JUPITER

Passing outwards from the zone of the minor planets, we come to the greatest and most magnificent member of the solar system, the giant planet Jupiter. To most observers, Jupiter will probably appear not only the largest, but also the most interesting telescopic object which our system affords. Some, no doubt, will put in a claim for Mars, and some will share Sir Robert Ball's predilection for Saturn; but the interest attaching to Mars is of quite a different character from that which belongs to Jupiter, and while Saturn affords a picture of unsurpassed beauty, there is not that interest of variety and change in his exquisite system which is to be found in that of his neighbour planet. Jupiter is constantly attractive by reason of the hope, or rather the certainty, that he will always provide something fresh to observe; and the perpetual state of flux in which the details of his surface present themselves to the student offers to us the only instance which can be conveniently inspected of the process of world-formation. Jupiter is at the very opposite end of the scale from such a body, for example, as our own moon. On the latter it would appear as though all things were approaching the fixity of death; such changes as are suspected are scarcely more than suspected, and, even if established, are comparatively so small as to tax the utmost resources of observation. On the former, such a thing as fixity or stability appears to be unknown, and changes are constantly occurring on a scale so gigantic as not to be beyond the reach of small instruments, at least in their broader outlines.

The main facts relating to the planet may be briefly given before we go on to consider the physical features revealed to us by the telescope. Jupiter then travels round the sun in a period of 11 years, 314·9 days, at an average distance of almost 483,000,000 miles. According to Barnard's measures, his polar diameter is 84,570, and his equatorial diameter 90,190 miles. He is thus compressed at the poles to the extent of ¹/16, and there is no planet which so conspicuously exhibits to the eye the actual effect of this polar flattening, though the compression of Saturn is really greater still. In volume he is equal to more than 1,300 earths, but his density is so small that only 316 of our worlds would be needed to balance him. This low density, not much greater than that of water, is quite in accordance with all the other features which are revealed by observation, and appears to be common to all the members of that group of large exterior planets of which Jupiter is at once the first and the chief.

The brilliancy of the great planet is exceedingly remarkable, far exceeding that of Mars or Saturn, and only yielding to that of Venus. In 1892 his lustre was double that of Sirius, which is by far the brightest of all the fixed stars; and he has been repeatedly seen by the unaided eye even when the sun was above the horizon. According to one determination he reflects practically the same amount of light as newly fallen snow; and even if this be rejected as impossibly high, ZÖllner's more moderate estimate, which puts his reflective power at 62 per cent. of the light received, makes him almost as bright as white paper. Yet to the eye it is very evident that his light has a distinct golden tinge, and in the telescopic view this remains conspicuous, and is further emphasized by the presence on his disc of a considerable variety of colouring.

Under favourable circumstances Jupiter presents to us a disc which measures as much as 50 in diameter. The very low magnifying power of 50 will therefore present him to the eye with a diameter of 2,500, which is somewhat greater than the apparent diameter of the moon. In practice it is somewhat difficult to realize that this is the case, probably owing to the want of any other object in the telescopic field with which to compare the planet. But while there may be a little disappointment at the seeming smallness of the disc even with a power double that suggested, this will quickly be superseded by a growing interest in the remarkable picture which is revealed to view.

Some idea of the ordinary appearance of the planet may be gained from Fig. 25, which reproduces a sketch made with a small telescope on October 9, 1891. The first feature that strikes the eye on even the most casual glance is the polar compression. The outline of the disc is manifestly not circular but elliptical, and this is emphasized by the fact that nearly all the markings which are visible run parallel to one another in the direction of the longest diameter of the oval. A little attention will reveal these markings as a series of dark shadowy bands, of various breadths and various tones, which stretch from side to side of the disc, fading a little in intensity as they approach its margin, and giving the whole planet the appearance of being girdled by a number of cloudy belts. The belts may be seen with very low powers indeed, the presence of the more conspicuous ones having repeatedly been evident to the writer with the rudimentary telescope mentioned in Chapter II., consisting of a non-achromatic double convex lens of 1½-inch aperture, and a single lens eye-piece giving a power of 36. Anything larger and more perfect than this will bring them out with clearness, and an achromatic of from 2 to 3 inches aperture will give views of the highest beauty and interest, and will even enable its possessor to detect some of the more prominent evidences of the changes which are constantly taking place.

The number of belts visible varies very considerably. As many as thirty have sometimes been counted; but normally the number is much smaller than this. Speaking generally, two belts, one on either side of a bright equatorial zone, will be found to be conspicuous, while fainter rulings may be traced further north and south, and the dusky hoods which cover the poles will be almost as easily seen as the two main belts. It will further become apparent that this system of markings is characterized by great variety of colouring. In this respect no planet approaches Jupiter, and when seen under favourable circumstances and with a good instrument, preferably a reflector, some of the colour effects are most exquisite. Webb remarks: 'There is often "something rich and strange" in the colouring of the disc. Lord Rosse describes yellow, brick-red, bluish, and even full-blue markings; Hirst, a belt edged with crimson lake; Miss Hirst, a small sea-green patch near one of the poles.' The following notes of colour were made on December 26, 1905: The south equatorial belt distinct reddish-brown; the equatorial zone very pale yellow, almost white, with faint slaty-blue shades in the northern portion; the north polar regions a decided reddish-orange; while the south polar hood was of a much colder greyish tone. But the colours are subject to considerable change, and the variations of the two great equatorial belts appear, according to Stanley Williams, to be periodic, maxima and minima of redness being separated by a period of about twelve years, and the maximum of the one belt coinciding with the minimum of the other.

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 3? 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.

The beautiful symmetry of this miniature system was broken in 1892 by Barnard's discovery of a fifth satellite—so small and so close to the great planet that very few telescopes are of power sufficient to show it. This was followed in 1904 by Perrine's discovery, from photographs taken at the Lick Observatory with the Crossley reflector, of two more members of the system, so that the train of Jupiter as at present known numbers seven. The fifth, sixth, and seventh satellites are, of course, far beyond the powers of any but the very finest instruments, their diameters being estimated at 120, 100, and 30 miles respectively. It will be a matter of interest, however, for the observer to follow the four larger satellites, and to watch their rapid relative changes of position; their occultations, when they pass behind the globe of Jupiter; their eclipses, when they enter the great cone of shadow which the giant planet casts behind him into space; and, most beautiful of all, their transits. In occultations the curious phenomenon is sometimes witnessed of an apparent flattening of the planet's margin as the satellite approaches it at ingress or draws away from it at egress. This strange optical illusion, which also occurs occasionally in the case of transits, was witnessed by several observers on various dates during the winter of 1905-1906. It is, of course, merely an illusion, but it is curious why it should happen on some occasions and not on others, when to all appearance the seeing is of very much the same quality. The gradual fading away of the light of the satellites as they enter into eclipse is also a very interesting feature, but the transits are certainly the most beautiful objects of all for a small instrument. The times of all these events are given in such publications as the 'Nautical Almanac' or the 'Companion to the Observatory'; but should the student not be possessed of either of these most useful publications, he may notice that when a satellite is seen steadily approaching Jupiter on the following side, a transit is impending. The satellite will come up to the margin of the planet, looking like a brilliant little bead of light as it joins itself to it (a particularly exquisite sight), will glide across the margin, and after a longer or shorter period will become invisible, being merged in the greater brightness of the central portions of Jupiter's disc, unless it should happen to traverse one of the dark belts, in which case it may be visible throughout its entire journey. It will be followed or preceded, according to the season, by its shadow, which will generally appear as a dark circular dot. In transits which occur before opposition the shadows precede the satellites; after opposition they travel behind them. The transit of the satellite itself will in most cases be a pretty sharp test of the performance of a 3-inch telescope, or anything below that aperture; but the transit of the shadow may be readily seen with a 2½-inch, probably even with a 2-inch. There are certain anomalies in the behaviour of the shadows which have never been satisfactorily explained. They have not always been seen of a truly circular form, nor always of the same degree of darkness, that of the second satellite being notably lighter in most instances than those of the others. There are few more beautiful celestial pictures than that presented by Jupiter with a satellite and its shadow in transit. The swift rotation of the great planet and the rapid motion of the shadow can be very readily observed, and the whole affords a most picturesque illustration of celestial mechanics.

A few notes may be added with regard to observation. In drawing the planet regard must first of all be paid to the fact that Jupiter's disc is not circular, and should never be so represented. It is easy for the student to prepare for himself a disc of convenient size, say about 2½ inches in diameter on the major axis, and compressed to the proper extent (¹/16), which may be used in outlining all subsequent drawings. Within the outline thus sketched the details must be drawn with as great rapidity as is consistent with accuracy. The reason for rapidity will soon become obvious. Jupiter's period of rotation is so short that the aspect of his disc will be found to change materially even in half an hour. Indeed, twenty minutes is perhaps as long as the observer should allow himself for any individual drawing, and a little practice will convince him that it is quite possible to represent a good deal of detail in that time, and that, even with rapid work, the placing of the various markings may be made pretty accurate. The darker and more conspicuous features should be laid down first of all, and the more delicate details thereafter filled in, care being taken to secure first those near the preceding margin of the planet before they are carried out of view by rotation. The colours of the various features should be carefully noted at the sides of the original drawings, and for this work twilight observations are to be preferred.

Different observers vary to some extent, as might be expected, in their estimates of the planet's colouring, but on the whole there is a broad general agreement. No planet presents such a fine opportunity for colour-study as Jupiter, and on occasions of good seeing the richness of the tones is perfectly astonishing. In showing the natural colours of the planet the reflector has a great advantage over the refractor, and observers using the reflecting type of instrument should devote particular attention to this branch of the subject, as there is no doubt that the colour of the various features is liable to considerable, perhaps seasonal, variation, and systematic observation of its changes may prove helpful in solving the mystery of Jupiter's condition. The times of beginning and ending observation should be carefully noted, and also the magnifying powers employed. These should not be too high. Jupiter does not need, and will not stand, so much enlargement as either Mars or Saturn. It is quite easy to secure a very large disc, but over-magnifying is a great deal worse than useless: it is a fertile source of mistakes and illusions. If the student be content to make reasonable use of his means, and not to overpress either his instrument or his imagination, he will find upon Jupiter work full of absorbing interest, and may be able to make his own contribution to the serious study of the great planet.