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.
[Illustration: 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.
Let us repeat in conclusion that our Earth becomes practically invisible for the inhabitants of the other worlds beyond the distance of Jupiter.
Turn back now for a moment to the plan of the Solar System.
We had to cross 775 million kilometers (480,000,000 miles) when we left the Sun, in order to reach the immense orb of Jupiter, which courses in space at 626 million kilometers (388,000,000 miles) from the terrestrial orbit. From Jupiter we had to traverse a distance of 646 million kilometers (400,000,000 miles) in order to reach the marvelous system of Saturn, where our eyes and thoughts must next alight.