Jupiter and satellites seen in a small glass.
The four Satellites.—When Galilei directed his telescope to Jupiter on the evening of Jan. 7, 1610, he saw three small star-like points near the planet; so:—
On Jan. 13 he discovered a fourth; thus:—
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:—
| No. and Name. | Mean Distance. | Sidereal Period. h m s | Mean Apparent Diameter. ″ | Real Diameter, in miles. | ||
|---|---|---|---|---|---|---|
| Diameters of Jupiter. | Miles. | |||||
| 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.