twelve o'clock Jupiter will have performed five complete rotations, and
the side originally viewed will be visible again.

Owing to the rotundity of the planet, only the central part of the disk
is sharply defined, and markings which can be easily seen when centrally
located become indistinct or disappear altogether when near the limb.
Approach to the edge of the disk also causes a foreshortening which
sometimes entirely alters the aspect of a marking. It is advisable,
therefore, to confine the attention mainly to the middle of the disk. As
time passes, clearly defined markings on or between the cloudy belts
will be seen to approach the western edge of the disk, gradually losing
their distinctness and altering their appearance, while from the region
of indistinct definition near the eastern edge other markings slowly
emerge and advance toward the center, becoming sharper in outline and
more clearly defined in color as they swing into view.

Watching these changes, the observer is carried away by the reflection
that he actually sees the turning of another distant world upon its axis
of rotation, just as he might view the revolving earth from a standpoint
on the moon. Belts of reddish clouds, many thousands of miles across,
are stretched along on each side of the equator of the great planet he
is watching; the equatorial belt itself, brilliantly lemon-hued, or
sometimes ruddy, is diversified with white globular and balloon-shaped
masses, which almost recall the appearance of summer cloud domes hanging
over a terrestrial landscape, while toward the poles shadowy expanses of
gradually deepening blue or blue-gray suggest the comparative coolness
of those regions which lie always under a low sun.

[Illustration: ECLIPSES AND TRANSITS OF JUPITER'S SATELLITES.

Satellite I and the shadow of III are seen in transit. IV is about to be
eclipsed.]

After a few nights' observation even the veriest amateur finds himself
recognizing certain shapes or appearances--a narrow dark belt running
slopingly across the equator from one of the main cloud zones to the
other, or a rift in one of the colored bands, or a rotund white mass
apparently floating above the equator, or a broad scallop in the edge of
a belt like that near the site of the celebrated "red spot," whose
changes of color and aspect since its first appearance in 1878, together
with the light it has thrown on the constitution of Jupiter's disk, have
all but created a new J