Although the strongest features of the disk of Jupiter are the great cloud belts, and the white or colored spots in the equatorial zone, yet the telescope shows many markings north and south of the belts, including a number of narrower and fainter belts, and small light or dark spots.  None of them is absolutely fixed in position with reference to others.  In other words, all of the spots, belts, and markings shift their places to a perceptible extent, the changes being generally very slow and regular, but occasionally quite rapid.  The main belts never entirely disappear, and never depart very far from their mean positions with respect to the equator, but the smaller belts toward the north and south are more or less evanescent.  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.

Nobody has yet suggested an altogether probable and acceptable theory as to its nature.  Some have said that it might be a part of the red-hot crust of the planet elevated above the level of the clouds; others that its appearance might be due to the clearing off of the clouds above a heated region of the globe beneath, rendering the latter visible through the opening; others that it was perhaps a mass of smoke and vapor ejected from a gigantic volcano, or from the vents covering a broad area of volcanic action; others that it might be a vast incandescent slag floating upon the molten globe of the planet and visible through, or above, the enveloping clouds; and others have thought that it could be nothing but a cloud among clouds, differing, for unknown reasons, in composition and cohesion from its surroundings.  All of these hypotheses except the last imply the existence, just beneath the visible cloud shell, of a more or less stable and continuous surface, either solid or liquid.