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Satellites of Saturn. Of these Huygens discovered the fourth in 1665; Cassini the fifth in 1671, the third in 1672, the first and second in 1684; and Herschel the sixth in 1787, and the seventh in 1788. These last are nearer to Saturn, than the other five; but, to prevent confusion in the numbers with regard to former observations, they are called the sixth and seventh. The tables exhibit their periods and distances from their primary. The third satellite is the largest of all; the first and fourth are nearly of the same size.
Satellites of Herschel. These are six in number. The second and fourth were discovered by Herschel in 1787; and, what is entirely singular in our system, he observed, that their orbits made an angle nearly perpendicular with the ecliptic of the primary. The other four were also discovered by Herschel. The first and fifth in 1790, and the other two in 1794. Their light is extremely faint; but the fourth is somewhat the brightest. The sixth, at its greatest distance, is farther removed from the earth than any body, if we except the comets, that is known to belong to our system. Of all the bodies hitherto described, the satellites of Herschel alone revolve from east to west, or in a retrograde direction.
Asteroids.* These bodies were entirely unknown, till the commencement of the present century. They appear of the size of stars of the 8th magnitude. It was owing to their diminutive size, that Herschel refused them a place among the planets, and gave them the name of Asteroids, though they are really primary planets, revolving round the sun.
Ceres was discovered by Joseph Piozzi, at the royal observatory at Palermo, January 1, 1801. It appears like a star of the 7th or 8th magnitude. Its diameter is estimated by Dr. Herschel at 160 miles, but this cannot be relied on as exact. All the asteriods are too small to be measured with precision. Their orbits are all between those of Mars and Jupiter. Ceres revolves in 4 years, 7 months, 10 days. Its mean distance from the sun is 263,663,000 miles.
Pallas was discovered by Dr. Olbers of Bremen, March 28, 1802. It appears sometimes like a star of the 7th magnitude, and sometimes considerably less. Its diameter is 110 miles. Its periodical revolution is 4 years, 7 months, 11 days; and its distance from the sun 267,438,000 miles. The orbits of Ceres and Pallas are said to cross each other.
Juno was discovered by Mr. Harding, at Lilienthal, near Bremen, September 18t. 1804. It appears like a star of the 8th magnitude. Its periodical revolution is a little longer than those of Ceres and Pallas. Its diameter is 119 miles, Its distance from the sun is 286,541,000 miles.
Vesta was discovered by Dr. Olbers, March 29, 1807. It may be seen by the naked eye, like a star of the fifth or sixth magnitude, and very much like the planet Herschel. The angle which its diameter subtends, is about half a second. Its periodical revo
* From mong star, and udos appearance,
lution is 3 years, 2 months, 5 days, and its mean distance, 206,596,000 miles. These elements all require to be corrected by future obe servations.
TABLE OF ASTEROIDS.
Names. When discovered. Periodical Distance from Inclination Eccen
of the Orbit.tricity.
y. m. d.
Vesta March 29, 1807 3 2 5206,596,000 7 8
10.095 10.097 0.246
Thus, of the 30 bodies, beside the comets, belonging to our system, only eight were known to the ancients; viz, the Sun, Mercury, Venus, the Earth, the Moon, Mars, Jupiter, and Saturn. Of the remaining 22, 4 were discovered in the 16th century; viz. Jupiter's 4 moons, by Galileo: 5 in the 17th centuryl; viz. Saturn's fourth moon by Huygens; and his first, second, third, and fifth, by Cassini : 9 in the 18th century; viz. Saturn's sixth and seventh moons, the planet Herschel, and his six moons, all by Dr. Herschel: and four already in the 19th ; viz. Ceres, by Piozzi ; Pallas, by Olbers; Jono, by Harding : and Vesta, by Olbers.
Comets. Comets are bodies revolving in very eccentric ellipses about the sun in one of the foci. When a comet is west of the sun, and moving towards it, it is said to be tailed; because a train of light follows it, in manner of a tail. When the sun and the comet are on opposite sides of the earth, the train is principally hid behind the body of the comet, and the little that appears has the form of a border of hair, or coma, whence it is called hairy; and whence the name comet is derived. The substance of the bodies of comets must be extremely solid, or they would be dissipated in their perihelion, or nearest approach to the sun. According to Sir Isaac Newton, the comet of 1680 endured a heat 28,000 times as great as that of the sun, in midsummer; or about 9,000 times as great as the heat of boiling water; or 2000 times as great as the heat of red hot iron. Little is ascertained respecting the real magnitodes
of comets. Their apparent magnitudes are also very various. That which appeared in the time of Nero, was, as Seneca relates, apparently as large as the sun; and that of 1652, according to Hevelius, did not seem to be less than the moon, though of a very pale, dim light.
The number of comets belonging to our system has never been ascertained. Conjecture has limited it to 450. The elements of 97 of them have been determined with some degree of accuracy. The angles, which their orbits made with the plane of the ecliptic, were found to vary from 1 to 88 degrees. The perihelion distance
of the comet of 1351, was just equal to the earth's mean distance. The perihelion distance of 24 of the others, was greater than this, and of the remaining 72, less. The least distance of the comet of 1680, was only 122,000 miles from the surface of the sun ; while its greatest distance was 12,189,000,000 miles. The perihelion distance of the comet of 1759 is about 52,000,000 miles ; its aphelion distance 3,342,500,000. These are the only two comets whose periods are known, That of the latter is about 76 years. It appeared in 1759, 1682, 1607, 1531, and 1456; and will probably reappear in 1835.
The period of the former is 575 years. It appeared in 1680, 1106, 531, and in 44, before Christ, and probably will not re-appear, till 2255. There is also strong reason to conclude, that the comet of 1264 was the same with that of 1556. If so, its period is 292 years; and it ought to appear again in 1848. Dr. Halley imagined, that the comet of 1661 was the same with that of 1532; and that its period was 129 years ; but in 1790, it was found to have violated its engagements. Dr. Halley had the honor first to foretel the return of a comet. It was the comet of 1759. The velocity of a comet increases as it approaches the sun. That of 1680, in its perihelion, moved with the amazing velocity of 880,000 miles an hour. The comet of 1744, had a tail of the length of 23,000,000 of miles; and that of 1759, of more than 40,000,000. The orbits of comets make very different angles with the plane of the ecliptic: 50 out of the 97, whose elements have been calculated, had a direct motion, or from west to east; and 47 from east to west. The comet of 1680, on the 11th November, at 1 hour, 6 minutes, P. M. was only 4000 miles north of the orbit of the earth. If the earth at that time, had been in the part of its orbit nearest to the comet, their mutual gravitation must have caused a change in the plane of the earth's orbit, and in the length of our year.
The following tables, taken, with some alterations, from Clarke's Commentary on the Bible, will present a full and interesting summary of the bodies in our solar system, together with their magnitudes, distances, periods, &c.
TABLE I. SUN AND PLANETS.
Bulk, J. Weight, l'imeoj ro-Inclina'n Hourly Weight Names. Diame-l'he Earth the Earth| tation on of ocis to motion in of ilbal
ler. being 1. being 1. Their acis. Equator. 'heir orbits.bsurface.
d. b, m. s.
883,246|1,380,000 333,928 25 14 8 0
27.my Mercury 3,224
0.1654 unknown unknown 111,256 1:0333 Venus 7,687
0.8899 23 22 0 75° 00' 81,398 p.9771 Earth 7,928
1 23 56 4 23 28
75,222 1.0000 Moon 2,180
27 7 43 5
2,335 0·1677 Mars 4,189
0.087 | 1 039 22 28 42 56,212 p.3355 Jupiter 89,170 1400 312.11 9 55 33 3 22 30,358 2:3287 Saturn 79,042 1000 97.76 10 16 14 30 00 22,351 1.0154 Sat, Ring|204,883
10 32 15 30 00 22,351 Herschell 35,11 | 90 16.84 unknown lunknown 15,846 9285
Veundistances Propor'n Inclınai on Periodical Revo-Sidereal Revo Vames. from the sun. of Light of orbits to lution.
lution. & Heat. the Ecliptic
y. d. h. m. s. y. d. h, m. s. Mercury 36,583,8256:25 7 0 0 0 87 23 14 33 0 87 23 15 40 Venos
68,360,0582.04 3 23 45 0 224 16 41 27 0 224 16 49 11 Earth 94,507,428 1
1 05 48 48 1 0 6 9 12 Moon 94,507,428 1: 15 9 0 0 27 743 5 0 27 743 13 Mars 144,000,0230-44375 1 51 0 1 321 22 18 27 1321 23 30 36 Pupiter 491,702,3010.0368751 19 15:11 315 14 39 211317 14 27 11 Saturn 901,668,908 0.01 106 2 30 45,29 164 7 21 50/29 176 14 36 43 Sat. Ring 901,668,908|0.01106
29 164 7 21 50 29 176 14 36 43 Herscheil 1,803,534,3920:00270 048 083 294 8 39 0184 29 0 29
Saleh Diam- Bulk, the Distance from Periodic Revo-Synodic Revo. Grea'stdis Viles. eler. Earth be Jupiter. lution lution. tance from
Satellites. Distance from Saturn. PeriodicRevolution. Synodic Revolut'n.
Saielktes. Distance from Herschel. Periodic Revolution. SynodicRevolu’n.
1. U. LII. IV. v. vi.
226,450 293,053 342,784 392,514
d. h. m. S. 5 21 23 29 5 21 25 0 8 16 57 439 8 17 1 19 10 22 58 20 10 23 4 0 13 10 56 29 13 11 5 1 38 0 39 41 38 1 49 0 107 7 35 10 107 16 40 0
OF THE FIXED STARS.
Those stars, which, when seen by the naked eye, or through telescopes, keep constantly in the same situation with respect to each other, are called fixed stars. They are easily distinguished from the planets by their twinkling. They appear of various magnitudes. This may arise from their different sizes, or distances, or both. Astronomers have distinguiühed them, from their apparent magnitudes, into six classes. The first contains those of the largest apparent size, the second those which appear next in bigness; and so on to the sixth, which includes all those that can just be seen without telescopes. Those, which can be seen only by the help of the telescope, are called telescopic stars.
Number of stars of each magnitude.
1st 2d 3d 4th 5th 6th Tota). In the Zodiac
5 16 44 120 183.6461014 In the Northern Hemisphere 6 24 95 200 291 635 1251 In the Southern Hemisphere 9 36 84 190 221 323 865
20 76 223 512 695 1601
The stars in the preceding table are so numerous, that it would be impossible to furnish names for them all and retain those names in the memory. To remedy this inconvenience the ancients distributed them into constellations, to which they gave the names of birds, beasts, fishes, &c. from an imaginary resemblance between the forms of the constellations, and of those animals. The stars of each constellation are numbered, according to their magnitude, by the letters of the Greek alphabet. a is the largest, B the second, g the third, &c. This division of the heavens was very ancient; for some of the constellations are mentioned by Homer and Hesiod, by Amos and Job.
The whole number of the constellations is 90. Of these 48 are ancient, and 42 modern; 33 north of the Zodiac, 12 in the Zodiac, and 45 south of it. Those stars, which have never been arranged into constellations, are called unformed stars. Those, whose distance from the nearest pole is less than the latitude of the place, never set below the horizon, and are called circumpolar stars. The eircles, which they appear to describe in consequence of the earth's rotation, are called circles of perpetual apparition. Those stars, whose distance from the farthest pole, is less than the latitude of the place, never rise above the horizon. They also receive the same name ; and the circles, which they appear to describe, are called circles of perpetual occultation.
The real number of the fixed stars cannot be ascertained. Before the invention of the telescope, it was not supposed to surpass 3000. But since that event it has been found, that the greater the