CHAPTER X. ASTRONOMICAL SCIENCE. SECTION I. THE SOLAR SYSTEM. 245. THIS Science derives its name from a5.mp STAR, and voμos, LAW or RULE. And it is divided into two parts. The first treats of the motions, magnitudes, and periods of the revolution of the heavenly bodies the second investigates the causes and laws by which those motions are regulated. The former is called PURE or PLAIN ASTRONOMY; the latter PHYSICAL ASTRONOMY. . The heavenly bodies which we shall chiefly notice, are the sun, the planets, comets, and stars. The phenomena of the celestial world will form a separate article, of which we shall afterwards treat. 246. The SUN, the source of light and heat, is by far the most considerable object that we behold suspended in space by the power of the Almighty Creator of the universe. And this splendid globe governs all the planetary motions, in as much as it is poised in or near the common centre of gravity, around which all the planets of the solar system are annually whirled. Illus. The sun is 337,086 times greater than the earth; it is surrounded by an atmosphere; it is sometimes covered with spots, and some of these spots have been observed four or five times as large as the earth. The observation of these spots shews that the sun moves in its axis, and 25 days is the duration of its entire sidereal rotation. The sun and planets seem to have a motion towards the constellation of Hercules. Obs. When we consider the sun as the fountain of light, that illuminates the world, and causes our earth to produce every thing desirable for man, may we not consider it also as an eminent, large, and lucid planet, evidently the primary one of our system; Mercury, Venus, the Earth, and all the others being its secondaries? And considering its solidity, its atmosphere and diversified surface, its rotation on its axis, and all the circumstances attending this glorious globe, may we not conclude that it is inhabited, like the other planets, by beings whose organs are adapted to the physical properties of the sun. This seems to be a more pleasing and agreeable way of making the tour of creation, than joining issue with fanciful poets, who have pronounced it the abode of blessed spirits; or closing with the chilling doctrines of angry moralists, who have fancied it the fittest residence for the punishment or purgation of the wicked from off the earth. SECTION II. THE PLANETS. 247. THERE are eleven planets belonging to our system. Six of these have been recognised from time immemorial: namely, Mercury, Venus, the Earth, Mars, Jupiter, and Saturn. But the remaining five, invisible to the naked eye, have lately been discovered by the help of the telescope; and are therefore called telescopic planets: namely, Uranus, discovered by Dr. Herschel, March 13, 1781. Juno, Vesta, ... M. Piazzi, January 1, 1801. ...M. Olbers, March 28, 1802. .M. Harding, Septem. 1, ....... 1803. M. Olbers,... March 29, 1807. 248. All these planets revolve round the sun, as the centre of motion: and in performing their revolutions they follow the laws of planetary motion discovered by Kepler; and confirmed by subsequent observations. These laws are, 249. The orbit of each planet is an ellipse; of which the sun occupies one of the foci. That focus is called the lower focus. If we suppose the plane of the earth's orbit, which passes through the centre of the sun, to be extended in every direction, as far as the fixed stars, it will mark out a great circle among them, which is the ecliptic; and with this the situations of the orbits of all the other planets are compared. The planes of the orbits of all the other planets must necessarily pass through the centre of the sun, but if extended as far as the fixed stars, they form circles different from each other, as also from the ecliptic; one part of each orbit, is on the north, and the other on the south side of the ecliptic. Therefore the orbit of each planet cuts the ecliptic in two opposite points, as A, B, which are called the nodes of that particular planet; and the nodes of one planet cut the ecliptic in planes different from the nodes of another planet. The line A B is called the line of the nodes. The ascending node is that where the planet passes from the south to the north side of the ecliptic. The angle E G is the inclination of the planes of the two orbits to each other. The descending node that where the planet passes from the north to the south side of the ecliptic. Perigee, when the sun and moon are nearest the earth: apogee, when at their greatest distance. The extremity of the major axis of this ellipse, nearest the sun, is called the perihelion; the opposite extremity of the same axis is call the aphelion. The line, which joins these two points, is called the line of the apsides. The radius vec tor is an imaginary line drawn from the centre of the sun to the centre of the planet, in any part of its orbit. The velocity of a planet in its orbit is always greatest at its perihelion. This velocity diminishes as the radius vector increases; till the planet arrives at its aphelion, when its motion is the slowest. It then increases in an inverse manner, till the plant arrives again at its perihelion. 250. The areas, described about the sun by the radius vector of the planet, are proportional to the times employed in describing them. These laws are sufficient for determining the motion of the planets round the sun; but it is necessary to know, for each of these planets, seven quantities; which are called the elements of their elliptical motion. The first five of these elements relate to the motion in an ellipse; the last two relate to the position of the orbit; since the planets do not all move in the same plane. 1. The duration of a sidereal revolution of the planet. 2. Half the major axis of the orbit; or the mean distance of the planet from the sun. 3. The eccentricity of the orbit; whence we deduce the greatest equation of the centre. 4. The mean longitude of the planet at a given epoch. 5. The mean longitude of the perihelion. 6. The longitude of the nodes at a given epoch. 7. The inclination of the orbit to the ecliptic. 251. The following tables from La Place's Exposition, present all these elements for the first moment of the present century; that is to say, for that point of time at midnight which separated the 31st of December 1800, and the 1st of January 1801; mean time at Paris.-[The observatory at Paris is in north latitude 48° 50′ 14", and in longitude 9′ 21′′ east from Greenwich observatory.] 1. Duration of a Sidereal Revolution. Days. ..87.92925804 | Ceres Days. 1681-53900000 .1681.70900000 ... ..365.25638350 | Jupiter ..224.70082399 Pallas ....686-97961860 | Saturn ...... 10758-96984000 .1335.20500000 Uranus .1590 99800000 ......30688.71268720 Mean Distance from the Sun. ....3870381 | Ceres 7233323 Pallas. 1.0000000 Jupiter ....2.7674060 .... ..2:3730000 .1.5236935 | Saturn ..2.6671630 .2.7675920 .5.2027911 ..9.5387705 19.1833050 4. Mean Longitude, January 1, 1801. Mercury. Venus Earth ... Mars Vesta Juno .197° 54244 .71° 24145 | Saturn ..........150° 38010 .297° 12990 Uranus.... .322° 79380 5. Mean Longitude of the Perihelion. 82° 6256 | Ceres..... 142° 9077 Mercury Venus Pallas. 162° 9565 .134° 7040 The examination of the first two tables will shew us that the duration of the revolutions of the planets increases with their mean distance from the sun. Whence Kepler discovered his third fundamental law; namely, 252. The squares of the times of the revolutions of the planets are to each other as the cubes of their mean distances. |