Saturn

SATURN ( ) is the sixth major planet in order of distance from the sun, and is the most remote planet that was known before the discovery of Uranus in 1781. Its mean distance from the sun is about 885,900,000 m. and its periodic time about 291 years. Its synodic period, or the interval between oppositions, is 378 days. To the naked eye, Saturn, when in opposition, always appears as a star brighter than the first magnitude, but in conse quence of the changing phases of its rings it varies greatly in brightness, viz., between 0.9 and 4.4, its light being more than trebled when the rings are open to their greatest extent. Con siderable modifications, however, arise from the rather large eccentricity (o.o56) of its orbit. As regards colour, the planet shines with a warm, yellowish light not unlike that of Arcturus.

The Globe.

In telescopic appearance the globe of Saturn exhibits strong resemblances to Jupiter. It is even more flattened at the poles, its polar and equatorial diameters being respectively about 67,00o and 75,00o m. ; it is less bright near the margin than at the centre of the disc ; and its surface is marked by dusky belts with light intermediate zones, but whereas these cloudlike bands are very conspicuous on Jupiter, they are usually feeble and ill defined—partly in consequence of the planet's greater distance— in the case of Saturn.

The volume of Saturn is about 75o times that of the earth, but the periodic times of its satellites show that it exceeds the earth only about 95 times in mass. Its mean density, therefore, is but 0.13 of that of the earth, or over 0.7 times that of water.

Rotation.

Owing to the difficulty of detecting individual features of a sufficiently definite nature, the rotation of the planet has been observed only on comparatively rare occasions. The first determination was made in 1794 by the elder Herschel, who derived a rotation period of 10 h. 16 m. In Dec. 1876 a bright spot appeared near the equator which was observed by Asaph Hall, at Washington, for more than a month, and which showed a rotation in to h. 14 m. 24 s. In 1893 and 1894 A. S. Williams deduced from

observations of dark spots in the northern hemisphere mean rotation periods of io h. 14 m. 45 s. and io h. 15 m. Jo s. respec tively, and in the same two years periods of io h. 12 M. 52 s. and 10 h. 12 M. 36 s. from a number of white equatorial spots. A further series of spots appeared in the planet's north temperate regions in 1903, and from the recorded observations of these ob jects Denning found a mean period of about i o h. 38 m. It seems certain, therefore, that, as in the case of Jupiter, there are varia tions in the motions of the spots from year to year, and also that the rotation period is shorter near the equator than in higher latitudes.

Physical Condition.—It is further clear that what we see of Saturn is not a solid surface, but a layer of cloudlike or vaporous matter; the mean density of the globe is, indeed, less than that of any other major planet, and it is further to be noted that con siderations based on the large ellipticity of the disc, which is greater than would be assumed by such a globe of anything like uniform density rotating with the angular velocity of Saturn, in dicate that the larger part of the planet's mass must be strongly concentrated towards the centre.

The radiometric observations of Coblentz and Lampland sug gest a surface temperature which, though considerably higher than can be accounted for by solar radiation, is a good deal lower than was formerly supposed to characterize the greater planets of the outer group. Conclusions based on such measurements, how ever, appear by themselves to be somewhat unreliable in conse quence of the absorption of certain long-wave radiations by the earth's atmosphere, but a comparatively low temperature is sup ported by the theoretical work of Jeffreys. For further references to this interesting problem, and the observational evidences of considerable energy presented by the surface features of the great planets, see the articles PLANET and JUPITER.