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.

[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.

SA