|Saturn is the outermost planet of the planets known in ancient times. The earliest known observations of Saturn, by the Babylonians, can be reliably dated to the mid-7th century BC, but it was probably noticed much earlier, since Saturn tends to shine brighter than most stars. To the naked eye it appears yellowish. The Greeks named it after Cronus, the original ruler of Olympus, who in Roman is the god Saturn.
Saturn is the 6th planet in order distance from the sun. It cannot approach the planet Earth closer than 1,190,000,000 kilometers. Its brightness is due to its large size. Saturn’s equatorial diameter is 120,660 kilometers, but its globe is kind of flattened, and the polar diameter is only 108,000 kilometers. The mass of Saturn is 95.17 times that of the Earth, and the escape velocity, which is the velocity which once attained it will enable the object to “coast” away from the planet, is 32.26 kilometers per second, more than three times that of the Earth. Saturn’s outer layers are made up of gas, it is a world quite unlike our own.
Saturn’s ring system is in a class of its own. While Jupiter and Uranus also have rings, those of Saturn are striking, and a telescope of moderate power will show them excellently. There can be no doubt that Saturn is one of the most beautiful objects in the sky.
The first telescopic observations of Saturn have been made by Galileo in July 1610. He saw the disk of the planet clearly, but his telescope gave only a magnification of 32 diameters and that was not good enough to show the ring system in the way we know it nowadays. Galileo thought that Saturn must be a triple planet and wrote that “Saturn is not one alone, but is composed of three, which almost touch one another.” Two years later, he found to his surprise that the “companions” had vanished, so that Saturn appeared as a single object. The ring system was then edge-on to the Earth, and this is why it could not be seen in Galileo’s telescope. The original aspect was seen again in the years following 1613, but Galileo was never able to interpret it correctly. Various strange theories were proposed to explain the planet’s unusual form. Hevelius of Danzig, for example, believed Saturn to be elliptical in shape, with two “appendages” attached to the surface.
The problem was solved by a Dutch astronomer, Christian Huygens, who began his observations in 1655. The telescopes that he used were much more powerful than Galileo’s, and gave a sharper defintion, so in a short time he concluded that “ Saturn is surrounded by a thin, flat ring which nowhere touches the body of the planet.” His theory was not widely excepted, but by 1665 it had been universally accepted, even though the nature of the ring system was not established until much later.
Among planetary orbits, that of Saturn is of fairly low eccentricity (0.056), though the difference in distance between perihelion (closest approach to the Sun) and aphelion (farthest retreat from the Sun) amounts to 160,000,000 kilometers. Because Saturn is so far from the Sun and the Earth Saturn always appears to be full, or nearly so, in the sense of the full moon.
The mean synodic period, the interval between successive oppositions, when the Earth passes approximately between Saturn and the Sun, is 378.1 days, so that Saturn is well placed for observation during several months in each year. Opposition dates up to 1990 are April 8, 1982; April 21, 1983; May 3, 1984; May 15, 1985; May 27, 1986; June 9, 1987; June 20, 1988; July 2, 1989; and July 14, 1990.
Like all the superior planets, that is all outwards from the Earth, Saturn moves for the greater part of each year eastward. Its average rate is about 1 in eight days. As it approaches opposition its motion seems to slow down and to stop all together for about 70 days before the opposition date. For a period that may be as little as 133 days or as great as 141 days, it then seems to move in a retrograde, or westward, direction before reaching another stationary point and resuming its eastward movement. This behavior does not indicate any real alteration in motion. The apparent regression is due to the fact that the Earth moving in a much smaller orbit at a greater velocity, is catching up with Saturn and passing it.
Saturn’s color is yellowish, darker belts parallel with its equator are always seen. These belts are not nearly as attracting as those on Jupiter, nor do they show so much detail. Saturn’s greater distance and smaller size make it less easy to study than Jupiter, the beauty of Saturn’s rings system tends to divert attention from its disk, particularly when the rings are wide open, thereby hiding a considerable part of the globe.
Saturn’s albedo, which is the proportion of incident light it reflects, is 0.61. The planet’s apparent magnitude, which is its brightness as it appears from Earth, depends largely upon the angle at which the ring system is displayed, largely because the rings are more reflective than the disk. When the rings are wide open, the magnitude attains -0.3, so that, of stars, only Sirius and Canopus appear brighter than Saturn. At oppositions when the rings are edge-on, as in 1980, the magnitude is as low as +0.08, though even at these times Saturn is still prominent.
The equatorial zone appears creamy, sometimes almost white. The polar regions are almost always less brilliant. The belts, unlike those of Jupiter, do not show obvious colors.
Well-defined spots on Saturn are rare. The most prominent example sighted during the present century was that discovered on August 3, 1933, by the English amateur W.T. Hay, using a six-inch refractor. The spot took the form of a large white oval patch in the equatorial zone, about one-fifth of the planet’s diameter in length, and with both ends well defined. During the next few weeks it lengthened rapidly, until by mid-September it had spread out so much that it could no longer be called a spot.
Pioneer 11 was the first unmanned space probe to meet with Saturn in September 1979. It had already passed by Jupiter, which had been its main objective, but valuable information about Saturn was obtained. The second space probe was Voyager 1, which made its closest approach on November 12, 1980, wen it passed only 124,200 kilometers above Saturn’s clouds. Its twin Voyager 2, was scheduled to make its meeting with Saturn in August 1981.
Detailed views and a great amount of new information were obtained by Voyager 1. Though Saturn’s disk is blander than that of Jupiter because of a greater amount of overlying “haze,” much detail was shown. A red spot was detected in the southern hemisphere