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Iapetus: East is Least and West is Best

Jane Houston Jones


Saturn's moon Iapetus is brighter at western elongation and fainter at eastern elongation, which makes it a great observing project when the Saturn system is in our evening right now.

I was reading about Saturn's satellites in the RASC 2005 Observers Handbook in early January, just as I was observing the Cassini orbiter's first close-up images of Iapetus on my computer. Not everyone who takes a look at Saturn observes Iapetus, although it's Saturn's third largest moon. 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. But its 79 day orbit takes Iapetus far outside the usual planetary eyepiece view. In fact Iapetus is 3 times further from Saturn than Titan, or 12 ring diameters from Saturn when it shines the brightest.

The magnitude of Iapetus varies from 10.1 at western elongation to 11.9 at eastern elongation. We have known for a long time that the leading side of Iapetus is dark as coal, while the trailing side is bright as snow. We are looking at the bright trailing side of tidally locked Iapetus when it is at western elongation, and we are looking at the dark leading side of the moon at eastern elongation. Why this is so is still under debate, as it has been for the past 334 years. Cassini discovered Iapetus in 1671 and he made the note that he could only see Iapetus on one side of Saturn and not on the other side. The dark area of Iapetus is called Cassini Regio, in his honor, and may be dark because the leading side of Iapetus collides with or alters dust from the moon Phoebe. Stay tuned as the Cassini instrument teams study the Iapetus data, and release their findings. Cassini will have one more flyby of Iapetus in September 2007. This year, on January 1, Cassini flew by Iapetus at a distance of 40,000 miles. The 2007 flyby will be from a distance of 763 miles.

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. SJAA's Akkana Peck created some Iapetus charts which should help you find Iapetus on March 15 and April 24, as it swings from eastern to western elongation. You'll find that and a few other dates charted here as well.

Below is a list of key Iapetus observing dates.


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