Saturn was the outermost planet known to ancient astronomers. Named after the father of Jupiter in Greek and Roman mythology, Saturn orbits the Sun at almost twice the distance of Jupiter. The planet's sidereal orbital period of 29.4 Earth years was the longest natural unit of time known to the ancient world. At opposition, when Saturn is at its brightest, it can lie within 8 A.U. of Earth. However, its great distance from the Sun still makes it considerably fainter than either Jupiter or Mars. Saturn ranks behind Jupiter, the inner planets, and several of the brightest stars in the sky in terms of apparent brightness.
The Saturn Data box presents orbital and physical data for the planet. Less than one-third the mass of Jupiter, Saturn is still an enormous body, at least by terrestrial standards. As with Jupiter, Saturn's many moons allowed an accurate determination of the planet's mass long before the arrival of the Pioneer and Voyager missions. Saturn's mass is 5.7 
1026 kg, or 95 times the mass of Earth. From Saturn's distance and angular size, the planet's radius—and hence the average density—quickly follow. Saturn's equatorial radius is 60,000 km, or 9.5 Earth radii. The average density is 700 kg/m3—less than the density of water (which is 1000 kg/m3). Here we have a planet that would float in the ocean—if only Earth had one big enough! Saturn's low average density indicates that, like Jupiter, it is composed primarily of hydrogen and helium. Saturn's lower mass, however, results in lower interior pressure, so these gases are less compressed than in Jupiter's case.
Saturn, like Jupiter, rotates very rapidly and differentially. The rotation period of the interior (as measured from magnetospheric outbursts, which trace the rotation of the planet's core) and at high planetary latitudes (determined by tracking weather features observed in Saturn's atmosphere) is 10h40m. The rotation period at the equator is 10h14m, or about 26 minutes shorter. Because of Saturn's lower density, this rapid rotation makes Saturn even more flattened than Jupiter. Saturn's polar radius is only 54,000 km, about 10 percent less than the equatorial radius. Careful calculations show that this degree of flattening is less than would be expected for a planet composed of hydrogen and helium alone.Astronomers believe that Saturn also has a rocky core, perhaps twice the mass of Jupiter's.
Saturn's best-known feature is its spectacular ring system. Because the rings lie in the equatorial plane, their appearance (as seen from Earth) changes in a seasonal manner, as shown in Figure 12.1. Saturn's rotation axis is significantly tilted with respect to the planet's orbit plane—the axial tilt is 27°, similar to that of both Earth and Mars. Consequently, as Saturn orbits the Sun the angles at which the rings are illuminated and at which we view them vary. When the planet's north or south pole is tipped toward the Sun, during Saturn's summer or winter, the highly reflective rings are at their brightest. During Saturn's spring and fall, the rings are close to being edge-on, both to the Sun and to us, so they seem to disappear altogether. One important deduction that we can make from this simple observation is that the rings are very thin. In fact, we now know that their thickness is less than a few hundred meters, even though they are over 200,000 km in diameter.
Figure 12.1 (a) Over time, Saturn's rings change their appearance to terrestrial observers as the tilted ring plane orbits the Sun. At some times during Saturn's 29.5-year orbital period the rings seem to disappear altogether as Earth passes through their plane and we view them edge-on. The two images are separated by a few months, having been taken (in true color) by the Hubble Space Telescope in the summer and fall of 1995. (b) The ring plane is precisely edge-on as viewed from Earth; several moons are visible, including Titan's shadow (black dot). (c) The rings have begun to tilt.
