Bright rings girdle Saturn's equator, making it the most beautiful of the planets. Those distinctive rings can be °spotted clearly in a small telescope; good binoculars, mounted steadily, will show the small outline of the planet elongated into an ellipse by the rings. Mounted binoculars should also pick out Saturn's largest moon, Titan, which orbits the planet every 16 days. Saturn's rings can reflect more light than the body of the planet itself, so that at its best Saturn appears of magnitude -0.3, outshone only by Sirius and Canopus. Without the rings, Saturn would be no more than magnitude 0.7, less than half as bright. Strange to say, from time to time Saturn really can appear to be without rings. The reason is the tilt of the planet's axis. As Saturn orbits the Sun, the rings are sometimes tipped towards us, while at other times the rings are presented edge-on. So thin are the rings that when edge-on (which happens about every 15 years, as in 2009 and 2025) they disappear from view in even the largest telescopes on Earth. The brightness of Saturn therefore depends not only on its distance from Earth, but also on the aspect of the rings. In many ways, Saturn is a smaller brother of Jupiter. Its equatorial diameter is 120,500 km, second only to that of Jupiter; its rotation period of 10¼ hours is second-fastest to that of Jupiter; and, like Jupiter, Saturn is composed mostly of hydrogen and helium. In one way, though, it is unique among the planets: its average density is less than that of water. This remarkable fact comes about because the planet's mass is less than a third that of Jupiter, so its gravity is less and hence its central regions are not compressed as densely. There probably is a rocky central core, but the region surrounding the core, in which hydrogen is compressed into a liquid metallic form, extends out to only half the planet's radius, against three-quarters of the radius in the case of Jupiter. That is not sufficient to compensate for the low density of Saturn's outer layers, so the planet's overall density is a mere 70 per cent of that of water. Saturn's low density is apparent from its outline, which is even more squashed than that of Jupiter. Saturn's pole-to-pole diameter is 109,000 km, fully 10 per cent less than its equatorial diameter; with Jupiter the difference is 6 per cent. Through a telescope Saturn appears as a tranquil, ochre-colored disk, darker at the poles and with some dusky horizontal bands. Pleasing views of the planet can be obtained with small telescopes, but serious study of Saturn requires at least 200 mm aperture, for there are none of the swirling, multicolored storm clouds that make Jupiter's globe so interesting in a small telescope, and no equivalent of the Great Red Spot. However, every 30 years or so a large white spot erupts in Saturn's northern hemisphere, when the planet's north pole is tilted at its maximum towards the Sun. The white spots are evidently storm clouds produced by solar heating. Such an outbreak occurred in 1990 and lasted a few months, with smaller spots appearing over the next few years. White spots apart, this is not to say that Saturn lacks activity in its atmosphere; it is just that the cloud patterns are usually concealed by high-altitude haze. Cameras aboard the probes Voyager 1 and 2 which reached Saturn in 1980 and 1981 recorded low-contrast cloud swirls resembling those on Jupiter. The meteorology of both planets should be similar, for they both have internal sources of heat. Like Jupiter, Saturn radiates twice as much heat as it receives from the Sun, a legacy of its birth. However, since Saturn is farther from the Sun, its clouds are about 30° colder than on Jupiter and form lower in the atmosphere. Tracking of cloud systems revealed that gales of up to 1800 km per hour blow on Saturn, over three times faster than on Jupiter. Inevitably, the attention of the space probes focused on the glorious rings. As seen from Earth, the rings look like a continuous disk encircling the planet, but appearances are deceptive. The Dutchman Christiaan Huygens in 1655 was the first to realize that the rings are not solid, but consist of a swarm of tiny particles orbiting Saturn. The central part of the rings, known as Ring B, is the widest and brightest. It is separated from the outer, fainter Ring A by a 5000-km-wide gap known as Cassini's Division, visible with a 75-mm telescope. Extending inwards from Ring B towards the planet is the faintest ring of all, the transparent Ring C, also known as the crepe ring. A narrow gap called Ancke's Division can be seen in Ring A, while observers looking through large telescopes under good conditions have reported apparent ripples within the rings, a sign that the density of ring material varies from place to place. Saturn's rings are extraordinarily thin in relation to their 270,000 km diameter. Voyager observations showed that the rings are no more than 100 m thick. On the same ratio of thickness to diameter, a gramophone record would be 5 km in diameter. The particles that make up the rings come in a wide range of sizes, from dust specks to lumps the size of a house or larger. Their composition is mostly frozen water, possibly mixed with dust, resembling loosely compacted snowballs. Saturn rings could have originated in several ways. One is from material that was prevented from forming into a moon by the overpowering force of Saturn's gravity. Alternatively, they may be the remains of a former moon that strayed too close to the planet and broke up, or a moon that was broken up by the impact of a comet. In fact, the rings may be replenished from time to time by icy material from impacting comets. In some paces, fine dust overlays the rings, apparently supported by electromagnetic forces in Saturn's magnetosphere, producing darker features known as spokes. Spoke-like features have been reported by ground-based observes, but it took the Voyager pictures to establish their reality. The Voyagers discovered several moons of Saturn too small to be seen from Earth, bringing the total of known Saturnian moons to 18, with others suspected. One of these tiny moons, Pan, actually orbits in Encke's Division in Ring A. Another, Atlas, patrols the outer edge of Ring A. Two of the moons, Prometheus and Pandora, orbit either side of the F Ring, shepherding its particles. Slightly farther out from Saturn, Janus and Epimetheus move along the same orbit, which initially confused astronomers who first spotted them from Earth in 1966. Orbit-sharing is common around Saturn. Tethys, a satellite visible from Earth, has two small siblings, Telesto and Calypso, moving on the same path. Dione, visible from Earth, shares its orbit with tiny Helene, another Voyager discovery. The gravitational effect of some of these satellites helps maintain the gaps in Saturn's rings. For instance, the gravitational tug of Mimas pulls particles out of the Cassini Division. Mimas itself, like most of the moons of Saturn, is a dirty snowball of frozen water and rock. It sports a remarkable crater 135 km wide, larger than Copernicus on the Moon, fully a third of its own 390 km diameter. The impact that caused this grotesque feature, called Herschel, must have nearly shattered Mimas. Saturn's largest moon, Titan, 5150 km in diameter, deserved to be considered as a planet in its own right. Bigger than Mercury (but slightly smaller than Jupiter's main moon Ganymede), it is the only moon to have a substantial atmosphere -- in fact, the atmospheric pressure on the surface of Titan is 50 per cent greater than at sea level on Earth. Titan's atmosphere is 90 per cent nitrogen, with methane making up most of the rest. Clouds of orange smog top the atmosphere, obscuring the surface from the prying eyes of space probes. Despite that thick atmosphere, Titan's surface temperature is very low, about -180°. Perhaps a rain of liquid methane falls from the rust-colored sky into the seas of methane on Titan's surface. A space mission called Cassini ids due to go into orbit around Saturn in 2004; it will drop a smaller probe, called Huygens, to land on the surface of Titan.