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disappointment at the seeming smallness of the disc even with a power double that suggested, this will quickly be superseded by a growing interest in the remarkable picture which is revealed to view.
FIG. 25.
Jupiter, October 9, 1891, 9.30 p.m.; 3⅞-inch, power 120.
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.
Jupiter, January 6, 1906—8 hours 20 minutes. Instrument, 9¼-inch Reflector.
λ = 238° (System 1); λ = 55° (System 2).