Basic Astronomical Data

Saturn orbits the Sun at a mean distance of 1,427,000,000 km (887 million miles). Its proximity to Earth is never less than about 1.2 billion km (746 million miles), and its phase angle—the angle that it makes with the Sun and Earth—never exceeds about 6°. Saturn seen from the vicinity of Earth thus always appears nearly fully illuminated, a limitation to observation finally overcome by the side-lit and backlit views enabled by deep-space probes.

Like Jupiter and most of the other planets, Saturn has a regular orbit—that is, its motion around the Sun is prograde (in the same direction that the Sun rotates) and has a small eccentricity (non-circularity) and inclination to the ecliptic, the plane of Earth's orbit. Unlike Jupiter, however, Saturn's rotational axis is tilted substantially—by 26.7°—to its orbital plane. The tilt gives Saturn seasons, as on Earth, but each season lasts more than seven years. Another result is that Saturn's rings, which lie in the plane of its equator, are presented to observers on Earth at opening angles ranging from 0° (edge on) to nearly 30°. The view of Saturn's rings cycles over a 30-year period. Earth-based observers can see the rings' sunlit northern side for about 15 years, then, in an analogous view, the sunlit southern side for the next 15 years. In the short intervals when Earth crosses the ring plane, the rings are all but invisible.

Saturn has no single rotation period. Cloud motions in its massive upper atmosphere trace out a variety of periods, which are as short as about 10 hours 10 minutes near the equator and increase with some oscillation to about 30 minutes longer at latitudes higher than 40°. Scientists have determined the rotation period of Saturn's deep interior from that of its magnetic field, which is presumed to be rooted in the planet's metallic-hydrogen outer core. Direct measurement of the field's rotation is difficult because the field is highly symmetrical around the rotational axis. Radio outbursts from Saturn, which appear related to small irregularities in the magnetic field, show a period of 10 hours 39.4 minutes at the time of the Voyager encounters; this value was taken to be the magnetic field rotation period.

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