Photographed by Voyager 1. The shadow of Saturn's three bright rings and Tethys are cast on the clouds tops of Saturn. The second largest planet in the Solar System, Saturn was first observed through a telescope by Galileo in 1610, but its rings were not identified until 1659, by Christiaan Huygens. It is a gas giant similar in atmospheric composition to Jupiter, and rotates very quickly, causing it to appear oblate (flattened at the poles). The rings are composed of ice and ice-coated dust and rock. Their origin and formation are not precisely understood, but it seems that tidal effects caused by some of Saturn's moons play a role in maintaining their structure. Tethys and Dione are the 9th and 12th of Saturn's 18 named moons. Many smaller, as yet unnamed, moons also exist.
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