Jupiter, February 17, 1906. J. Baikie, 18-inch Reflector.
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.
CHAPTER XI
SATURN
At nearly double the distance of Jupiter from the sun circles the second largest planet of our system, unique, so far as human knowledge goes, in the character of its appendages. The orbit of Saturn has a mean radius of 886,000,000 miles, but owing to its eccentricity, his distance may be diminished to 841,000,000 or increased to 931,000,000. This large variation may not play so important a part in his economy as might have been supposed, owing to the fact that the sun heat received by him is not much more than 1⁄100th of that received by the earth. The planet occupies twenty-nine and a half years in travelling round its immense orbit. Barnard's measures with the Lick telescope give for the polar diameter 69,770, and for the equatorial 76,470 miles. Saturn's polar compression is accordingly very great, amounting to about 1⁄12th. Generally speaking, however, it is not so obvious in the telescopic view as the smaller compression of Jupiter, being masked by the proximity of the rings.