Observing Saturn

Viewing the ringed planet with a telescope is relatively easy for most of the same reasons that observing Jupiter is easy. The planet spends a large amount of time in a favorable portion of the sky for viewing, about six months out of each 12.5-month apparition. Saturn's rings span a total area about as large as the disk of Jupiter, though the disk of the planet itself rarely exceeds 20 arc seconds. In one area the planet is not as easy to view as Jupiter. Since Saturn is about twice as far from the Sun as Jupiter is, the planet receives far less light than Jupiter does and so its surface brightness is much lower than is Jupiter. This in turn can make the planet a bit of a challenge for the amateur astrophotographer to make crisp images. Saturn's maximum possible brightness is only about magnitude -0.5 with the rings fully open and when the rings are closed, it may not reach zero magnitude even at opposition.

Your attention will be quickly drawn to the ring system. Most of the light you will see comes from the B ring area, which has an albedo of nearly 60%. Mid-size telescopes should easily be able to pick up the A ring and the Cassini division in between the two. The A ring is considerably darker with an albedo value of about 30%. The C ring is more difficult to see inside the B ring and makes a good challenge. The C ring is much darker with an albedo value of some 20%. None of Saturn's other rings are visible through amateur telescopes. How spectacular the rings are will be dependent upon Saturn's position in its orbit. When the planet is near one of its solstices, the rings are at their maximum visibility and in fact the back side of the rings are tipped above the planet' summer time pole. When Saturn is near an equinox, then the rings appear edge on and may in fact disappear completely. For a time during the edge on period (which will occur next in 2010) the rings will actually present their unilluminated side towards Earth allowing them to be glimpsed in silhouette in front of the planet. The rings will also cast shadows on the planet, which gives Saturn a remarkable three-dimensional appearance not evident on any other planet. When Saturn is near quadrature, watch as the planet casts its shadows on the rings, further enhancing the three-dimensional aspect of our view.

Saturn's globe is itself somewhat bland in nature. A hydrocarbon haze shrouds the planet's dynamic atmosphere from our view, leaving Saturn a plain butterscotch color. When Saturn's poles are tipped towards Earth, you may be able to notice a discoloration in the haze at the poles. Here the hydrocarbon smog is

30 To many in the media, this is a reference to Cassini's immense size. The Battlestar Galactica is a fictional space-going aircraft carrier nearly a mile long. But there was a more insidious meaning to the insult. The Battlestar Galactica was also the last ship of her kind.

thinner in nature, broken down by continuous exposure to sunlight over the nearly fifteen uninterrupted years of sunlight. The process is similar to what causes the naturally occurring "ozone holes" in Earth's atmosphere.

Many of Saturn's moons are visible in modest telescopes. Titan shines at magnitude +8 and is visible to even the smallest of telescopes. Several other moons are within reach of telescopes of six inches or larger. Titan takes about two weeks to make a leisurely swing around Saturn. Unlike Jupiter's moons, which move back and forth in a straight line, Titan makes big loops around the planet unless Saturn's rings are edge on. So most of the time you will see Titan pass well north or south of the planet's disk. Inward of Titan, the moons Rhea, Tethys and Dione are all approximately magnitude +10 and should be visible with a 4-inch telescope under dark skies and easily with a 6-inch. Enceladus at magnitude +12 starts to push the limits of 8-inch scopes under a dark sky and Mimas is a challenge even for the largest of scopes because its feeble magnitude +13 glow is buried very close to Saturn itself. All of these moons orbit in the ring plane as Titan does so they follow the same types of path around the planet that Titan does. They swing behind the planet over one pole, move out to a greatest elongation then swing back in to pass in front of the planet passing the opposite pole. There are also moons to see outward of Titan. Iapetus is one of the great odd balls of the solar system. When Iapetus is west of Saturn, it points its bright trailing hemisphere towards Earth and the moon brightens to magnitude +10. But as it travels around to the east side of the planet, it points more and more of its tar-black leading side towards Earth and the moon fades to magnitude +14. Hyperion is magnitude +14 at all times and requires a telescope of 10 inches to be just barely visible. The last of Saturn's tele-scopically discovered moons is Phoebe. This moon, likely a captured asteroid orbiting Saturn in the reverse direction of all the other moons is fainter than magnitude +16 and more likely a target for professionals.

As Jupiter and Saturn ride high into the evening sky at opposition time, it's time to mount our scopes for an observing journey to the Kings of Worlds and explore their wonders. Each is a mini solar system in its own right, ruling its gravitational domain with an iron fist. Let's go view the wonders of Jupiter and Saturn.

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