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:— +—————————————+—————————————————————+————————+————————-+————————-+ | | Mean Distance. | | | | |No. and Name.+———————————+————————-+Sidereal|Mean |Real | | |Diameters | Miles. |Period. |Apparent |Diameter,| | |of Jupiter.| | |Diameter.|in miles.| +—————————————+—————————————————————+————————+————————-+————————-+ | | | | h m s| ″ | | | I. Io | 3·03 | 267,000| 1 18 29| 1·02 | 2390 | | II. Europa | 4·72 | 425,000| 3 13 18| 0·91 | 2120 | |III. Ganymede| 7·71 | 678,000| 7 4 0| 1·49 | 3480 | | IV. Callisto| 13·55 |1,193,000|16 18 5| 1·27 | 2970 | +—————————————+—————————————————————+————————+————————-+————————-+ 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.