Yet there are some considerations that tend to disprove the cloud theory. Barnard mentions that the semi-permanent nature often shown by the markings is against it. The great Red Spot has been visible at intervals for a hundred years or more, and other markings may be traced with but little change of form for months, although these objects are free to drift around and their rotation periods are not constant. Professor Barnard, using the powerful Yerkes telescope, said that when seen under the finest conditions the appearance of the surface is more of a pasty nature. The great oval-shaped spot mentioned above and famed as the "Great Red Spot" appeared in the southern hemisphere in 1878, or at least it appeared more distinctly on that date, for early drawings show that traces of it had been observed long before that. It was at first of a very strong red color and stretched above the equator for a distance of 30,000 miles. It is now of a delicate pinkish tinge, extremely faint but still visible. The cause of this prominent marking is not known. It has been suggested that it might have been a gigantic eruption. Gigantic indeed, if it covered 30,000 miles! It also has a curious repellent nature, for all the belts and spots that come near it, instead of passing onto it or over it, seem to be forcibly pushed to one side and held at a certain distance until they have completely passed this whatever-it-is. It has also been suggested that a new continent may be forming here, but the fact that this feature like other features on Jupiter's apparent surface does not seem to have a fixed position has caused it to be likened rather to a vast drifting island. This "vast drifting island" sometimes moves fast, sometimes slowly and sometimes remains perfectly stationary! The last suggestion was brought forward before the more or less irregular drift was noticed which seems to prove after all that it could not be a true part of the real surface of Jupiter. If Jupiter were composed of materials as dense and solid as the earth, he would not be as large a planet as he now is. The materials of which this great world is composed are only about one fourth as dense as the earth or 1.25 as dense as water. [Figure 150: JUPITER AND FOUR OF HIS MOONS. Photograph by the Yerkes Observatory.] The axis of the planet is nearly perpendicular to the plane of its motion, the inclination being only about three degrees from the perpendicular. Thus there is no great variety of seasons on Jupiter although his year is twelve times as long as our year. If his axis were inclined as much to his ecliptic as the axis of the earth, his polar regions would remain in darkness for six years at a time. JUPITER'S SATELLITES