Rev. T. E. R. Phillips.
These changes in colouring bring us to the fact that the whole system of the Jovian markings is liable to constant and often very rapid change. Anyone who compares drawings made a few years ago with those made at the present time, such as Plates [XXII.] and [XXIII.], cannot fail to notice that while there is a general similarity, the details have changed so much that there is scarcely one individual feature which has not undergone some modification. Indeed, this process of change is sometimes so rapid that it can be actually watched in its occurrence. Thus Mr. Denning remarks that 'on October 17, 1880, two dark spots, separated by 20° of longitude, broke out on a belt some 25° north of the equator. Other spots quickly formed on each side of the pair alluded to, and distributed themselves along the belt, so that by December 30 they covered three-fourths of its entire circumference.' The dark belts, according to his observations, 'appear to be sustained in certain cases by eruptions of dark matter, which gradually spread out into streams.'
Even the great equatorial belts are not exempt from the continual flux which affects all the markings of the great planet, and the details of their structure will be found to vary to a considerable extent at different periods. At present the southern belt is by far the most conspicuous feature of the surface, over-powering all other details by its prominence, while its northern rival has shrunk in visibility to a mere shadow of what it appears in drawings made in the seventies. Through all the changes of the last thirty years, however, one very remarkable feature of the planet has remained permanent at least in form, though varying much in visibility. With the exception of the canals of Mars, no feature of any of the planets has excited so much interest as the great red spot on Jupiter. The history of this extraordinary phenomenon as a subject of general study begins in 1878, though records exist as far back as 1869 of a feature which almost certainly was the same, and it has been suggested that it was observed by Cassini two centuries ago. In 1878 it began to attract general attention, which it well deserved. In appearance it was an enormous ellipse of a full brick-red colour, measuring some 30,000 miles in length by 7,000 in breadth, and lying immediately south of the south equatorial belt. With this belt it appears to have some mysterious connection. It is not actually joined to it, but seems, as Miss Clerke observes, to be 'jammed down upon it'; at least, in the south equatorial belt, just below where the spot lies, there has been formed an enormous bay, bounded on the following side (i.e., the right hand as the planet moves through the field), by a sharply upstanding shoulder or cape. The whole appearance of this bay irresistibly suggests to the observer that it has somehow or other been hollowed out to make room for the spot, which floats, as it were, within it, surrounded generally by a margin of bright material, which divides it from the brown matter of the belt. The red spot, with its accompanying bay and cape, is shown in Fig. 25 and in Plate [XXII.], which represents the planet as seen by the Rev. T. E. R. Phillips on January 6, 1906. The spot has varied very much in colour and in visibility, but on the whole its story has been one of gradual decline; its tint has paled, and its outline has become less distinct, as though it were being obscured by an outflow of lighter-coloured matter, though there have been occasional recoveries both of colour and distinctness. In 1891 it was a perfectly easy object with 3⅞ inches; at the present time the writer has never found it anything but difficult with an 18-inch aperture, though some observers have been able to see it steadily in 1905 and 1906 with much smaller telescopes. The continued existence of the bay already referred to seems to indicate that it is only the colour of the spot that has temporarily paled, and that observers may in course of time witness a fresh development of this most interesting Jovian feature.
The nature of the red spot remains an enigma. It may possibly represent an opening in the upper strata of Jupiter's dense cloud-envelope, through which lower strata, or even the real body of the planet, may be seen. The suggestion has also been made that it is the glow of some volcanic fire on the body of the planet, seen through the cloud-screen as the light of a lamp is seen through ground-glass. But, after all, such ideas are only conjectures, and it is impossible to say as yet even whether the spot is higher or lower than the average level of the surface round it. A curious phenomenon which was witnessed in 1891 suggested at first a hope that this question of relative height would at least be determined. This phenomenon was the overtaking of the red spot by a dark spot which had been travelling after it on the same parallel, but with greater speed, for some months. It appeared to be quite certain that the dark spot must either transit the face of the red spot or else pass behind it; and in either case interesting information as to the relative elevations of the two features in question would have been obtained. The dark spot, however, disappointed expectation by drifting round the south margin of the red one, much as the current of a river is turned aside by the buttress of a bridge. In fact, it would almost appear as though the red spot had the power of resisting any pressure from other parts of the planet's surface; yet in itself it has no fixity, for its period of rotation steadily lengthened for several years until 1899, since when it has begun to shorten again, so that it would appear to float upon the surface of currents of variable speed rather than to be an established landmark of the globe itself. The rotation period derived from it was, in 1902, 9 hours 55 minutes 39·3 seconds.
The mention of the changing period of rotation of the red spot lends emphasis to the fact that no single period of rotation can be assigned to Jupiter as a whole. It is impossible to say of the great planet that he rotates in such and such a period: the utmost that can be said is that certain spots upon his surface have certain rotation periods; but these periods are almost all different from one another, and even the period of an individual marking is subject, as already seen, to variation. In fact, as Mr. Stanley Williams has shown, no fewer than nine different periods of rotation are found to coexist upon the surface; and though the differences in the periods seem small when expressed in time, amounting in the extreme cases only to eight and half minutes, yet their significance is very great indeed. In the case of Mr. Williams's Zones II. and III., the difference in speed of these two surface currents amounts to 400 miles per hour. Certain bright spots near the equator have been found to move so much more rapidly than the great red spot as to pass it at a speed of 260 miles an hour, and to 'lap' it in forty-four and a half days, completing in that time one whole rotation more than their more imposing neighbour. It cannot, therefore, be said that Jupiter's rotation period is known; but the average period of his surface markings is somewhere about nine hours fifty-two minutes.
Thus the rotation period adds its evidence to that already afforded by the variations in colour and in form of the planet's markings that here we are dealing with a body in a very different condition from that of any of the other members of our system hitherto met with. We have here no globe whose actual surface we can scrutinize, as we can in the case of Mars and the moon, but one whose solid nucleus, if it has such a thing, is perpetually veiled from us by a mantle which seems more akin to the photosphere of the sun than to anything else that we are acquainted with. The obvious resemblances may, and very probably do, mask quite as important differences. The mere difference in scale between the two bodies concerned must be a very important factor, to say nothing of other causes which may be operative in producing unlikeness. Still, there is a considerable and suggestive general resemblance.
In the sun and in Jupiter alike we have a view, not of the true surface, but of an envelope which seems to represent the point of condensation of currents of matter thrown up from depths below—an envelope agitated in both cases, though more slowly in that of Jupiter, by disturbances which bear witness to the operation of stupendous forces beneath its veil. In both bodies there is a similar small density: neither the sun nor Jupiter is much denser than water; in both the determination of the rotation period is complicated by the fact that the markings of the bright envelope by which the determinations must be made move with entirely different speeds in different latitudes. Here, however, there is a divergence, for while in the case of the sun the period increases uniformly from the equator to the poles, there is no such uniformity in the case of Jupiter. Thus certain dark spots in 25° north latitude were found in 1880 to have a shorter period than even the swift equatorial white markings.
One further circumstance remains to be noted in pursuance of these resemblances. Not only does the disc of Jupiter shade away at its edges in a manner somewhat similar to that of the sun, being much more brilliant in the centre than at the limb, but his remarkable brilliancy, already noticed, has given rise to the suggestion that to some small extent he may shine by his own inherent light. There are certain difficulties, however, in the way of such a suggestion. The satellites, for example, disappear absolutely when they enter the shadow of their great primary—a fact which is conclusive against the latter being self-luminous to anything more than a very small extent, as even a small emission of native light from Jupiter would suffice to render them visible. But even supposing that the idea of self-luminosity has to be abandoned, everything points to the fact that in Jupiter we have a body which presents much stronger analogies to the sun than to those planets of the solar system which we have so far considered. The late Mr. R. A. Proctor's conclusion probably represents the true state of the case with regard to the giant planet: 'It may be regarded as practically proved that Jupiter's condition rather resembles that of a small sun which has nearly reached the dark stage than that of a world which is within a measurable time-interval from the stage of orb-life through which our own Earth is passing.'
Leaving the planet itself, we turn to the beautiful system of satellites of which it is the centre. The four moons which, till 1892, were thought to compose the complete retinue of Jupiter, were among the first-fruits of Galileo's newly-invented telescope, and were discovered in January, 1610. The names attached to them—Io, Europa, Ganymede, and Callisto—have now been almost discarded in favour of the more prosaic but more convenient numbers I., II., III., IV. The question of their visibility to the unaided eye has been frequently discussed, but with little result; nor is it a matter of much importance whether or not some person exceptionally gifted with keenness of sight may succeed in catching a momentary glimpse of one which happens to be favourably placed. The smallest telescope or a field-glass will show them quite clearly. They are, in fact, bodies of considerable size, III., which is the largest, being 3,558 miles in diameter, while IV. is only about 200 miles less; and a moderate magnifying power will bring out their discs.