This giant gravitates in space accompanied by a suite of five satellites. These are:
| Names. | Distance from surface of Jupiter. | Time of revolution. | ||
| Kilometers. | Miles. | Days. | Hours. | |
| 5. | 200,000 | 124,000 | 11 | |
| 1. Io | 430,000 | 266,000 | 1 | 18 |
| 2. Europa | 682,000 | 422,840 | 3 | 13 |
| 3. Ganymede | 1,088,000 | 674,560 | 7 | 4 |
| 4. Callisto | 1,914,000 | 1,186,680 | 16 | 16 |
The four principal satellites of Jupiter were discovered at the same time, on the same evenings (January 7 and 8, 1610), by the two astronomers who were pointing their telescopes at Jupiter: Galileo in Italy, and Simon Marius in Germany.
On September 9, 1892, Mr. Barnard, astronomer of the Lick Observatory, California, discovered a new satellite, extremely minute, and very near the enormous planet. It has so far received no name, and is known as the fifth, although the four principal are numbered in the order of their distances.
Fig. 46.—Jupiter and his four principal satellites.
The four classical satellites are visible in the smallest instruments (Fig. 46): the third is the most voluminous.
Such is the splendid system of the mighty Jupiter. Once, doubtless, this fine planet illuminated the troop of worlds that derived their treasure of vitality from him with his intrinsic light: to-day, however, these moons in their turn shed upon the extinct central globe the pale soft light which they receive from our solar focus, illuminating the brief Jovian nights (which last less than five hours, on account of the twilight) with their variable brilliancy.
At the distance of the first satellite, Jupiter exhibits a disk fourteen hundred times vaster than that of the Full Moon! What a dazzling spectacle, what a fairy scene must the enormous star afford to the inhabitants of that tiny world! And what a shabby figure must our Earth and Moon present in the face of such a body, a real miniature of the great solar system!
Our ancestors were well inspired when they attributed the sovereignty of Olympus to this majestic planet. His brilliancy corresponds with his real grandeur. His dominion in the midnight Heavens is unique. Here again, as for Venus, Mars, and Mercury, astronomy has created the legend of the fables of mythology.