An interesting Object.—Brightness and Position.—Period &c.—Belts and Spots on the Planet.—Observations of Hooke, Cassini, and others.—The “Ellipse” of 1869-70.—The Red Spot, its appearance, dimensions, and rotation-period.—Bright equatoreal Spots.—Dark Spots in N. hemisphere.—Rotation-period.—Nature of the Red Spot and of the bright and dark equatoreal Spots.—New Belts.—Changes on Jupiter.—Further Observations required.—Occultations of Jupiter.—The four Satellites, and their phenomena.—Occultation of a star by Jupiter.

Of all the planets, Jupiter is the most interesting for study by the amateur. It is true that Saturn forms an exquisite object, and that his wonderful ring-system is well calculated to incite admiration as a feature unique in the solar system. But when the two planets come to be repeatedly observed, and the charm of first impressions has worn away, the observer must admit that Jupiter, with his broad disk and constantly changing markings, affords the materials for prolonged study and sustained interest. With Saturn the case is different. His features are apparently quiescent; usually there are no definite spots upon the belts or rings. There is a sameness in the telescopic views; and this ultimately leads to a feeling of monotony, which causes the object to be neglected in favour of another where active changes are in visible progress.

Brightness and Position.—Jupiter is a brilliant object in the heavens, his lustre exceeding that of Mars or Saturn, though not equal to that of Venus. I have occasionally seen the planet with the naked eye in the daytime, about half an hour after sunrise; and it has been frequently observed by Bond, in America, with the Sun at a considerable altitude. Humboldt and Bonpland, at Cumana, 10° N. lat., saw Jupiter distinctly with the naked eye, 18 minutes after the Sun had appeared in the horizon, on Sept. 26, 1830. The planet is favourably visible for a considerable time every year, and is only beyond reach near the times of his conjunctions with the Sun, when he usually evades observation for about three months. As regards his altitude, Jupiter becomes exceptionally well placed at intervals of 12 years; thus in 1859, 1870-1, and 1882 his declination was 22° or 23° N., and his height therefore very great when passing the meridian. In 1894 he will occupy a similarly auspicious region to observers in the N. hemisphere. In 1865, 1877, and 1889 his declination was 23° S., and he was favourably presented to southern astronomers.

The image of Jupiter as seen in a telescope is involved in a slight yellow tinge, and with the naked eye the same colour is often apparent. But when observed through a very pure transparent atmosphere, his light nearly approaches the silvery lustre of Venus or the Moon. The planet shines with unusual splendour, considering his great distance from the Sun, and his atmosphere must be highly reflective and possibly intensified by inherent light from the planet himself. The central parts of Jupiter’s disk are usually the brightest, as there is a faint shading-off and indefiniteness at the limbs. These and other facts support the view that Jupiter is still incalescent and sufficiently self-luminous to emit a small amount of light.

Period &c.—This planet revolves round the Sun in 4332d 14h 2m, which is equal to more than 11-3/4 years. His orbit is somewhat eccentric, so that his distance from the Sun varies from 506,500,000 to 460,000,000 miles, and the mean is 483,300,000 miles. His apparent diameter ranges from a max. of 50 at a good opposition to 30·4 in conjunction. The planet’s diameter measured along the equator is 88,000 miles, and the polar compression is very marked, amounting to 1/16, or, more exactly, to 1/15·82, according to Engelmann, from a mean derived from eleven observers. When Jupiter is in quadrature there is a slight phase evident in the shading-off of the limb furthest from the Sun.

Belts and Spots on the Planet.—From the time that the telescope became available as a means of astronomical research, it may be readily surmised that an object coming so well within the reach of ordinary appliances, and one displaying so many prominent and variable features, should absorb a large share of attention, and that many facts of interest should have been gleaned as to his physical peculiarities. But it must be confessed that, though something has been learned as to the visible behaviour of the markings, there is much that is perplexing in their curious vagaries. No doubt the vast changes affecting the Jovian envelope, the diversity of the markings, and their proper motions result from the operations of a peculiarly variable atmosphere, affected probably by a heated and active globe beneath it, and by the very rapid movement of rotation to which it is subject.

The telescope, on being turned towards Jupiter, reveals at once an array of dark and light stripes or belts stretching across the disk in a direction parallel to one another and to the equator of the planet. These belts are supposed to have been first detected by Zucchi in 1630. Usually there are two broad and prominent dark belts, one on either side of the equator; while towards the poles other belts appear, some of them very narrow, partly by the effects of foreshortening. The equatoreal zone of the planet is of a lighter tint, and variegated with white and dark spots and streaks, liable to rapid changes, and indicating that this region is in a highly disturbed condition.

Observations of Hooke, Cassini, and others.—Hooke and Cassini were amongst the first to find definite spots on the surface of Jupiter. From 1664 to 1667 a particularly large and distinct spot was frequently seen in the planet’s S. hemisphere. This object disappeared in the latter year, but returned in 1672, and was seen until the close of 1674, when it again temporarily vanished, to reappear at subsequent epochs. Cassini was enabled to determine the rotation-period from this spot. He found that the markings in the immediate vicinity of the equator moved with greater celerity than those in higher latitudes, the difference in their rotation-periods being nearly 6 minutes. A century later Sir W. Herschel confirmed these results: he saw a bright spot which completed a rotation in nearly 5 minutes less time than several dark spots. SchrÖter also made many observations, and noted frequent changes in the spots and differences in their rotation-periods. He watched a bright object near the equator which had a period more than 5 minutes less than some dark spots. In later years MÄdler and others followed up the investigation of these markings, and with nearly similar results. The various spots were undoubtedly affected by proper motions, enabling them to yield discordant rotation-periods. Bright forms near the equator moved with great rapidity and effected a rotation in about 9^h 50^m, while dark spots on either side of it occupied between 9^h 55^m and 9^h 56^m. The markings were evidently controlled by currents of different velocities in the planet’s atmosphere.

Dawes, in 1849 and following years, noted luminous spots, like satellites in transit, on a belt in the planet’s S. hemisphere. In October 1857 he observed a group of eleven of these objects; and in 1858 Lassell saw many similar appearances in a bright belt near the equator.

The Ellipse of 1869-70.—In 1869 and 1870 Gledhill, of Halifax, and Prof. Mayer, of the Lehigh University, saw a remarkable formation just south of the great belt lying on the S. side of the equator. It was in the form of a perfect ellipse, ruddy in colour, and very distinct in outline. Its major axis was parallel with the belts. It was first observed on Nov. 14, 1869, and had disappeared in July 1870, though on Dec. 1, 1871, a similar elliptic ring was seen resting on the S. equatoreal dark band.

The Red Spot.—In July 1878 a large spot, of oval form and intense red colour, appeared in about the same latitude as the ellipse seen by Gledhill and Mayer in 1869-70. It was first announced by Dennett of Southampton, though it appears to have been seen a few weeks earlier by Prof. Pritchett, of Missouri, U.S.A. The object alluded to soon attracted general notice; and as it continued visible during the oppositions of 1879, 1880, and 1881 under the same striking aspect, it created a considerable stir among telescopists, and the “great red spot on Jupiter” became familiarly known both in appearance and in title.

No planetary marking in modern times has enlisted half the amount of attention that has been devoted to this object. It has endured amid all the turmoils of the Jovian atmosphere for twelve years, and has preserved an integrity of form and size which prove it to have been singularly capable of withstanding disruption. But its tint has varied greatly; so that at times the oval outline of the spot could hardly be discerned amongst the contiguous belts. In the winter of 1881 the interior of the ellipse began to lose tone, and in 1882 it faded rapidly, so that the central region of the spot assumed nearly the same light tint as the outlying bright belts. Apparently the spot had either been filled up with luminous cloudy material or had been partially obscured by the interposition of matter situated higher in the Jovian atmosphere. The elliptical contour of the object was still intact, however, though it had quite lost its bold and prominent character. Only the skeleton of its former self remained, and its entire disappearance seemed imminent. But further decadence was fortunately averted by influences unknown to us, and the spot has continued visible to this day, though shorn of the attributes which roused so much enthusiasm amongst observers more than ten years ago.

From measures at Chicago, in the years from 1879 to 1884, Prof. Hough found the mean dimensions of the spot to be:—Length 11·75, breadth 3·71. These figures represent a real length of 25,900 miles and a diameter of 8200 miles. The latitude of the spot was 6·97 S.

This object has served an important end in attracting wide-spread observation, not only to itself, but to the general phenomena occurring on the surface of Jupiter. Observers, in studying the red spot, were also led to study the bright equatoreal spots and other features so plentifully distributed over the disk. It was most important this should be done; for since the time of Herschel and SchrÖter not much progress had been made in elucidating the proper motions of the spots and finding an accurate rotation-period for the planet. Dawes, Lassell, and many others had, it is true, secured some interesting observations and drawings, but not of the special kind required, and thus no fresh light had been thrown upon the vagaries in the behaviour of the spots, as described by the old observers. But a mass of new facts were now to be realized. Schmidt at Athens, Prof. Hough at Chicago, A. Stanley Williams at Brighton, and many others, including myself at Bristol, began systematic observations of Jupiter, with a view to learn something more of the periods, changes, and general characteristics of the spots and other features. The results were of an interesting nature, though too extensive for more than bare mention here. In 1879 the red spot gave a rotation-period of 9^h 55^m 34^s·2, but this increased to 9^h 55^m 35^s·6 in 1880-1 and to 9^h 55^m 38^s·2 in 1881-2. During the ensuing three years the period was almost stationary at 9^h 55^m 39^s·1, but in 1885-6 it further augmented to 9^h 55^m 41^s·1, since which year it has ranged between 9^h 55^m 40^s and 41^s. From ten years’ observations, the mean period of the red spot is as nearly as possible 9^h 55^m 39^s.

Bright Equatoreal Spots.—The bright spots near the equator rotated in 9^h 50^m 6^s in 1880; but in subsequent years the time slightly increased, for in 1882 I found it 9^h 50^m 8^s·8, and in 1883 9^h 50^m 11^s·4. The bright spots therefore perform a rotation in 5½ minutes less time than the red spot. The former move so much more swiftly than the latter that they pass it at the rate of 260 miles per hour, and in 44½ days have effected a complete circuit of Jupiter relatively to it. Thus a brilliant white spot, if noticed in the same longitude as the red spot on one night, will, on subsequent nights, be observed to the W. of it, and, after an interval of about 44½ days, the same objects will again occupy coincident longitude.

Dark Spots in N. hemisphere.—In the autumn of 1880 there was a confluent outbreak of dark spots from a belt in about 25° N. latitude, and these exhibited a rotation-period of only 9^h 48^m, so that they travelled more rapidly than the white spots on the equator. Some short dusky belts were also remarked slightly S. of the latitude of the red spot, and these indicated a period of 9^h 55^m 18^s. It is clear from these various results that the motion of the Jovian markings does not decrease according to their distance from the equator.

Rotation-Period.—Below are given the times of rotation ascertained by some previous observers:—

A: Bright spots near the equator of Jupiter.

B: Herschel’s observations embraced few rotations, and the periods he derived differed considerably.

C: SchrÖter also alleges he saw spots return to the same part of the disk in 7^h 7^m, 7^h 36^m, and 8^h 1^m!

D: From ten years’ observations of the red spot.

The foregoing list is by no means complete, for, owing to the large number of recent determinations, I have thought it advisable to omit some of them.

It should be mentioned here that the above times of rotation are derived from atmospheric features more or less volatile in nature, and that therefore the actual sphere of Jupiter rotates in a period which we have not precisely discovered. No doubt the motion of the real surface is not very different from that of the atmospheric markings above it. There is reason to think that, whatever the character of the planet’s crust may be, we have never yet obtained a glimpse of it. A dense veil of impenetrable vapours appears to surround the globe on all sides, and this is subject to violent derangement from the evolution of heated material or gaseous fluids from the surface below. These disturbances seem to be very durable in some instances as to their observed effects. The atmosphere would, in fact, appear to possess a singular capacity for retaining the impressions of its changes. The permanency of certain spots can hardly be due to continued action from those parts of the disk immediately underlying them; for their variable motions soon transport them far from the places at which they were first seen, and prove their existence to be quite independent of their longitude.

Nature of the Red Spot.—There is much in connection with the red spot that remains in mystery. Its dimensions, form, and motion have severally been ascertained within small limits of error, and the alterations in its tint and degree of visibility have been recorded with every care. But we can only conjecture as to the origin, character, and end of this remarkable formation. What agency produced it, and moulded the definite elliptical outline it has always preserved—what forces control its oscillations of speed, and keep it suspended so long in the aerial envelope of Jupiter—are matters of pure theory. When, in July 1878, it first came under notice it was a well-developed object, and though Russell in 1876, Lord Rosse and others in 1873, and Gledhill and Mayer in 1869-70 had delineated forms suspiciously like the red spot and situated in the same latitude, yet the several features may not have been absolutely identical, for nothing was seen of the spot in 1877 or in some other years. But there is a strong probability that the red ellipse of 1869-70 must have been the red spot in an incipient stage of its formation. The object may have undergone temporary obscuration, similarly to Cassini’s spot two centuries ago.

My own opinion of the spot is that it represents an opening in the atmosphere of Jupiter, through which, in 1878-82, we saw the dense red vapours of his lower strata, if not his actual surface itself. Its lighter tint in recent years is probably due to the filling-in of the cavity by the encroachment of durable clouds in the vicinity. Parts of some of the more prominent belts display an intense red hue like that formerly shown by the red spot, and they may be due to the same causes. Extensive fissures are probably formed in the atmosphere, and quickly distended in longitude by the natural effect of the planet’s tremendous velocity of rotation. It is curious, however, that these rents, after a certain distention, assume a durable outline until they lose their colouring and are temporarily if not finally obliterated.

When the red spot was visible under its best conditions I frequently examined it, hoping to detect some mark well in its interior which might serve as a clue to the true rotation-period of the sphere of Jupiter. For if the spot consisted of a clear patch in the planet’s atmosphere, I thought it possible some real object on the surface might be discerned through it, in which case the difference in its motion and that of the red spot would enable the rate of motion of the globe to be found. If the spot moves more slowly than the planet, then a surface-marking must appear to pass from the E. to the W. side of the spot; but no such evidence could be obtained, owing to the absence of suitable markings. The red tint of the great spot seemed very general over the entire area of the ellipse until its central regions paled in 1882. There were two dark specks, one at the E. and another at the W. extremity of the spot; but these were unchangeable as regards position.

The spot, though placed very near the border of the great S. belt, has never been connected with it, though in Jan.-Feb. 1884, May 1885, and March-April 1886 the spot became temporarily attached to a belt on its S. side. There was some controversy as to this feature, Prof. Hough, from observations with the 18½-inch refractor at Chicago, alleging that at no time had the spot coalesced with or been joined to any belt in its vicinity. But in 1886 many observers succeeded in detecting the junction of the markings alluded to, and Prof. Young gave a drawing of the appearance as seen with power 790 on the 23-inch objective at Princeton (see ‘Sidereal Messenger,’ vol. v. p. 292). The spot and belt were probably at different heights in the Jovian atmosphere, so that there was no commingling of material, one object being simply projected on the other, for the elliptical form of the red spot remained visible all the time. The latter moves more slowly than the connecting belts, and, when clear of them, is often seen with a white aureola fringing its environs.

Bright Equatoreal Spots.—These are affected by rapid changes of form, brightness, and motion. Sometimes they are exceedingly bright; at other periods they are quite invisible. This intermittency is not occasioned (as I assured myself by many observations) by the total extinction of spots and appearance of new ones, but is due to the temporary obscuration of the same objects. The variations are irregular, and probably depend upon phenomena also irregular. The motion of these objects often shows great deviations from their average rate; they are sometimes much in advance of or behind their computed positions. One fine spot of this class was closely watched in 1880 and following years. It was usually in the shape of a brilliant oval, well defined, and occasionally quite as large as the third satellite of Jupiter; but it was sometimes seen as a diffused white patch, apparently emerging from the N. edge of the belt. Whenever the spot was very bright, there was a trail of light or luminous matter running eastwards from it, as though there were an eruption of shining material from the spot, which the rapid rotation of the planet from W. to E. caused to drift in an opposite direction.

Dark Equatoreal Spots.—Closely contiguous to the white spots there are almost invariably seen very dark spots, much deeper in tone than the dark belt upon which they appear to be projected. It has been suggested that these dark spots are shadows from the white spots, which may be elevated formations protruding through the envelope of Jupiter. This idea seems to me untenable; for the dark spots have been distinguished under a vertical Sun, and sometimes they are found one on each side of a white spot. Again, an intensely brilliant spot is occasionally seen without any outlying condensation of dark matter. But though they are not shadows, the dark equatoreal spots certainly have an intimate relation with the brighter markings near them and move with the same velocity.

It is proved from many observations that the longer an object is observed the slower becomes its rate of rotation. Sir W. Herschel found the converse. In discussing his results of 1778 and 1779, he said:—“By a comparison of the different periods it appears that a spot gradually performs its revolutions in less time than at first” (Phil. Trans. 1781, p. 126). But his periods were each based on less than fifty rotations, so that no certain conclusions could be derived from them.

In recent years the rapidly moving bright spots have usually appeared in the equatoreal side of the great S. dark belt. The polar side of the great N. belt also exhibits bright spots, but these rotate in a period only a few seconds less than that of the red spot. Bright spots are also observed to the S. of the latter object and on other portions of the disk.

As to the belts, they are usually straight; but cases are recorded of slant-belts, in which the direction has been very oblique. One of these was noticed in the planet’s N. hemisphere in Mar.-April 1860, and another was seen in the S. hemisphere in Jan. 1872. I observed one near the N. polar shading in Dec. 1881.

New Belts.—The formation of the dark belts seems to be brought about gradually, and they appear to be sustained in certain cases by eruptions of dark matter, which gradually spread out into streams. On Oct. 17, 1880, two dark spots, separated by 20° of longitude, broke out on a belt some 25° N. of the equator. Other spots quickly formed on each side of the pair alluded to, and distended themselves along the belt so that by Dec. 30 they covered three fourths of its entire circumference. At the middle of January the spots formed a complete girdle round the planet; but they became much fainter, and were soon eradicated by combination with the belt on which they had appeared.

Changes on Jupiter.—Prof. Hough, of Chicago, is adverse to the opinion that rapid changes occur on Jupiter, and mentions the stability of the red spot and other markings in support of his views. He believes that the erroneous statements about sudden changes made by both ancient and modern astronomers are largely due to differences in the telescopic images due to atmospheric variations. No doubt such an explanation will suffice to meet some instances, and the swift rotation of the planet may also have been the unsuspected cause of some of the extraordinary changes described; but there are real variations as well. These are very frequent in the planet’s equatoreal zone.

Further Observations required.—Drawings of Jupiter obtained under the highest powers that may be employed with advantage, and with a cautious regard to faithful delineation, will probably throw much light on the phenomena occurring in this planet’s atmosphere. And it is most desirable to pursue the various markings year after year with unflagging perseverance; for it is only by such means that we can hope to unravel the extraordinary problem which their visible behaviour offers for solution. Too much stress cannot possibly be laid on the necessity of observers being as precise as possible in their records. The times when an object comes to the central meridian should be invariably noted; for this affords a clue to its longitude, and a means of determining its velocity. Its position, N. or S. of the equator, should be either measured or estimated; and alterations in tone, figure, or tint described, with a view to ascertain its real character.

The climate of England is very ill-adapted to an investigation of this sort, where the most needful point consists in frequency of observation. If the markings on Jupiter could be re-examined every night, and traced through their changes, an explanation of certain phenomena exhibited by them would soon be forthcoming. The interrupted character of previous observations destroys much of their value. Closely consecutive results are necessary to remove doubts as to the identity of the objects observed; so that, in such a research, natural advantages of position are more desirable than instrumental advantages, for the latter are impotent in a cloudy atmosphere.

The red spot must be watched as long as any vestiges of it remain. Its variations of speed may ultimately yield indications of periodicity39; so may its alterations of tint. The belts in the vicinity of the spot demand an equal share of attention; for it may be possible to divine from their changes whether there are any links of association between them and the red spot. In recent years the latter has apparently repulsed the belts on its N. side, though suffering encroachments from those on its S. side.

The equatoreal spots also deserve continued vigilance on the part of observers. It has already been stated that the bright spots vary rapidly; their motions are not uniform in rate, and what is now wanted is a large number of new observations. Does accelerated velocity occur with increased brilliancy of these objects? Are their alternating disappearances and revivals uniform in period? and are they really due to transitory obscurations of the same durable forms? Are the dark spots which frequently border the white spots implicated in effacing the latter? Many other questions like these are suggested by the curious behaviour of the markings, and the discriminating observer will know how to gather the materials likely to aid in answering them. The rotation-period has been already found in regard to many features; but this element may be re-investigated with profit, for the velocity of the spots offers a very complex problem for solution. Do the markings generally exhibit a retardation of speed as long as they subsist? Abnormal spots, such as those which made their apparition in the autumn of 1880, should be traced through any vagaries they may present; and peculiar shape or direction in the belts will also merit study, as possibly supplying facts of consequence. It will be important to learn whether objects in a certain latitude have a common rotation-period, or whether different forms give different times. The rate of motion shown by certain features may depend upon their character, and not so much upon their position in latitude.

The altitudes of the various markings affords another promising line of research. The appearances and changes of closely contiguous features may be expected to furnish useful data in this connection. Owing to their proper motions they apparently overlap each other at times, and in their alterations of aspect the observer may discover the clue to their relative heights. The subject is discussed in a practical and interesting way by Mr. Green (Memoirs R. A. S. vol. xlix. p. 264) and by Mr. Stanley Williams (‘Zenographic Fragments,’ i. p. 112), and these works should be consulted by everyone engaged in the study of Jovian phenomena.

It is unfortunate that the observer, in delineating this object, must perforce adopt an extremely hurried method of representing what he sees at the telescope. The planet turns so quickly upon his axis that forms near the central meridian become sensibly displaced in a few minutes; indeed, it has been stated that an interval of two minutes only is sufficient to introduce a change obvious to simple eye-estimation. In order, therefore, to complete a sketch, the utmost dispatch is requisite; for this object cannot be depicted from the combined outcome of several evenings of observation. The proper motions of the different features prevent this. With Mars, or any orb exhibiting markings relatively constant, collective results are extremely valuable, and more trustworthy than pictures depending upon an isolated observation.

Amateurs, in entering upon these observations, should be prepared for rapid changes in the apparent aspect of Jupiter caused by his rotation, and not hastily infer them to be real. They should also hesitate before placing confidence in any anomalous results obtained under indifferent seeing; for bad images have been directly responsible for many misleading announcements.

Occultations of Jupiter by the Moon.—Phenomena of this kind are always awaited with keen interest by the possessors of telescopes; but it is rarely that all the circumstances are favourable. The first recorded instance appears to have been in A.D. 847. In 1792, on April 7, SchrÖter observed an occultation of this planet, with a desire to verify his suspicion of a lunar atmosphere. He saw that “some of the satellites became indistinct at the limb of the Moon, while others did not suffer any change of colour. The belts and spots of Jupiter appeared perfectly distinct when close to the limb of the Moon.” On Jan. 2, 1857, an occultation took place under conditions which rendered it visible to many observers in this country, and the most interesting fact elicited was that at emersion a dark border was seen attached to the arc of the Moon projected on the planet. Mr. Lassell described this dark border as “a shadowy line, in character, magnitude, and intensity extremely like Saturn’s obscure ring projected on the ball.” During the thirty years following 1859 only two occultations visible in England occurred, and the last of these, on August 7, 1889, was widely observed. On this occasion Capt. Noble and others redetected the shadowed edge of the Moon seen by Lassell in 1857. “It was a strongly marked shading, following the outline of the Moon’s limb.” At Bristol I recorded that, at the disappearance, the outer margin of our satellite was fringed with light where it crossed the planet; but at the reappearance this effect had vanished, and the appearance was perfectly normal. The disk of Jupiter, where it met the edge of the lunar disk, looked dusky by the effects of contrast; but I saw no marked shading with a sharply terminating boundary, such as appears to have been remarked elsewhere. As the planet emerged definition was superb, the belts were lividly distinct, and the spectacle was one of the prettiest that could be imagined. The red spot was going off the W. limb, and the disk was covered with belts; many of them near the poles were extremely narrow, like fine lines drawn with a sharp lead pencil. I used a 4-inch refractor, powers 65 and 145, with this instrument the foregoing sketch was made. The exceptional distinctness of the Jovian markings on this occasion shows that the proximity of the Moon has certainly no tendency to efface planetary details, but rather to intensify them40.

On Sept. 3, 1889, an occultation of Jupiter was visible in America, and observed by Mr. Brooks at Geneva, N.Y., with a 10-1/8-inch equatoreal. His drawing, made from a photograph and eye-observations, shows nothing of a dark fringe bordering the Moon’s limb.

The four Satellites.—When Galilei directed his telescope to Jupiter on the evening of Jan. 7, 1610, he saw three small star-like points near the planet; so:—

On Jan. 13 he discovered a fourth; thus:—

and ascertaining that these bodies followed Jupiter in his course, concluded them to be moons in attendance upon him. At first the discovery was discredited by others; but it soon had to be accepted as an incontestable fact of observation. These satellites are usually among the very first objects which the amateur views in his telescope, and they form, in combination with their primary, an exquisite picture, the impression of which is not soon forgotten. The periods, distances, &c. of the satellites are as follows:—

The third satellite is much the largest, and its brightness is about equal to that of a star of the 6th mag. The other three may be rated as generally 7th mag., though their brightness is variable, especially that of the fourth satellite, which has been seen exceedingly faint.

It is customary to distinguish these objects, not by their names, as in the case of the moons of Mars, Saturn, and Uranus, but by the Roman numbers affixed to them progressively according to their distances from Jupiter.

The satellites are just visible to the naked eye when the conditions favour their detection; but they are so much involved in the rays of the planet, and often so near to him, that it may be regarded as an exceptional feat to discern them without telescopic aid. When III. and IV. are near their max. elongation and on the same side of the planet, they have been occasionally observed separately. I. and II., though much closer to Jupiter and more within the influence of his glare and rays, have been similarly seen. When attempting such observations it is best to hide the bare disk of the planet behind some terrestrial object, as this will cut off the obnoxious rays and prevent the brilliant light from dazzling the eye. An opera-glass, or any small portable telescope, reveals the whole retinue of satellites, and enables them to be traced through their revolutions. The ‘Nautical Almanack’ gives diagrams of their diurnal positions, and with this work as a reference observers will find no difficulty in identifying them apart.

Sir W. Herschel, in the years 1794 to 1796, found that the satellites revolve on their axes in the same time that they revolve about the planet. He was led to this conclusion by a study of the variations in the light emitted by the satellites in different parts of their orbits, and described I. as “of a very intense bright, white, and shining light,—brighter than II. or IV. (not larger). IV. inclines to red, and nearly as bright as II. The latter is of a dull ash-colour. III. is very white.” Modern observers have selected II. as relatively the most highly reflective, while IV. is the least. Spots exist on the surfaces of these objects, and probably occasion many of the differences observed.

The eclipses, occultations, and transits of the satellites afford a very fertile and attractive series of phenomena for telescopic review. The exact times of occurrence are tabulated in the ‘Nautical Almanack’ and asterisks are affixed to such as are visible in this country. Prior to the date of opposition of Jupiter the eclipses occur of course on the W. side of the disk, while after opposition they take place on the E. side. The durations are as follow for the several satellites:—I. = 2^h 20^m, II. = 2^h 56^m, III. = 3^h 43^m, IV. = 4^h 56^m. In reference to III. and IV. the entire phenomenon may be generally observed; but this is not so in regard to II., as the emersions are frequently effected behind the planet. Only the immersions of I. are visible before opposition, from the same cause; for the satellite enters the cone of shadow close to the planet’s limb, and only comes out of it when the globe of the planet is interposed in the line of sight. In such cases the satellite emerges soon after from the limb of Jupiter; so that its obscuration has been compounded of two separate phenomena, viz. an eclipse and an occultation. After opposition this satellite is first occulted and then eclipsed. IV. sometimes escapes eclipse altogether, by passing above or below the shadow.

The motion of light was discovered, and its velocity determined, by means of the eclipses of Jupiter’s satellites. These phenomena are also useful in ascertaining longitudes. A spectator on Jupiter himself would see a vast number of solar and lunar eclipses—about 4500 of each—during the Jovian year of 4332·6 days, because the three inner satellites exhibit these phenomena at every revolution, their orbits being very slightly inclined to Jupiter’s equator, and the latter being but little inclined to the plane of the ecliptic.

The occultations of the satellites are comparatively frequent, and may be well observed in a good telescope. A tolerably high magnifier is required to witness these occurrences with the best effect, the disks of the satellites being small and not clearly traceable through the various stages of their disappearances unless much amplified. With considerable telescopic power the disks are well seen, and it then becomes feasible to watch the satellites, first as they come into contact with the limb, then as the globe of the planet overlaps more and more of their diminutive forms, and finally as they reach last contact and withdraw their narrow unobscured segments behind the expansive sphere of their primary. Both the beginning and end phase of these occultations is generally observable in regard to Sat. IV., and frequently also in the case of III. But with reference to II. and I. it often happens that only the disappearance or reappearance can be witnessed. These occultations have furnished some singular and unexplained facts of observation. On meeting the limb of Jupiter Sats. I. and II. have not always disappeared in a normal way. On April 26, 1863, Wray, with an 8-inch objective, saw II. distinctly projected within the limb for nearly 20^s. Other similar cases are recorded. The satellites have been seen apparently “through the edge of the disk.” One observer mentions that II. appeared and disappeared several times before occultation. The explanation appears to be that there is so much irradiation round the disk of Jupiter that it produces a false limb, and it is through this the satellites have been seen. A very tremulous image, in bad air, may also be responsible for some of the anomalies recorded.

The transits of the Jovian moons offer the most attractive phenomena of all, and they come well within the reach of small telescopes. On entering upon the planet they are visible as bright round spots projected on the dusky limb, and subsequently present some eccentric features. II. is invisible, except on the limbs; I. is often seen as a grey spot threading along the belts; III. appears as a large dark spot41, nearly as black as its shadow; IV. seems to be black, and scarcely to be recognized from its shadow. The appearances are certainly to some extent variable. Mr. Stanley Williams has seen III. as a brilliant disk at mid-transit. I. sometimes crosses the whole disk as a white spot; at certain other times it is invisible; at others, again, it is seen as a faint grey spot. IV. is not always black, its aspect depending upon the chord it traverses. Thus, on the evening of Sept. 12, 1889, Mr. Williams, Mr. G. T. Davis of Reading, and myself were observing Jupiter when IV. was in transit on a belt in the N. hemisphere, but not a vestige of the satellite was seen by any of us. On the morning of May 23, 1890, at 3^h 30^m A.M., however, while observing the red spot on Jupiter, I noticed a black circular spot on the great N. equatoreal belt; and this proved to be IV. in transit. These peculiarities have been accounted for as partly due to contrast and partly to dusky spots on the surfaces of the satellites. Dr. Spitta has made a number of experiments to elucidate this subject, and concludes that “the perpetual whiteness of the second satellite, and the darkened tints of the others during transit, are due to differences in their relative albedo [reflective power] as compared with that of Jupiter, and are not dependent upon the relative quantity of light reflected by one or the other, or upon any physical peculiarities of the Jovian system.”

The shadows of the satellites transit the disk as dark spots larger than the satellites themselves, owing to the penumbral fringes. Before opposition these shadows precede the satellites; after opposition the latter come first. The shadow of II. appears to be much lighter than the others, and is usually of a pale chocolate-colour; and I saw it thus at the opening of the year 1885:—

Sat. II. is probably involved in an atmosphere sufficiently dense to enable it to present undue luminosity relatively to the others; and if so, the feeble shadow it transmits on Jupiter may be partly explained by the effects of refraction. On the day of opposition both satellites and shadows are projected on the same part of the disk, and the latter are occulted by the former. On Jan. 14, 1872, Mr. F. M. Newton saw I. centrally placed on its shadow; so that the satellite was apparently surrounded with a ring of shade. On May 13, 1876, Mr. G. D. Hirst saw Sat. I. partly occulting its own shadow; a black crescent was seen in the bright zone N. of the equator. On Feb. 18, 1885, Dr. R. Copeland, at Dun Echt, saw the shadow of I. “almost totally occulted by the satellite itself; as the satellite approached Jupiter’s limb it came out quite bright and large, with a mere crescent of the shadow showing on its southern edge.” This phenomenon was also observed at Bristol.

Occasionally all the satellites become invisible at the same time, being either eclipsed, occulted, or in transit. An instance of this kind was recorded by Molyneux on Nov. 2, 1681 (O.S.). Sir W. Herschel observed a similar occurrence on May 23, 1802; also Wallis on April 15, 1826, and Dawes and others on Sept. 27, 1843, and Aug. 21, 1867. A visible repetition of the event was narrowly avoided on the morning of Oct. 15, 1883. On this occasion the planet should, according to the ‘Nautical Almanack,’ have been denuded of his satellites for a period of 19 minutes; but this disappearance did not occur, for at no time were all the satellites included within the margin or shadow of Jupiter. I observed that Sat. III. entered upon the disk just as IV. released itself, and the two formed a curious configuration at 4^h A.M., hanging close upon the planet’s limbs.

Spots have been seen on the satellites both in transit and while shining on the dark sky. This particularly refers to III. and IV. II. has never given indications of such markings on its bright uniformly clear surface. Dawes, Lassell, and Secchi frequently observed and drew the spots. Secchi described III. as similar in aspect to the mottled disk of Mars as seen in a small telescope; his drawings exhibit no analogy, however, to those by Dawes of the same object. III. has been remarked of a curious shape, as if dark spots obliterated part of the limbs. Sat. I. was observed in transit on Sept. 8, 1890 by Barnard and Burnham, and it appeared to be double, being divided by a bright interval or belt. They used a 12-inch refractor, powers 500 and 700, and the seeing was very fine.

Many other curious points have been noticed in the various aspects and phenomena of the Jovian satellites. Further observations will doubtless throw new light on some of the puzzling records of the past.

Occultation of a Star.—An occultation of the 7th mag. star 4 Geminorum by Jupiter took place on Nov. 7, 1882, and it was observed by Prof. Pritchett, of Glasgow, Missouri, with a 12-1/4-inch equatoreal, power 200. “The images of both planet and star were very steady. The margin of Jupiter’s disk was very sharply defined. The immersion was very near the N. border of the broad S. equatoreal belt. At 11^h 28^m 10^s·65 local mean time the star was apparently within the dark outline of the disk, apparent geometric contact having occurred at 11^h 20^m 24^s·49. For a moment the star seemed to disappear, but a moment later was plainly seen, as if through a well-defined notch in the otherwise continuously even margin. This notch lasted 46^s·26, and at 11^h 28^m 56^s·91 it vanished, and the light of the star was entirely extinguished.” The emersion of the star could not be observed, as clouds supervened.