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date: 18 November 2017


A Dictionary of Astronomy

Ian Ridpath


(♄) The sixth planet from the Sun. Its mean opposition magnitude is between +0.7 and −0.3, depending on the tilt of the rings towards us, the faintest of the five planets known since antiquity. Saturn is the most flattened in shape of all the planets, with an equatorial diameter of 120 536 km and a polar diameter of 108 728 km. It is also the least dense of all the planets (0.69 cm3) and the only planet less dense than water. The rotation period of the visible surface ranges between about 10h 14m near the equator and 10h 40m at 60° south. The rotation of the planet's interior, derived from observations of Saturn's radio emissions by the Cassini probe, is 10h 47m 6s.


Physical data

Diameter (equatorial)


Inclination of equator to orbit

Axial rotation period (sidereal)

120 536 km



10.233 hours

Mean density

Mass (Earth = 1)

Volume (Earth = 1)

Mean albedo (geometric)

Escape velocity

0.69 g/cm3




36.1 km/s

Orbital data

Mean distance from Sun

106 km


Eccentricity of orbit

Inclination of orbit to ecliptic

Orbital period (sidereal)





29.447 years

Saturn has a thick atmosphere composed of about 96% hydrogen and 3% helium (molecular percentages), with traces of methane and ammonia. The temperature near the top of theatmosphere is around −190°C. Internally, Saturn is thought to possess a rocky high-temperature core, perhaps containing iron, about 20 000 km across. This is possibly surrounded by a layer of icy materials 5000 km thick, and a layer of metallic hydrogen and helium over 10 000 km thick. Convection currents within the conductive hydrogen layer are probably responsible for Saturn's magnetic field, which has an equatorial field strength of about 2 × 10−5 tesla, somewhat weaker than the Earth's. Surrounding this layer is normal molecular liquid hydrogen and helium, which gradually merges into a gaseous layer near the visible surface. Like that of the Earth, Saturn's magnetic field is oriented parallel to the planet's spin axis, but with opposite polarity.

Like Jupiter, the visible surface of Saturn is crossed by dark belts or bands of cloud, with bright zones between, although the atmosphere is generally calmer than Jupiter's. Dark and bright spots occur, but are fainter and far less frequent than on Jupiter. Wisps and festoons suggestive of turbulence in the atmosphere are visible on spacecraft images. There is a ‘jet stream’ in the equatorial zone, where the rotation period is nearly half an hour faster than elsewhere. There are no long-lived features, but occasional spectacular outbursts of huge white spots occur in the equatorial zone. The first to be well-observed was in 1933 August, soon spreading over much of the equatorial zone. Similar outbreaks occurred in 1960 March and 1990 October.

Saturn's most distinctive feature is its bright rings. They have an albedo of up to 0.60, far higher than any other planetary rings. Through telescopes, three main rings are visible: the outer A Ring, 14 600 km wide, extending out to 136 800 km from Saturn's centre; the central B Ring, the brightest, 25 500 km wide; and the much fainter inner C Ring or crêpe ring, 17 500 km wide. Darker spokes are faintly visible on the B Ring. Between the A and B Rings lies a prominent gap, the Cassini Division, and the A Ring itself is divided by the Encke Gap and the Keeler Gap. Space probes have revealed that every ring has dozens of tiny subdivisions.

There are a further four named rings: the D Ring, which lies inside the C Ring; the narrow F Ring, which lies outside the A Ring; the more distant G Ring; and the outermost, the wide and diffuse E Ring. The innermost edge of the D Ring lies 67 000 km from the centre of Saturn, while the outer rim of the E Ring is 480 000 km from Saturn's centre. Despite their great extent, the rings are extremely thin, a few hundred metres at most, and disappear in all but the largest telescopes when edgewise-on to the Earth, which happens every 15 years or so. Saturn has over 60 known satellites.

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