Even when viewed through a small telescope, Saturn and its ring system is one of the most unique objects in the sky. With a large modern telescope in good observing conditions, the planet appears as a light yellow and gray banded oblate spheroid. Like the other giant planets�Jupiter, Uranus, and Neptune�the visible planet is the cloud top of an extensive gaseous atmosphere. THE PLANET Saturn orbits the Sun at a mean distance of 1.427 billion km (0.893 billion mi) with a period of 29.4577 tropical years. The orbit is inclined 2.49� to the ecliptic, or Earth-orbital, plane and has an eccentricity of 0.0556. At Saturn's distance from the Sun, it receives only 0.01 of the unit solar radiation flux that the Earth does. Among planets in the solar system, Saturn is second in size only to Jupiter; Saturn has an equatorial diameter of 120,660 km (74,980 mi). Its volume would enclose about 769 Earth-sized bodies. Saturn's internal rotation period, defined by periodic radio emissions, is 10.657 hours. This fast rotation is responsible for Saturn's equatorial bulge and oblate shape. The equatorial-polar-diameter ratio is 1.12 to 1. Saturn's mass is 5.686 X 106{ kg (12.54 X 106{ lb), or 95.147 times the Earth's. Thus the average density is only 0.69 g/cm7 (43 lb/ft7), which is much less than water, indicating a very deep atmosphere and a very small core. Atmosphere Saturn is one of the giant outer planets, which are characterized by their large size, low density, and corresponding extensive atmospheres. Current models of the interior indicate that below the relatively thin opaque cloud layer is an extensive, clear hydrogen-helium atmosphere. Data on the internal heat flux, the detailed gravity field, and the observed upper-atmosphere hydrogen-helium ratio satisfy a model of the interior where the ratio of hydrogen to helium decreases with depth. The gas density gradually increases downward, and the gas transforms into a liquid. Further down the pressures increase to a critical level, and there the hydrogen becomes metallic. A small core of silicate material probably exists at the center. Saturn's white rings were first seen by Galileo Galilei in 1610; his small, imperfect telescope showed the planetary disk flanked by what he first interpreted as being two smaller bodies. Christiaan Huygens correctly theorized (late 1650s) the ring nature of these alleged "companions." James Clerk Maxwell mathematically demonstrated (1857) that the rings were composed of many small, unconnected particles, each orbiting near Saturn's equatorial plane. The classical designations for the rings are based on the gross ring components identified from the ground, but the Voyager spacecraft have shown the ring system to be highly structured. The radial particle-density distribution changes over distances of hundreds of meters. Estimated sizes of individual particles range from tens to hundreds of centimeters. The ring plane has a maximum thickness of 1 to 2 km (0.6 to 1.2 mi). Spectroscopy shows the presence of water ice, which probably covers rocky silicate cores.