Saturn, the most distant planet known to ancient astronomers, is in fact the second largest in the Solar System and the easiest one to recognize through a telescope because of the broad, bright rings that encircle it equator. Like Jupiter, it has a cloudy atmosphere that overlies an interior of liquid hydrogen and helium. Saturn has 30 moons, more than any other planet.
To the naked eye, Saturn looks like a bright, yellowish star. Binoculars just reveal a small disk, with a hint of elongation caused by its rings. With a small telescope, the rings, which are over twice as wide as the planer itself, can be seen clearly. Of the five planets visible to the naked eye, Saturn is the e slowest moving, its oppositions (see p.15) occurring a year and two weeks apart. When Saturn is viewed at a magnification of 100 times at opposition, its globe looks the same size as the full Moon seen with the naked eye. Saturn’s disk is crossed by cloud belts, but these are not as prominent as those of Jupiter, nor is there an equivalent of the Great Red Spot. However, white spots appear in the equatorial zone about every 30 years, during summer in the planet’s Northern Hemisphere.
VIEWING SATURN’S RINGS
As Saturn orbits the Sun, we see its rings from various angles because of the combined effect of Saturn’s axial tilt and the inclination of its orbit to our own. At most, the rings are tipped toward us at about 27 degrees.
Twice in each orbit of Saturn around the Sun the rings appear to us edge-on, when they disappear from view because they are so thin.
STRUCTURE AND ATMOSPHERE
Saturn’s cloudy atmosphere is similar to that of Jupiter, only colder and with an overlying haze, which gives Saturn it butterscotch color and smoother look. Beneath the clouds is an interior of liquid hydrogen and helium, plus a rock core. Saturn’s average density is only about 70 percent that of water, the lowest of any planet. This low density, combined with its rapid rotation, gives it a highly elliptical shape, its equatorial diameter being nearly 7,500 miles (12,000 km) greater than its polar diameter.
RINGS AND MOONS
Saturn’s rings consist of an orbiting swarm of icy chunks, each no more than several feet across. The orbits of the innermost moons lie among the rings. Pan orbits in a gap called the Encke Division in there outer part of Saturn’s A ring. The next moon, Atlas, orbits at the edge of ring A, while Prometheus and Pandora orbit either side of ring F. Some moons actually share the same orbits.
The outermost part of Saturn’s rings system visible from the Earth is ring A, which is nearly 170,000 miles (275,000) across. This is separated from the brightest and widest part, ring B, by the Cassini Division, a gap of 2,800 miles (4,500 km), visible with a 3 in (75 mm) telescope. Next is the partly transparent ring C (or crepe ring). Fainter rings, called D and F, lie inside and outside the visible rings. Two other very faint rings, known as G and E, lie beyond ring F but are not shown here.
Saturn has 30 known moons. The largest is Titan, 3,200 miles (5,150 km)in diameter. It is the second largest moon in the Solar System (after Jupiter’s Ganymede) and the only one with a substantial atmosphere. At magnitude 8, it can be seen with a small telescope as tit orbits the plate every 16 days. An aperture of 6 in (150 mm) should show several others moons-notably Rhea, Tethys, Dion, Lappets, Enceladus, and perhaps Mimas. Iapetus is unusual in being over four times fainter when on one side of Saturn than on the other, because it has one bright and one dark hemisphere.