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 1/16th, 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disappointment at the seeming smallness of the disc even with a power double that suggested, this will quickly be superseded by a growing interest in the remarkable picture which is revealed to view. [Illustration: FIG. 25. JUPITER, October 9, 1891, 9.30 p.m.; 3-7/8-inch, power 120.]