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.

On favourable occasions the brilliancy of Jupiter is very considerable; so much so that it rivals Venus and Mars. And besides this, there appears to be something special in the nature of Jupiter’s surface, for not only does it seem to radiate a much larger proportion of the solar light which falls on it than do the planets generally, but some observers have expressed the opinion that it possesses inherent light of its own. Speculations, however, such as this must always be received with reserve, because of the evident difficulty of making sure of the facts on which they must be based. One thing, however, seems less open to doubt. Bearing in mind the small amount of heat which reaches Jupiter from the Sun, there is reason to infer that the clouds which certainly exist on Jupiter must owe their origin to the influence of some other heat than solar heat; in other words that Jupiter possesses sources of heat within itself.

Jupiter has satellites, 5 in number. The discovery of four of these, was one of the first fruits of the invention of the telescope, for they were found by Galileo in January, 1610. The 5th satellite is so small that it escaped notice until as recently as 1892, having been discovered on September 9 of that year by Professor Barnard, with the great Lick telescope in California. It is, however, so minute that one can count on one’s fingers the telescopes capable of showing it.

The four old satellites of Jupiter shine as stars of about the 7th magnitude; in other words, they are sufficiently bright to be visible with telescopes however small: indeed several instances are on record of persons gifted with very good sight, having been able to see them with the naked eye. For the study of their physical appearance very powerful optical assistance is necessary, but their movements are so rapid, and the phenomena which result from those movements are so interesting, that these bodies may be considered to occupy the first place in the stock-in-trade of every amateur astronomer, who lays himself out for planet-gazing, with the object of profiting himself or his friends. The phenomena here alluded to are known as eclipses, transits, and occultations.

The four old satellites do not bear any names, but are numbered from the innermost outwards, and are always alluded to by their numbers as I, II, III and IV.

An eclipse of a Jovian satellite is identical in principle with an eclipse of the Moon; that is to say, just as an eclipse of the Moon happens when the Moon passes into and is lost in the Earth’s shadow, so an eclipse of a Jovian satellite happens when such satellite becomes lost in the shadow cast by the planet into space. The Ist IInd and IIIrd satellites in consequence of the smallness of the inclination of their orbits, undergo eclipse once in every revolution round their primary, but the IVth is less often eclipsed, owing to the joint effect of its considerable orbital inclination, and of the distance to which it recedes from its primary.

An occultation of a Jovian satellite is akin in principle to an occultation of a star by the Moon. As the Moon moving forwards suddenly covers a star, so the planet, on occasions, suddenly covers one of its satellites. If the satellite in question is the IVth, its disappearance behind the planet and its reappearance from behind the planet will both be visible in due succession. This is often true also of the IIIrd satellite, but for reasons connected with the proximity to their primary of the Ist and IInd satellites, only their disappearance or reappearance (not both) can, as a rule, be observed on the same occasion. The most interesting, by far, however, of the phenomena connected with Jupiter’s satellites are their transits in front of, that is across, the visible disc of the planet. Though these transits are of frequent occurrence, yet they are always interesting because of the diverse appearances which the satellites exhibit at different times, and which cannot be said to be in accordance with any recognised laws. Moreover, in observing the transit of a satellite, we may often see the black shadow cast by the satellite on the planet’s disc; and this shadow will sometimes precede and sometimes follow the satellite itself. From the fact that the satellite generally appears as a bright spot on a bright background whilst the shadow is black, or blackish, an inexperienced observer is apt to look at the shadow and think he is seeing the satellite.

Jupiter revolves round the Sun in not quite 12 years at a mean distance of 483 millions of miles. Its apparent diameter varies between 50 and 30 according to its position with respect to the Earth. Its true diameter is about 88,000 miles. Owing to its large size and rapid rotation, as has already been mentioned, Jupiter is very much flattened at the poles. The amount of this (the polar “compression” as it is called) is about ¹/₁₆.