Saturn is the sixth planet from the Sun and the second largest in the Solar System, after
Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an
eighth the average density of Earth, but is over 95 times more massive. Even though Saturn
is almost as big as Jupiter, Saturn has less than a third its mass. Saturn orbits the Sun at a
distance of 9.59 AU (1,434 million km), with an orbital period of 29.45 years.
Saturn is named after the Roman god of wealth and agriculture, who was the father of the
god Jupiter. Its astronomical symbol ( ) has been traced back to the Greek Oxyrhynchus
Papyri, where it can be seen to be a Greek kappa-rho ligature with a horizontal stroke, as
an abbreviation for Κρονος (Cronus), the Greek name for the planet ( ).[29] It later came
to look like a lower-case Greek eta, with the cross added at the top in the 16th century to
Christianize this pagan symbol.
The Romans named the seventh day of the week Saturday, Sāturni diēs, "Saturn's Day", for
the planet Saturn.[30]
Saturn's interior is thought to be composed of a rocky core, surrounded by a deep layer of
metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and an outer
layer of gas. Saturn has a pale yellow hue, due to ammonia crystals in its upper
atmosphere. An electrical current in the metallic hydrogen layer is thought to give rise to
Saturn's planetary magnetic field, which is weaker than Earth's, but has a magnetic
moment 580 times that of Earth because of Saturn's greater size. Saturn's magnetic field
strength is about a twentieth that of Jupiter. [27] The outer atmosphere is generally bland and
lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can
reach 1,800 kilometres per hour (1,100 miles per hour).
The planet has a bright and extensive system of rings, composed mainly of ice particles,
with a smaller amount of rocky debris and dust. At least 274 moons orbit the planet, of
which 63 are officially named; these do not include the hundreds of moonlets in the rings.
Titan, Saturn's largest moon and the second largest in the Solar System, is larger (but less
massive) than the planet Mercury and is the only moon in the Solar System that has a
substantial atmosphere.[28]
Saturn is the most distant of the five planets easily visible to the naked eye from Earth, the
other four being Mercury, Venus, Mars, and Jupiter. (Uranus, and occasionally 4 Vesta, are
visible to the naked eye in dark skies.) Saturn appears to the naked eye in the night sky as a
bright, yellowish point of light. The mean apparent magnitude of Saturn is 0.46 with a
standard deviation of 0.34.[24] Most of the magnitude variation is due to the inclination of the
ring system relative to the Sun and Earth. The brightest magnitude, −0.55, occurs near the
time when the plane of the rings is inclined most highly, and the faintest magnitude,
1.17, occurs around the time when they are least inclined. [24] It takes approximately 29.4
years for the planet to complete an entire circuit of the ecliptic against the background
constellations of the zodiac. Most people will require an optical aid (very large binoculars
or a small telescope) that magnifies at least 30 times to achieve an image of Saturn's rings
in which a clear resolution is present.[48][154] When Earth passes through the ring plane, which
occurs twice every Saturnian year (roughly every 15 Earth years), the rings briefly disappear
from view because they are so thin. Such a "disappearance" will next occur in 2025, but
Saturn will be too close to the Sun for observations.[180]
Saturn and its rings are best seen when the planet is at, or near, opposition, the
configuration of a planet when it is at an elongation of 180°, and thus appears opposite the
Sun in the sky. A Saturnian opposition occurs every year—approximately every 378 days—
and results in the planet appearing at its brightest. Both the Earth and Saturn orbit the Sun
on eccentric orbits, which means their distances from the Sun vary over time, and
therefore so do their distances from each other, hence varying the brightness of Saturn
from one opposition to the next. Saturn also appears brighter when the rings are angled
such that they are more visible. For example, during the opposition of 17 December 2002,
Saturn appeared at its brightest due to the favorable orientation of its rings relative to the
Earth,[181] even though Saturn was closer to the Earth and Sun in late 2003. [181]
From time to time, Saturn is occulted by the Moon (that is, the Moon covers up Saturn in
the sky). As with all the planets in the Solar System, occultations of Saturn occur in
"seasons". Saturnian occultations will take place monthly for about a 12-month period,
followed by about a five-year period in which no such activity is registered. The Moon's
orbit is inclined by several degrees relative to Saturn's, so occultations will only occur
when Saturn is near one of the points in the sky where the two planes intersect (both the
length of Saturn's year and the 18.6-Earth-year nodal precession period of the Moon's orbit
influence the periodicity).[182]
