People instantly recognize Saturn, the sixth planet from the sun, by its rings, even though there are times when stargazers can't see them from Earth. They disappear periodically as a result of Saturn's orbital motion and its tilt relative to the plane of its orbit. Saturn is farther from the sun than Earth, and it moves more slowly in its orbit, but it spins faster, experiencing two days in less than 24 hours.
Like all the planets, Saturn has an elliptical orbit, and its distance from the sun varies. Its closest approach, or perihelion, is 1.35 billion kilometers (839 million miles), and its greatest separation, or aphelion, is 1.5 billion kilometers (934 million miles), making it almost 10 times as far from the sun as the Earth. It completes one orbit in 29.45 Earth years, and since it travels almost 9 billion kilometers (5.6 billion miles) in that orbit, its velocity is almost 34,701 kilometers per hour (21,562 miles per hour). That sounds fast, but it's only about a third of the orbital velocity of Earth.
Saturn isn't solid, like the Earth, and different parts of it rotate at different velocities. Specifically, the area around the equator rotates at a different speed than the regions above and below it. The equator makes a complete rotation in 10 hours and 14 minutes, but the regions above and below the equator rotate in 10 hours and 39 minutes. Moreover, both Voyager spacecraft measured the period of rotation of the magnetic field in 1980 and 1981 and found it to be 10 hours and 39 minutes. When Cassini measured it in 2004, however, it had slowed to 10 hours and 45 minutes.
Like Earth's axis, Saturn's axis is tilted relative to the plane of its orbit, and by almost the same amount -- its 26.7-degree tilt is just 3.2 degrees more than Earth's. Saturn therefore experiences seasons as first one pole and then the other face the sun as it orbits. At a certain point in its orbit, its tilt relative to the Earth is such that its rings appear head-on and seem to disappear. Galileo was the first to notice this phenomenon, which occurs every 14 to 15 Earth years.
Saturn's atmosphere, containing 75 percent hydrogen and 25 percent helium with trace amounts of other compounds, such as methane and water, is host to some of the fastest winds in the solar system. The Voyager spacecraft measured wind speeds of 1,800 kilometers per hour (1,118 miles per hour) at the equator. Winds generally blow in an easterly direction, although they can also blow westerly, as a team of planetary scientists from Great Britain and the United States discovered in 2009. They made this discovery after determining that the rotational period is faster than scientists previously believed.
- Getty Images/Lifesize/Getty Images