clouds of Jupiter, partaking the great velocity of his diurnal motion, are formed into strata, parallel to his equator, that the clouds reflect more light than the body of Jupiter, and that the belts are the body seen through the parallel interstices of the clouds. "But whatever be the nature of these belts," says Mr. Dick, "the sudden changes, to which they are occasionally subject, seem to indicate the rapid operations of some powerful physical agency-for some of these are more than 5,000 miles in breadth-and, since they have been known to disappear in the space of an hour or two, and even during the time of a casual observation, agents more powerful than any with which we are acquainted, must have produced so extensive an effect." ELEMENTS OF JUPITER. Mean diameter, 89,170 miles. Mean diameter, as seen from the Sun, 37′′.7. Motion of the aphelion in longitude for 100 years, 1° 34' 33". Diurnal rotation, 9 h. 55 m. 37 s. Mean distance from the Sun, 490,000,000 miles. SATELLITES OF JUPITER. Jupiter is attended by four satellites. They are reckoned the first, second, third, and fourth, beginning with the one nearest to the primary. These satellites were discovered by Galileo, on the 8th of January (1610, and called by him Medica sidera, Medicean stars, in honor of his patrons, the family of the Medici. By some, we are told these satellites are not to be seen by the naked eye. But Prior, in his Lectures on Astronomy, informs us, that, "with the exception of the third and fourth, they are never visible to the naked eye instances of these two being so seen are extremely rare, although they have been known to occur." He tells us, in another place, that," according to Dr. Herschel, the third is the largest; the second the least; and the first and fourth are nearly of the same size. They are all of them supposed to be considerably larger than the Earth; but their dimensions are not exactly known.” Through a good telescope, the satellites of Jupiter present a delightful prospect. They seem generally ranged in a straight line, parallel, or nearly parallel, to his belts. Jupiter and his satellites eclipse each other. Like the Moon, they throw their dark shadows upon their primary; and, like her, they fall into his shadow and are eclipsed. These phenomena are a demonstration, that those distant luminaries are in themselves opaque, and shine not by their own light, but by rays borrowed from the Sun. The eclipses of Jupiter's satellites are of great utility to us. By these it is found that light is progressive, which, before their discovery, was supposed to be instantaneous. By them, the relative distances between the Earth, the Sun, and Jupiter, can be ascertained. But the greatest benefit derived from these eclipses, is to geography and navigation. They afford one of the best methods yet known for ascertaining longitude. It could not have occurred to Galileo, when he first discovered these satellites, that, by an act so simple, he was rendering so great a benefit to mankind. Here is verified the observation of a celebrated traveller, that the Deity, every where, brings the greatest events from causes apparently the least. How is Jupiter in magnitude and brilliancy? What is his form? What is his character? What is the most remarkable phenomenon in the disk of Jupiter? How do his belts appear? What have been visible in the belts? What do some of the spots appear to do? What opinions have been formed respecting the cause of the apparent spots and belts on the disk of Jupiter? What do the sudden changes in Jupiter's belts seem to indicate? How broad are some of the belts? By whom were the satellites of Jupiter discovered? What year? Can these satellites be seen by the naked eye? What does Prior say of their magnitude? What prospect do they present through a good telescope? Of what are the eclipses of Jupiter and his satellites a demonstration? Why are the eclipses of Jupiter's satellites of great use to us? What is verified by them? SECTION XII. Of Saturn. Beyond Jupiter, in the solar system, is Saturn, formerly considered the most remote of the planets.) He shines with a dull, pale, leaden light.) The character of Saturn, h, is a scythe, rudely represented; according to some, an old man leaning on a staff. In heathen mythology, Saturn was the father of Jupiter. Belts and dark spots have been discovered on the disk of Saturn. Five belts, nearly parallel to the equator, were observed by Huygens. Several, nearly parallel to the ring, and more extensive in proportion to the body of the planet, than those of Jupiter, were seen by Dr. Herschel. By the spots of Saturn changing their position, his diurnal rotation was determined by Dr. Herschel to be 10 h. 16 m. 0.44 s. Guy, in his Astronomy, informs us, "later accounts say, 12 h. 134 m.” To an inhabitant of Saturn, the Sun's light and heat must be about 90 times less than they are to us.) Viewed with a good telescope, Saturn appears of a (spheroidical figure A remarkable circumstance is, the flatting at the poles does not seem to commence till the high latitude of 43° 20' According to Dr. Herschel, the proportion of his disk is, Diameter of the greatest curvature, Polar diameter, 36 35 32 The most remarkable phenomenon of Saturn is a ring with which he is encompassed. Something extraordinary in the appearance of this planet was discovered by Galileo. It seemed a large globe between two smaller globes. This discovery he announced in 1610. Continuing his observations till the year 1612, to his surprise the smaller globes disappeared, and the larger remained apparently alone. But, after some time, the smaller globes again appeared on each side of the larger globe, changing their form as he continued his observations; appearing, at different times, round, semicircular, oblong like an acorn, with horns towards the globe, becoming, gradually, so long and wide as to encompass it with an elliptical ring. Upon this, Huygens set about improving the art of grinding object-glasses, and made telescopes which magnified two or three times more than any which had been before made, with which he discovered very clearly the ring of Saturn; and, having observed it for some time, he published the discovery in 1656.” The ring of Saturn is double, or, rather, consists of two concentric rings, detached from each other, and from the body of the planet. In Plate ii, Saturn and his double ring are represented as in the largest view, when seen from the Earth. In Plate iii, Fig. 10, he appears as if viewed by a spectator at right angles to the plane of the ring. In Plate iii, Fig. 11, the ring is represented very obliquely to the view of the observer. Its inclination to the ecliptic is 31°. The two parts of the ring lie in the same plane, performing a revolution round an axis perpendicular to that plane, in 10 h. 32 m. 15 s. It is visible to us when the Sun is on the same side of its plane with the Earth, but at no other times. A deep shadow is cast by the ring on that part of Saturn which is opposite to the Sun. In this dark shadow, each half of the planet, in succession, must be enveloped for almost fifteen of our years, or during one half of Saturn's annual revolution. During the same term, each, in succession', must be illuminated by the double ring, the light of which is more brilliant than that of the planet itself. The ring of Saturn is considered by Dr. Herschel, not as a shining matter, or aurora borealis, as supposed by some, but solid and dense, as the body of the planet. Mean diameter, 79,042 miles. Mean diameter, as seen from the Sun, 18′′. Inclination of his orbit to the ecliptic, 2° 29′ 34.8". Tropical revolution, 29 y. 162 d. 11 h. 30 m. 0 s. Sidereal revolution, 29 y. 167 d. 0 h. 27 m. 0 s. Place of the ascending node, Cancer, 22° 12' 57". Place of the descending node, Capricorn, 22° 12' 57". Motion of the nodes in longitude, for 100 years, 52′ 35′′. Retrograde motion of the nodes in 100 years, 30' 57". Place of the aphelion, 8 s. 2° 38′ 18′′. |