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.
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
With a telescope magnifying thirty-three times which he had constructed himself, Galileo discovered the moons of Jupiter. Three of them were first seen on the 7th of January, 1610, and the fourth on the 13th of the same month. These moons, which are sometimes called the "Galilean quartet," were the first heavenly bodies which had not been known from time immemorial.