Round or oblong spots,
as distinguished from belts, are still more variable and transient. The
main belts themselves show great internal commotion, frequently
splitting up, through a considerable part of their length, and sometimes
apparently throwing out projections into the lighter equatorial zone,
which occasionally resemble bridges, diagonally spanning the broad space
between the belts.
[Illustration: JUPITER AS SEEN AT THE LICK OBSERVATORY IN 1889. THE
GREAT RED SPOT IS VISIBLE, TOGETHER WITH THE INDENTATION IN THE SOUTH
BELT.]
Perhaps the most puzzling phenomenon that has ever made its appearance
on Jupiter is the celebrated "great red spot," which was first noticed
in 1878, although it has since been shown to be probably identical with
a similar spot seen in 1869, and possibly with one noticed in 1857.
This spot, soon after its discovery in 1878, became a clearly defined
red oval, lying near the southern edge of the south belt in latitude
about 30 deg. Its length was nearly one third of the diameter of the disk
and its width almost one quarter as great as its length. Translated into
terrestrial measure, it was about 30,000 miles long and 7,000 miles
broad.
In 1879 it seemed to deepen in color until it became a truly wonderful
object, its redness of hue irresistibly suggesting the idea that it was
something hot and glowing. During the following years it underwent
various changes of appearance, now fading almost to invisibility and now
brightening again, but without ever completely vanishing, and it is
still (1901) faintly visible.
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