Besides this the axial turning of Jupiter brings an entirely different part of the planet into view every two or three hours; so that in making a map or chart of the planet, an arbitrary meridian must be selected. Even then the process is not an easy one, and it is found that spots on Jupiter's equator turn round in 9 hours 50 minutes, while other regions take a few minutes longer, the nearer the poles are approached. The Great Red Spot, about 30,000 miles long and a quarter as much in breadth has been visible for about half a century. Bolton, an English observer, has made interesting studies of it very recently. The four moons, or satellites, which a small telescope reveals, are exceedingly interesting on many accounts. They were the first heavenly bodies seen by the aid of the telescope, Galileo having discovered them in 1610. They travel round Jupiter much the same as the moon does round the earth, but faster, the innermost moon about four times per week, the second moon about twice a week, the third or largest moon (larger than the planet Mercury) once a week, and the outermost in about sixteen days. The innermost is about 260,000 miles from Jupiter, and the outermost more than a million miles. From their nearness to the huge and excessively hot globe of Jupiter, some astronomers, Proctor especially, have inclined to the view that these little bodies may be inhabited. Jupiter has other moons; a very small one, close to the planet, which goes round in less than twelve hours, discovered by Barnard in 1892. Four others are known, very small and faint and remote from the planet, which travel slowly round it in orbits of great magnitude. The ninth, or outermost, is at a distance of fifteen and one-half million miles from Jupiter, and requires nearly three years in going round the planet. It was discovered by Nicholson at the Lick Observatory in 1914. The eighth was discovered by Melotte at Greenwich in 1908, and is peculiar in the great angle of 28 degrees, at which its orbit is inclined to the equator of Jupiter. The sixth and seventh satellites revolve round Jupiter inside the eighth satellite, but outside the orbit of IV; and they were discovered by photography at the Lick Observatory in 1905 by Perrine, now director of the Argentine National Observatory at Cordoba. The ever-changing positions of the Medicean moons, as Galileo called the four satellites that he discovered--their passing into the shadow in eclipse, their transit in front of the disk, and their occultation behind it--form a succession of phenomena which the telescopist always views with delight. The times when all these events take place are predicted in the "Nautical Almanac," many thousand of them each year, and the predictions cover two or three years in advance.