elongation, and compare its brightness to nearby stars, use your favorite planetarium program to calculate the extreme magnitudes of Iapetus, and to compare it to nearby stellar magnitudes.

See below for some key Iapetus observing dates from the 2013 Royal Astronomical Society of Canada (RASC) Observer's Handbook, page 237. The highlighted dates link to finder charts. South is up in these charts to match many telescopic views. Make your own charts for your precise location and observing time.



Eastern elongation (dark side of Iapetus faces earth, magnitude 11.9) Inferior conjunction (Iapetus is north of Saturn) Western elongation (bright side of Iapetus is facing earth, magnitude 10.1) Superior conjunction (Iapetus is south of Saturn)
Feb. 2 Feb. 21 March 13 April 2
April 22 May 10 (JPG, 18 KB) May 30 (JPG, 16 KB) June 19 (JPG, 17 KB)
July 9 (JPG, 19 KB) July 28 (JPG, 15 KB) Aug. 17 (JPG, 14 KB) Sept. 7 (JPG, 17 KB)
Sept. 27 Too near the sun Too near the sun Too near the sun
Dec. 18 Jan. 6    


Editor's note: originally posted here in June 2011, this "What's Up" blog was updated to show related events in 2013 on April 29, 2013


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About: Jane Houston Jones
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Jane is an astronomer, NASA science podcast developer, writer, educational and outreach artisan and social media enthusiast. She and her husband have an asteroid (22338 Janemojo) named after them.
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Last Updated: 24 Jun 2013