In the High Heavens/Chapter 5

by Robert Stawell Ball

CHAPTER V.

THE FIFTH SATELLITE OF JUPITER.

 

INCE the invention of the telescope some two hundred and eighty years ago the great planet Jupiter has never been the object of so much attention as it was during the autumn of 1892. It will be remembered that among the first-fruits of the new instrument for looking at objects which were a long way off, was the great discovery of the system of which Jupiter was the centre. The four satellites lie just on the dividing line between objects which can be seen with the unaided eye, and objects which require optical assistance to make them visible. It seems to be certain that there have been individuals gifted with rare powers of vision who under exceptionally favourable circumstances have been able to discern one or other of the satellites of Jupiter without optical aid. Testimony has been adduced which seems to show that long before the invention of Galileo's tube for studying the heavens, one or two of these satellites had been seen by the Chinese. But it would be futile to say that these glimpses of the moons of Jupiter really amounted to any anticipation of the great discovery of Galileo, Such mere casual observations never thoroughly demonstrated the existence of the little bodies, still less did they yield such a volume of accurate knowledge as would enable us to determine their movements, so as to say when they would again be likely to be visible. No astronomer ever seriously entertained the notion that there was such a system of attendants revolving around Jupiter until their existence had been demonstrated once and for all by the telescope of Galileo.

There can be no doubt that the moons of Jupiter are in themselves quite bright enough to be ordinarily seen by the unaided eye were it not for a single circumstance. They lie too close to the great planet. At first sight it might seem that the very fact that they are placed in the brilliant illumination which Jupiter radiates should rather tend to make them more easily discerned. The nearer an object is held to a source of light the better it can generally be seen. Does it not therefore appear somewhat paradoxical to say that the reason we are generally unable to see the moons of Jupiter with the unaided eye, is because they lie so close to a lustrous globe? Ought not that to be the very reason why they should be seen with all the greater facility? This is a point which may require a few words of explanation because it is intimately connected with the recent great discovery.

We hold a book near a candle when we want to read, because under the circumstances supposed the only light available comes from the candle. The type has no other illumination, and the nearer it is to the candle the clearer the printing appears. But this is not at all analogous to the case of Jupiter and his satellites. We cannot think of Jupiter as the candle, and a satellite as a page of the book. If such were indeed the analogy, then the nearer the satellite lay to the body of the planet the more brightly would it be illuminated, and under certain circumstances the more easily would it be seen. The very opposite is, however, notoriously the case. It will be instructive to see wherein the difference lies.

It is, of course, quite true that Jupiter is a brilliant source of light, but in this respect the light that emanates from the planet is of a very different character from the light which radiates from the sun. Jupiter is not a sun-like object diffusing radiance around in virtue of his own intrinsic brilliancy. If the satellites revolving around the planet were really in the condition of the planets themselves as they revolve around the sun, then the closer the satellite was to the planet the greater would be the illumination it would receive. But of course this is not at all the case. Although we have excellent reasons for believing that Jupiter is in truth a highly heated globe, yet we are perfectly assured that the temperature falls far short of that which would be required if the great planet were to dispense its beams around in the same manner as a miniature sun. Indeed, there are facts connected with the satellites themselves which render it perfectly clear that Jupiter, so far from possessing a sun-like radiance, is absolutely devoid of any intrinsic light whatever. It shines merely as the earth itself shines, or as the moon shines, or the planet Mars, or Venus, or Saturn, by the sunbeams which fall upon it. As to the light which illumines the satellites of Jupiter, it also can only come from the sun. We cannot, indeed, say that the light-radiating power of a certain area on Jupiter is the same as that from an equal area on one of his moons. There are certainly intrinsic differences between the material constitution of the great planet and of his satellites which prevent us from affirming that they are at all times equally capable of reflecting light. Some portions are whiter than others, and therefore return a larger fraction of the sunlight which falls upon them. Still, however, we may for the purpose of the present argument remember that as both Jupiter and the moons are illuminated by the same sun, they are both sufficiently nearly of the same brightness. It therefore follows that there would be no gain of lustre to the satellite in being near Jupiter. Note, then, the difference between what would have happened if Jupiter were sun-like, and what does actually happen when Jupiter is merely a planet. In the former case there would be a distinct accession of brightness to the satellite the closer it made its approach. In the latter case there would be no variation of brightness at all.

To follow the argument a step further we must think of what takes place in the eye of the astronomer who is observing the planet. On the retina an image is formed oi the great globe. The image is extremely minute, but owing chiefly to the disturbances of our atmosphere, the genuine image is surrounded with a region in which the nerves of the retina are more or less affected by the light, which ought, if all the sources of disturbance could be excluded, to be entirely concentrated within the image itself. Of course, vision of other objects on this affected part of the retina will be correspondingly impaired. If the image of a satellite fall upon it, then whether it will be perceived or not depends upon whether the brilliance of the little object is sufficient to excite those nerves whose sensibility is somewhat lessened by the stray light referred to As the satellite acquires no increased brilliance from Jupiter's own lustre, it is obvious that the further it be away the more will its image stand out on that part of the retina where there is no temporary diminution of sensibility, and the better it will be seen. Such is the reason why the moons of Jupiter cannot be seen with the unaided eye except under conditions that need not be again referred to. Were objects of no greater brilliance quite aloof from such a bright orb as one of the great planets, they would easily be discerned without optical aid. In the ordinary language of the astronomer, one of Jupiter's satellites would be reckoned as bright as a star of the fourth magnitude.

Since the discovery of the satellites of Mars by Professor Asaph Hall at Washington, in 1877, there has been no event in the astronomical world which has possessed the same interest as that of which we are now speaking. From one point of view it might appear that the announcement of an addition of another satellite to Jupiter's system was no very significant matter after all. No doubt the new attendant of the great planet is a very trifling object, as far as dimensions are concerned. It is not nearly so large as many of those minor planets the discoveries of which are constantly being announced. How comes it that people are talking about and thinking about Jupiter's fifth satellite, while there are thousands or rather millions of stars lying unnoticed through space? and yet any one of these stars is perhaps a million times as big as this little satellite, besides being a sun, which may presumably be a source of light and heat to planets which circulate around it. What is the ground for so much excitement about the discovery of an object which is probably among the most minute, if not itself actually the most minute, of all the objects of which our telescopes can take cognisance? Why is it that an apparition of a great comet with a blazing tail half across the sky would not have for astronomers half the interest possessed by this little stranger? Why is it that sun-spots, lunar craters, the ice-caps of Mars, and his newly discovered lakes, the revelations as to Venus, were for the moment forgotten or unheeded; why did everyone become so eager to learn all about this tiny little moon of Jupiter, and on the tiptoe of expectation for every further item of intelligence from the Lick Observatory?

Let me say at once that this extreme interest in the little object appears most natural. I am glad to share in it, for there seem to be certain very good reasons why such interest should be felt. In the first place, Jupiter has always been a favourite telescopic object. The globe itself is so vast that the features are on a sufficiently large scale to be discernible with comparatively small instrumental power. Thus it is that the cloud belts on the great planet are familiar objects to everyone who has ever used a telescope. Then, too, the ever- varying positions of the four older satellites make them a spectacle that is always attractive. At one time we are watching an eclipse, in which a satellite plunges into Jupiter's shadow and disappears for a while. At another time we note how the wanderer vanishes by occultation behind the body of the great globe around which it revolves. Then, too, there are the singularly delicate and beautiful phenomena of the transits of the satellites in front of the planet. In this case not only is the satellite itself often to be traced in the act of crossing, but, as a far more striking manifestation, its dark shadow is thrown on the brilliant globe. The movements of the satellites are so rapid that the different phenomena we have referred to are repeated frequently, and many of them can be discerned with a comparatively small telescope.

But the Jovian system of satellites has also a claim on those astronomers who devote themselves rather to mathematical research than to telescopic observation. Each of the moons is, of course, mainly guided in its movement by the attraction of the great globe itself. If there were only a single moon and if there were no other interference, then the determination of its movement would be a comparatively simple matter, and the places occupied by the satellite at every date could be predicted with complete confidence. In no one of the planets, however, can so simple a condition of things as we have supposed be realised. It is no doubt true that the earth is attended by but a single moon, but then the movement of the earth's companion is rendered highly complex by the fact that the attraction of the sun constantly tends to make it swerve from the simple elliptic path which it would otherwise pursue. The movement of our moon, however embarrassing a problem it may present to mathematicians, is, nevertheless, simplicity itself in comparison with the movement which has to be performed by one of Jupiter's moons. In the case of each of these little bodies there is not only a force exerted by the sun with the effect of disturbing the satellite's motion, but each one of the four globes attracts each one of the others, and is in turn attracted by it.

The consequence of this is that the movements of Jupiter's satellites form one of the most troublesome problems which the mathematical astronomer has ever been called upon to solve. It presents peculiar features of difficulty arising from the exceptional character of the Jovian system; but these very difficulties, so far from deterring mathematicians from the study, have in some cases acted as a stimulus. A considerable part of Laplace's famous book, the "Mécanique Céleste," is devoted to the study of the system of Jupiter's satellites. He has contrived certain analytical methods for encountering the many points that arise, and he has succeeded in explaining some of the most remarkable dynamical features of the system. Many other mathematicians have also essayed the task of a thorough elucidation of the problem. Indeed, on more than one occasion the question has been propounded by the Academy of Sciences at Paris, and a considerable prize has been offered for a satisfactory discussion.

The test of the completeness of such a theory would be sought in the precision with which it would enable the movements of the satellites to be predicted. No doubt a good deal has been done in this way. Our "Nautical Almanac," for instance, announces, with all needful details, the various eclipses and occultations of the satellites, as well as their transits and the movements of the shadows across the disc. It is quite possible, even with our present knowledge of the subject, to predict such phenomena for some years in advance. The accuracy with which these indications can be made is amply sufficient for the ordinary purposes for which they are required. But in such investigations the requirements of science demand a much closer degree of approximation between what is observed and what is calculated than is possible in the present state of our knowledge of Jupiter's satellites. There is much work yet to be done before the movements of this system can be reduced to satisfactory order.

The object in now mentioning these matters is not assuredly to attempt, in these pages, any contribution towards the task of improving the tables of Jupiter's satellites. My purpose is to show how much attention has been paid to the system by astronomers of every class. It would be utterly impossible to obtain any accurate notion of how often Jupiter has been carefully observed, let us say, within the last hundred years. We can, however, obtain some sort of estimate which will help to explain the profound impression that the announcement of a discovery of a new satellite is calculated to make. The great planet is visible for some months every year. We shall certainly be well within the mark if we say that it must have been scrutinised carefully by skilful observers at least a thousand times each year. For, remember how many observatories there are in the world, where special attention is given to such work, and also of late years how many excellent telescopes there are in private hands. Considering, too, that the Jovian system is one of such intense interest to all observers, and that, except the moon, there is no object in the sky more frequently and more carefully studied, it is not at all an undue estimate to assume that Jupiter and his moons must have been carefully examined, I do not mean merely looked at, at least one hundred thousand times during the last hundred years.

Now we shall be able to understand the extraordinary interest which the announcement of the detection of a fifth satellite has created. Here was this system which everyone knew, which had occupied so much attention, and now we are told on the best authority that there is something to be seen in it which had eluded all the eyes that ever looked at it before. This is indeed a surprise. Those who have good telescopes will think on the fortune which might conceivably have smiled on them, if perchance the satellite had presented itself on one of those rare occasions when it might have come within the reach of instruments less powerful than that by which it was actually discovered. The mathematical astronomers to whom the problem of Jupiter has ever been an attractive though a very difficult subject will find that the new satellite imparts an entirely fresh aspect to the question. It will now, doubtless, be attacked again with a quickened interest, and it is certain that the movements of the newly discovered body will suggest considerations of great theoretical importance. It is even quite possible that its detection may have the effect of removing some of the difficulties that have hitherto been experienced in the attempts to interpret the movements of the four older bodies.

The Lick Observatory had already become famous from the numerous valuable observations which have been made within the last few years. We certainly mean no disparagement to its previous achievements when we say that they have been altogether cast in the shade by the announcement of the last discovery which has been made on the summit of Mount Hamilton. We fully appreciate the splendid series of double-star discoveries by Burnham. We recognise the value of the observations of Mars, of the beautiful lunar photographs, of the admirable and instructive spectroscopic work of Keeler, but from henceforth it would seem that the Lick telescope must be chiefly remembered, not for these admirable labours, but as the instrument with which Jupiter's fifth moon has been found.

In this respect the Lick telescope may he compared with another celebrated instrument of America, the great Washington refractor. Doubtless much excellent work has been done by this latter instrument, besides that achievement by which its name will be specially handed down. It was with this superb glass at Washington that Professor Asaph Hall discovered the two satellites of Mars in 1877. This at once raised the name of Hall to a high rank in the list of famous astronomical discoverers. Now we have a triumph of the same high order won with the Lick telescope. This entitles the name of Barnard to be inscribed on the same select roll as that which contains the name of Asaph Hall.

On such an occasion astronomers of all countries freely offer their hearty congratulations to those who pursue their science in America. In no other country can there be found such a lavish and splendid endowment of astronomical observatories. Nowhere else is there such abundant provision for the carrying on of astronomical work of all kinds. It were fitting that the rewards should go to the credit of the country which has done most to earn them. There is no civilised nation whose inhabitants would not have experienced a thrill of pride if such a discovery as that of the two moons of Mars or of the fifth satellite of Jupiter had been achieved within its borders by one of its own people. As it happens, both these distinctions belong to America, and those who are fully acquainted with the matter know how valiantly the American astronomers have struggled with their difficulties and how triuphantly they have overcome them. Nor should it be forgotten in this connection that the great Lick telescope as well as the Washington telescope is of American manufacture. Both are the products of the consummate optical skill of Messrs. Alvan Clark, of Massachusetts. Those who provided these grand instruments, those who made them, those who used them, and the nation which owns them, are all to be sincerely congratulated on the splendid results of their joint efforts.

The orbit of Jupiter so nearly resembles a circle that the distance from the earth to the planet does not greatly alter. Accordingly there is not much variation in the distance from the earth to the planet at one opposition and another. It does so happen that in the opposition through which Jupiter passed in 1892, the actual distance attained was almost the smallest possible. Even that, however, is nearly four times as great as the distance from the earth to the sun. In the case of Jupiter, the most important question, so far as the advantages for observation are concerned, is the season of the year when the opposition takes place. For observations of a planet it is specially desirable to have the body as high as possible in the heavens. The atmospheric difficulties, which are always so embarrassing to the astronomer, are lessened with every increase of the altitude. This consideration will show how the opposition referred to offered exceptionally favourable advantages for the observation of Jupiter. As the great planets move in planes which are nearly coincident with the ecliptic, it follows that the best time for observing the planet will be during the winter season. Of course, the most suitable moment, so far as altitude is concerned, would be when the opposition took place on midwinter day, while the lowest altitude would be reached in an opposition on Midsummer Day. It is true that the ideally perfect opposition was not reached in 1892. The opposition took place in October, that is to say, two months before the most suitable time. But on the whole the conditions were unusually favourable.

Professor E. E. Barnard had already obtained deserved fame as a skilful astronomical observer, and therefore it is that his announcement of this new discovery has been at once accepted by astronomers. It is the extreme minuteness of the body which is the cause of its having hitherto escaped notice. We are told that the fifth satellite appears as a star of the thirteenth magnitude, if not even very much less. An object possessing no greater brilliance than is thus indicated can only be perceived by a good telescope under the most favourable circumstances. When, however, the difficulties of seeing the satellite are enhanced by the fact that it is located close to so brilliant a globe as the great planet, then it is only the exceptional powers of the Lick telescope, and the exceptional excellence of the situation in which the telescope is placed, which have enabled it to be detected at all. So far as we can estimate the lustre of the new satellite, it can hardly be the five-hundredth part of the lustre of even the faintest of its older companions in the same system. Indeed not improbably the proportion must be expressed by a figure considerably greater than that which I have written. If one of the older satellites were crushed into a thousand equal fragments, the bulk of one of these fragments would be comparable with that of the new satellite.

The distance of this new moon from the centre of the planet appears to be about 112,500 miles, and the period of each revolution is about 11 hours 57 minutes. It will thus be noticed that the satellite revolves round Jupiter in a period which exceeds that required by Jupiter to accomplish a rotation on his own axis, namely, 9 hours 551/2 minutes. The new satellite is so close to the surface of Jupiter that the difficulty of this detection is greatly enhanced by the fact that it is so frequently hidden by the great globe. Only for a comparatively small part of each revolution does the little body appear well away from the margin of the planet. When most remote it will be at a distance of 36 seconds from the edge, that is, about two-thirds of the diameter of Jupiter. Then six hours later it will be at a similar distance on the opposite side of its orbit. It is often difficult to observe one of the large satellites when in the act of transit across the planet's disc, so that we can hardly be surprised that the transits of an object which is such an extremely small fraction of their size should not be perceived.

Of course, there is a notable difference between the case of a transit of a satellite over its primary and that of a planet, like Venus or Mercury, in front of the sun's disc. In the latter case the planet appears as a black spot against the brilliant background. In fact, it may be remembered that an unsuccessful search for an intra-Mercurial planet has actually been conducted in the manner thus suggested by seeing if it could not be observed during the progress of the transit. But the case is very different when a satellite of Jupiter transits over the face of the planet. The lustre of the satellite, arising as it does from sunbeams only, is equal to the lustre of the face of the planet, except in so far as inequalities in the intrinsic reflecting powers of the two bodies may suffice to cause a difference. The shadow of the new satellite on the globe of the planet, though no doubt it would be an extremely small point, would still nevertheless be intensely black in comparison with the surrounding surface, and therefore it might be expected that it ought to be comparatively easy to see when sufficient optical power was available. It must, however, be observed that the diameter of this shadow is considerably less than the tenth of a second, and therefore far too minute to be recognisable as a dark spot. As there is but little variation in Jupiter's distance from the sun, it will be almost equally well displayed at every opposition, if not to observatories in the British Islands, then to observatories elsewhere. Thus, for instance, if the opposition happened to be in June, as will sometimes occur, then, though the planet will be very low down for observers in our latitudes, yet it will be very favourably placed for astronomers in the southern hemisphere. Thus we may hope that we shall speedily accumulate a considerable quantity of observations relative to the new object.

To realise all that is implied by this discovery of an additional moon to the four previously known members of Jupiter's system, it will be necessary to refer to another point. Every one who knows anything of astronomy is aware that the distances of the several satellites from the centre of the planet, and the periodic times in which they revolve, are connected by a definite relation. This is, of course, an immediate inference from Kepler's famous law. It may be worth while to refer to this, as it leads to the recognition of certain important facts in connection with the new discovery. Let us consider, for instance, the innermost of the four well-known satellites of Jupiter. It revolves round its primary in a period which, according to the best determination, may be taken as 1 day, 18 hours, 27 minutes, 34 seconds. We may regard this orbit as circular and the distance of the satellite from the centre of Jupiter as 262,000 miles. In like manner the outermost of the four satellites revolves around Jupiter in a period of 16 days, 16 hours, 32 minutes, 11 seconds, and the length of its radius is 1,170,000 miles. There is a certain relation between the four magnitudes I have named, which is expressed by saying that the squares of the times are proportional to the cubes of the distances. As this law depends upon gravitation it must be obeyed by any new satellite, and here we can foresee that the Barnard moon, whatever else it may do, must at all events revolve in an orbit under such conditions that the cube of its radius bears to the square of the periodic time the same relation as in the case of each of the other satellites.

In estimating the distance of a satellite from its primary, the most natural unit of measurement to adopt is not to be expressed in miles or in thousands of miles. It should rather be given in terms of the equatorial radius of the planet. The sense of proportion is gratified in this way of looking at the matter. This is specially advantageous in the case of Jupiter's moons, and we shall proceed to illustrate it by pointing out the movements that would be appropriate for moons placed at different distances from the centre of Jupiter. The critical case of a moon which was as close as possible to the surface of the planet so as just to graze it, is one of peculiar interest. The moon so circumstanced would have to hurry round its vast circle in something less than three hours. If its pace fell short of that the body would fall into the planet. Were it greater, then the body would fly off into a different path altogether. If a satellite were situated at a distance from Jupiter exactly equal to the radius of the planet at its equator, then the time of revolution would be just a little more than eight hours. With every increase in the distance there is a corresponding increase in the period. I need not follow the matter into any further detail beyond stating that if the distance of the satellite were ten times the radius of the planet, then the periodic time would be about 92 hours.

There is, however, one special case of so much interest that it must not be passed over. We have hitherto said nothing as to the rotation of Jupiter on its axis. Were Jupiter a rigid body throughout its mass, and did it contain neither oceans nor an atmosphere, then the speed at which the planet rotates would have no significance so far as the movements of the satellites were concerned. But, of course, the supposition just made is anything but correct with regard to the constitution of Jupiter. It is doubtful if there are any parts of its vast globe which could be described as rigid, while it is certain that it is enveloped by a prodigious atmosphere. In other words, Jupiter is composed of materials which are liable to tidal influence. This being so, the speed of Jupiter's rotation on his axis is a very important element in the consideration of the movement of his satellites. We may take the period of rotation at 9 hours 551/2 minutes. This would be the periodic time of a satellite which was situated at a distance above the surface of the planet which was about one and a quarter times as much as his radius; more accurately, this magnitude would be 2·273 equatorial radii of the planet distant from the centre of Jupiter. A satellite which revolved in this critical orbit would occupy quite an exceptional position, as the time of its revolution would equal that of the planet's rotation, so that the satellite would be constantly over the same spot of the planet. The planet would, in fact, always bear the same face towards an observer situated on the satellite, just as the moon always turns the same face to the earth. It is, however, certain that the new satellite is somewhat outside this critical position.

The fact that the period of revolution of the new satellite happens to be so near twelve hours leads to a somewhat singular difficulty in determining its movements. It is plain that to find the orbit which it pursues with any precision it would be desirable to combine observations made when it was now at one side and now at the other side of Jupiter. Let us suppose that' at midnight the satellite is at its greatest distance to the east of the planet. It can then, of course, be observed under the most favourable circumstances. Six hours later the satellite, having accomplished half a revolution, will be at the greatest elongation west; but then Jupiter will be so placed that it cannot be observed. Daylight, of course, follows, and in twenty-four hours after the first observation the satellite will have resumed the position which it had at midnight the preceding night. It thus follows that at one observatory it can only be possible for many nights in succession to observe the satellite at one of the sides of Jupiter. If the period of revolution had happened to be an hour or two different from what it actually is, then there would have been no such difficulty. A few nights after one elongation had been observed the other would have been presented in a convenient phase for measurement. As it is, however, Professor Barnard must have his observations supplemented by those at some other observatory in a considerably different longitude. Here is the difficulty. Telescopes of sufficient power to show the new object there may be, but the exceptionally favourable conditions for observation presented on the top of Mount Hamilton are not easily to be paralleled elsewhere. It is, however, generally found that once an object has been discovered it is frequently possible to observe it again with telescopic advantages greatly inferior to those with which the discovery was made.

As to the physical character of the new satellite it seems difficult to offer any surmise. It seems probable that so small an object must be in the solid state. We are in the habit of accounting for the obviously non-solid condition of Jupiter himself by the excessive heat which he still contains. But the new satellite bears to Jupiter a proportion, let us say, of a grain of mustard-seed to a cocoa-nut. It therefore appears that though such a vast bulk as Jupiter may not yet have had time to cool down into the solid form, the same can hardly be averred of its tiny companion. If there should be any fluid materials on the small satellite they must be distracted by the most terrific tides. It is certain that seas on its surface would be submitted to tidal forces at least thirty times as potent as those on the earth.

There cannot be a doubt that Barnard's discovery will afford much occupation for mathematicians. It is hardly possible that it will not be the means of lending a fresh impetus to the study of the entire Jovian system. At the same time we must remember that the detection of the new body does not offer to us, so far as we are at present advised, any interesting information of the same character as that which the satellites of Mars presented. The mass of Mars was an element not very confidently known until the satellites had been discovered, and their distances and periods measured. The mass of Jupiter, however, is one of the most carefully determined elements of the solar system. It has been accurately ascertained by the movements of its satellites, especially of the fourth. There can hardly be a doubt that the value assigned to it is right to within one-thousandth part of the whole. We have, therefore, nothing further of this kind to expect from the new satellite.

It will also be remembered that one of the most astonishing features of the Martian system of moons was the extraordinarily rapid motion of the inner of the two, by which it coursed three times round the planet before the planet turned round once. This fact at the time of its announcement was unique in the whole solar system. There was never a case known before in which a secondary planet accomplished a revolution in less time than the primary accomplished a rotation. And so far as this discovery in regard to Jupiter goes, the peculiar feature of the inner moon of Mars still remains unique. Even though the period of the satellite is only about twelve hours, it is still about two hours longer than the time which Jupiter requires to spin round his axis. We ought, however, here to notice that the rotation of Jupiter is exceptionally rapid. If the great planet required as much time for one of his rotations as does Mars, or the earth, then the new satellite of Jupiter would present the same anomalous feature to its primary as we actually find in Mars and its inner satellite.

There are so many mysteries about Jupiter that we are specially glad to welcome the new satellite in the hope that somehow it will afford a clue to their explanation. We are still ignorant of the true nature of the great red spot which has a period of rotation different from that of the planet itself. Then there are the small black spots which appear to revolve round the planet in a few minutes less time than the planet itself requires for its revolution. Professor Barnard's discovery is one of those achievements which is often succeeded by others of the same class. This has, indeed, been the case. The Fifth Satellite has been followed by a Sixth and a Seventh which have been detected by Photography. Professor Perrine, at the Lick Observatory, has the high honour to have added to our knowledge of the Solar system by the detection of these delicate objects.