Jupiter presents a magnificent appearance in a good telescope. Its oblong disk is seen crossed in an east and west direction, and parallel to its equator, by broad, vari-coloured bands, called belts. These frequently change in form and, to some extent, in situation, as well as in number. But there are always at least two wide belts, one on each side of the equator. In 1878 a very remarkable feature was noticed just south of the principal south belt of Jupiter, which has become celebrated under the name of the Great Red Spot. In a few years after its discovery its colour faded, but it still remains visible, with varying degrees of distinctness, as an oblong marking, about 30,000 miles long and 7000 miles broad. The outer border of the great south belt bends away from the spot, as if some force of repulsion acted between them, or as if the spot were an elevation round which the clouds of the belt flowed like a river round a projecting headland. The nature of this curious spot is unknown. Other smaller spots, sometimes white, sometimes dusky, occasionally make their appearance, but they do not exhibit the durability of the Great Red Spot.

Jupiter has eight satellites, four of which, known since the time of Galileo, are conspicuous objects in the smallest telescope. All but one of these four are larger than our moon, while the other four are extremely insignificant in size. The four principal satellites are designated by Roman numerals, I, II, III, IV, arranged in the order of distance from the planet. They also have names which are seldom used. Satellite I (Io) has a diameter of 2452 miles, and revolves in a period of 1 day, 18 hours, 27 min., 35.5 sec., at a mean distance of 261,000 miles; II (Europa) is 2045 miles in diameter, and revolves in 3 days, 13 hours, 13 min., 42.1 sec., at a mean distance of 415,000 miles; III (Ganymede) has a diameter of 3558 miles, a period of seven days, 3 hours, 42 min., 33.4 sec., and a mean distance of 664,000 miles; IV (Callisto) is 3345 miles in diameter, has a period of 16 days, 16 hours, 32 min., 11.2 sec., and a mean distance of 1,167,000 miles. The object of giving the periods with extreme accuracy will appear when we speak of the use made of observations of Jupiter's satellites. The first of the four small satellites, discovered by Barnard in 1892, is probably less than 100 miles in diameter, and has a mean distance of 112,500 miles, and a period of only 11 hours, 57 min., 22.6 sec. The other small satellites are much more distant than any of the large ones, the latest to be discovered, the eighth, being situated at a mean distance of about 15,000,000 miles, but travelling in an orbit so eccentric that the distance ranges between 10,000,000 and 20,000,000 miles. The period is about two and a fifth years. But the most remarkable fact is that this satellite revolves round Jupiter from east to west, a direction contrary to that pursued by all the others, and contrary to the direction which is almost universal among the rotating and revolving bodies of the solar system.

The large satellites are very interesting objects for the telescope. When they come between the sun and Jupiter their round black shadows can be plainly seen moving across his disk, and when they pass round into his shadow they are suddenly eclipsed, emerging after a time out of the other side of the shadow. These phenomena are known as transits and eclipses, and their times of occurrence are carefully predicted in the American Ephemeris and Nautical Almanac, published at Washington for the benefit of astronomers and navigators, because these eclipses can be employed in comparing local time with standard meridian time. They were formerly utilised to determine the velocity of light, in this way:

As the earth goes round its orbit inside that of Jupiter the latter is seen in opposition to the sun at intervals of 399 days. When it is thus seen the earth must be between the sun and Jupiter, and the distance between the two planets is the least possible. But when the earth has passed round to the other side of the sun from Jupiter this distance becomes the greatest possible. The increase of distance between the two planets, as the earth goes from the nearest to the farthest side of its orbit, is about 186,000,000 miles. Now it was noticed by the Danish astronomer, Roemer, that as the earth moved farther and farther from Jupiter the times of occurrence of the eclipses kept getting later and later, until when the earth arrived at its greatest distance the eclipses were about 16 minutes behind time. He correctly inferred that the retardation of the time was due to the increase of the distance, and that the 16 minutes by which the eclipses were behindhand when the distance was greatest represented the time taken by light to cross the 186,000,000 miles of space by which the earth had increased its distance from Jupiter. In other words, light must travel 186,000,000 miles in about sixteen minutes, from which it was easy to calculate its speed per second—which we now know to be 186,330 miles. Our knowledge of the velocity of light furnishes one of the means of calculating the distance of the sun.

We come next to the beautiful planet Saturn, whose mean distance from the sun is 886,000,000 miles. The distance varies between 911,000,000 and 861,000,000 miles. Saturn's year is equal to 29.46 of our years. It comes into opposition every 378 days. The most surprising feature of Saturn is the system of immense rings surrounding it above the equator. The globe of the planet is 76,470 miles in equatorial diameter, and 69,780 miles in polar diameter, a difference of 6690 miles, so that Saturn is even more compressed at the poles and swollen at the equator than Jupiter. The axis of rotation is inclined 27° from a perpendicular. The rings are three in number, very thin in proportion to their vast size, and placed one within another in the same plane. The outer diameter of the outer ring, called Ring A, is about 168,000 miles. Its breadth is about 10,000 miles. Then comes a gap, about 1600 miles across, separating it from Ring B, the brightest of the set. This is about 16,500 miles broad, and at its inner edge it gradually fades out, blending with Ring C, which is called the crape, or gauze, ring, because it has a dusky appearance, and is so translucent that the globe of the planet can be seen through it. This ring is about 10,000 miles broad, and its inner edge comes within a distance of between 9000 and 10,000 miles of the surface of the planet. Ring A apparently has a very narrow gap running round at about a third of its breadth from the outer edge. This, known as Encke's Division, is not equally plain at all times. Occasionally observers report the temporary appearance of other thin gaps.

The mean density of Saturn is less than that of any other planet, being but 0.13 that of the earth, or 0.72 that of water. It follows that this great planet would float in water. The weight of bodies at its surface would be a little less than three-quarters of their weight on the surface of the earth. The globe of Saturn, like that of Jupiter, is marked by belts parallel with the equator, but they are less definite in outline and less conspicuous than the belts of Jupiter. The equatorial zone often shows a beautiful pale salmon tint, while the regions round the poles are faintly bluish. Light spots are occasionally seen upon the planet, and it appears to rotate more rapidly at the equator than in the higher latitudes. There seems to be every reason to think that Saturn, also, is of a vaporous constitution, although it may have a relatively condensed nucleus.

But while the globe of the planet appears to be vaporous, the same is not true of the rings. We have already mentioned the fact that they are exceedingly thin in proportion to their great size and width. The thickness has not been determined with exactness, but it probably does not exceed, on the average, one hundred miles. There appear to be portions of the rings which are thicker than the average, as if the matter of which they are composed were heaped up there. This matter evidently consists of an innumerable multitude of small bodies. In other words, the rings are composed of swarms of what may be called meteors. That their composition must be of this nature, although the telescope does not reveal it, has been proved in two ways: first, by mathematical calculation, which shows that if the rings were all of a piece, whether solid or liquid, they would be destroyed by the contending forces of attraction to which they are subject; and, second, by spectroscopic observation, which proves, in a way that will be shown when we come to deal with the stars, that the rings rotate with velocities proportional to the distances of their various parts from the centre of the planet. Hence it is inferred that they must consist of a vast number of small bodies or particles.

Saturn has ten satellites, all revolving outside the rings. The names of nine of these in the order of increasing distance are: Mimas, distance 117,000 miles; Enceladus, distance 157,000 miles; Tethys, distance 186,000 miles; Dione, distance 238,000 miles; Rhea, distance 332,000 miles; Titan, distance 771,000 miles; Hyperion, distance 934,000 miles; Japetus, distance 2,225,000 miles; and Phœbe, distance 8,000,000 miles. The last, like the eighth satellite of Jupiter, revolves in a retrograde direction. Only Titan and Japetus are conspicuous objects. The period of Mimas is only about 22½ hours; that of Titan is 15 days, 22 hours, 41 min., and that of Japetus about 79 days, 8 hours. Barnard's measurements indicate for Titan a diameter of 2720 miles. Japetus is probably about two-thirds as great in diameter as Titan.

Photographs of Mars
Made at the Yerkes Observatory by E. E. Barnard, with the forty-inch refractor, September 28, 1909.