Leaving the planet itself, we turn to the beautiful system of satellites of which it is the centre. The four moons which, till 1892, were thought to compose the complete retinue of Jupiter, were among the first-fruits of Galileo's newly-invented telescope, and were discovered in January, 1610. The names attached to them--Io, Europa, Ganymede, and Callisto--have now been almost discarded in favour of the more prosaic but more convenient numbers I., II., III., IV. The question of their visibility to the unaided eye has been frequently discussed, but with little result; nor is it a matter of much importance whether or not some person exceptionally gifted with keenness of sight may succeed in catching a momentary glimpse of one which happens to be favourably placed. The smallest telescope or a field-glass will show them quite clearly. They are, in fact, bodies of considerable size, III., which is the largest, being 3,558 miles in diameter, while IV. is only about 200 miles less; and a moderate magnifying power will bring out their discs. [Illustration: PLATE XXIII. 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