into the field of view, with
his _cortege_ of satellites, like a yellow-and-red moon, attended by
four miniatures of itself. You instantly comprehend Jupiter's mastery
over his satellites--their allegiance is evident. No one would for an
instant mistake them for stars accidentally seen in the same field of
view. Although it requires a very large telescope to magnify their disks
to measurable dimensions, yet the smallest glass differentiates them at
once from the fixed stars. There is something almost startling in their
appearance of companionship with the huge planet--this sudden
verification to your eyes of the laws of gravitation and of central
forces. It is easy, while looking at Jupiter amid his family, to
understand the consternation of the churchmen when Galileo's telescope
revealed that miniature of the solar system, and it is gratifying to
gaze upon one of the first battle grounds whereon science gained a
decisive victory for truth.

The swift changing of place among the satellites, as well as the
rapidity of Jupiter's axial rotation, give the attraction of visible
movement to the Jovian spectacle. The planet rotates in four or five
minutes less than ten hours--in other words, it makes two turns and four
tenths of a third turn while the earth is rolling once upon its axis. A
point on Jupiter's equator moves about twenty-seven thousand miles, or
considerably more than the entire circumference of the earth, in a
single hour. The effect of this motion is clearly perceptible to the
observer with a telescope on account of the diversified markings and
colors of the moving disk, and to watch it is one of the greatest
pleasures that the telescope affords.

It would be possible, when the planet is favorably situated, to witness
an entire rotation of Jupiter in the course of one night, but the
beginning and end of the observation would be more or less interfered
with by the effects of low altitude, to say nothing of the tedium of so
long a vigil. But by looking at the planet for an hour at a time in the
course of a few nights every side of it will have been presented to
view. Suppose the first observation is made between nine and ten o'clock
on any night which may have been selected. Then on the following night
between ten and eleven o'clock Jupiter will have made two and a half
turns upon his axis, and the side diametrically opposite to that seen on
the first night will be visible. On the third night between eleven and