The Story of the Solar System

by George F. Chambers

The planet Jupiter occupies, in one sense, the first position in the planetary world, it being the largest of all the planets. Moreover, with the exception of Venus, it is the brightest of the planets. As with Mars, and for the like reason, Jupiter, when in the positions known as the Quadratures (or near thereto), exhibits a slight phase, but owing to the far greater distance from the Sun of Jupiter, compared with Mars, the deviation of the illuminated surface from that of a complete circle is very small; it is, however, perceptible at or near the time of quadrature, a slight shading off of the limb farthest from the Sun being traceable.

Jupiter is noteworthy on account of two features, both of them more or less familiar, at least by name, to most people—its belts and its satellites,—both of which will be described in due course.

The belts are dusky streaks, which vary from time to time both in breadth and number: most commonly two broad belts will be seen with two or three narrower ones on either side; but sometimes all are rather narrow, and their narrowness is made up for by an increase in their number.

Fig. 14.—Jupiter, November 27, 1857 (Dawes).

Under all circumstances they lie practically parallel, or nearly so, to the planet’s equator. It is generally thought that the planet, whatever may be its actual structure or constitution, is surrounded by a dense cloudy envelope, and that the shaded streaks which we call belts are rifts in this atmosphere, which expose to view the solid body of the planet underneath. Whether, however, the term “solid body” is an accurate one to be used in this connection is thought by some to be open to doubt. The laws which regulate the existence of these belts are quite unknown; indeed it seems doubtful whether any laws exist at all, for the belts at one time appear to undergo constant change, whilst at another time they remain almost unchanged for several months. It has been suggested that when the changes are rapid it must be presumed that great atmospheric storms are to be considered as in progress, and possibly this may be the true explanation. Belts are commonly non-existent immediately under the equator; whilst north and south of this void space it most usually happens that there is one broad belt and several narrower ones in each hemisphere. At each pole the planet’s brightness is less than the average brightness, but it cannot exactly be said that this is due to the existence there of belts properly so called.

It was formerly considered that no tinges of colour could be traced on Jupiter except a silvery gray of different degrees of intensity; but during the last thirty years there can be no doubt that shades of brown, red, and orange, of no great depth, but yet quite definite have been traceable. Many observers concur in this opinion. Whether this detection of colour is due to an absolute development of colour during the period in question; or whether its detection is merely the result of more careful scrutiny with better instruments is a matter as to which the evidence is not clear. Though the general position of the belts is such that they are parallel to the planet’s equator, yet there are sometimes exceptions to this rule, for in a few very rare instances a streak in the nature of a narrow belt has been seen, inclined to the equator at a decided angle, perhaps 20° or even more.

It occasionally happens that spots are seen on Jupiter’s belts. Sometimes these remain visible for a considerable period. They are either dark or luminous, and their origin is unknown. Besides these casual spots, which are always small in size, there was visible during many years following 1878 a very remarkable and conspicuous large spot, strongly red in colour for several years, though it afterwards became much fainter. This spot exhibited an oval outline and was about 27,000 miles long and 8000 miles broad. For about 4 years it maintained its intense red colour and its shape almost unaltered; but after 1882, the shape remaining, the colour sensibly faded. The observations which were made on this spot during 1886 by Professor Hough at Chicago, U. S., with an 18-inch refractor, led him to the opinion that the persistence of the red spot for so many years rendered untenable the generally accepted theory that the phenomena seen on the surface of the planet are due to atmospheric causes.

Some astronomers have thought that a relationship subsists between the spots on the Sun and the spots on Jupiter. There certainly seems an apparent identity in point of time between the two classes of spots, and on the assumption that the spots on Jupiter are indicative of disturbances on the planet, Ranyard broached the idea that both classes of phenomena are dependent on some extraneous cosmical change; and are not related as cause and effect. Browning suggested many years ago that the red colour of the belts is a periodical phenomenon coinciding with the epoch of the greatest display of sunspots, but this thought does not appear to have been followed up by any one. Spots on Jupiter seem to have been first recorded by Robert Hooke in 1664. In the following year Cassini saw a spot which he found to be in motion, and by following it attentively he inferred that the planet rotated on its axis in 9h. 56m. It is a remarkable illustration of the great care bestowed by Cassini on his astronomical work that the best modern determinations of Jupiter’s rotation-period differ from Cassini’s estimate by only half a minute.

Bearing in mind the enormous size of Jupiter compared with the Earth, whilst its period of rotation is considerably less than half the Earth’s, it will be at once seen that the velocity of matter at the planet’s equator is immensely great—466 miles per minute against the Earth’s 17 miles per minute. One result of this is the great intensity of the centrifugal force at the equator, and likewise the greatness of the compression of the planet’s body at the poles. Hind has suggested that the great velocity which thus evidently exists may have the effect, by reason of the development of the heat which it gives rise to, of compensating the planet for the small amount of heat which owing to its distance it receives from the Sun.