Patrick Moore's Astronomy: A Complete Introduction by Patrick Moore & Percy Seymour

Author:Patrick Moore & Percy Seymour
Language: eng
Format: epub, mobi
Tags: Patrick Moore's Astronomy: A Complete Introduction
Publisher: Hodder Education. An Hachette UK company
Published: 2015-04-27T16:00:00+00:00

Saturn

Next in order of distance from the Sun comes Saturn (see Plate 9), arguably the most beautiful object in the entire sky. To the naked eye it looks ordinary enough; it takes the guise of a bright, rather yellowish star, whose ‘dull’ glare led the ancients to name it after the God of Time (also Jupiter’s father, and the previous ruler of Olympus). It takes 29.5 years to complete one orbit and is therefore a slow mover in the sky; once identified, it is easy to find again.

In size and mass, Saturn is second only to Jupiter. Though it is of the same basic nature, there are some important differences, and in particular Saturn’s overall density is lower; it is actually less than that of water, so, although the mass is 95 times greater than that of the Earth, the surface gravity is less than 1.2 times greater. The surface is gaseous, and hydrogen is (predictably) the main constituent, with most of the rest of the atmosphere being made up of helium. Below the clouds come deep layers of liquid hydrogen, and finally a silicate core not a great deal larger than the Earth. The core temperature may be as high as 15,000°C, but the outer clouds are bitterly cold, at around −180°C. Belts and bright zones are visible with a small telescope, but they are less pronounced than those of Jupiter, and there is nothing to match Jupiter’s Great Red Spot.

There is a magnetic field, much stronger than that of the Earth, though not nearly so powerful as that of Jupiter; and Saturn is a quick spinner, which means that its yellowish globe is very obviously flattened. The axial inclination is almost 27 degrees to the perpendicular, much greater than with Jupiter, and the magnetic and rotational axes are almost coincident.

Occasional outbreaks are seen in the form of white spots. There were two of special note during the twentieth century, one in 1933 (discovered by a skilful amateur astronomer named W. T. Hay) and the other in 1990. Both of these became very conspicuous and lasted for some time. Undoubtedly, they were due to material gushing up from below cloud level.

Four spacecraft have so far encountered Saturn. The first of these was Pioneer 11, which had already passed by Jupiter (in 1974) and was sent on to Saturn more or less as an afterthought. It sent back good pictures, but better results have come from the Voyagers, both of which were amazingly successful. Voyager 1 approached Saturn to within a distance of 125,502 km (78,000 miles) on 12 November 1980, and Voyager 2 passed by at a mere 101,370 km (63,000 miles) on 25 August 1981. At least the radiation danger was known to be much less than with Jupiter, because Saturn’s zones are much weaker.

Excellent views of the globe were obtained, but of course the main glory of Saturn lies with its rings, which are made up of vast numbers of particles of water ice, all spinning around

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