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The first six are called northern signs; and the last six southern.
Zodiac. The Zodiac is a broad belt in the heavens, 16 degrees wide; in the middle of which is the ecliptic. It comprehends the orbits of all the planets.
Horizon. The horizon is either sensible or real. The sensible horizon is the small circle which limits our prospect, where the sky and land or water seem to meet. The real horizon is a great cir. cle, parallel to the former, which divides the earth into upper and lower hemispheres.
The point of the heavens directly over our heads is called the Zenith ; and the opposite point, or that directly under our feet, is called the Nadir. The straight line connecting these two, passes through the centre of the horizon and is called its axis. The zenith and nadir of any place, therefore, are the poles of its horizon.
Declination. The declination of a heavenly body is its distance north or south of the equator, measured on a meridian.
Tropics. The Tropics are two small circles, drawn parallel to the equator, at the distance of 23 28 on each side of it. The northern is called the tropic of Cancer ; the southern, the tropic of Capricorn. The sun never passes these circles ; but, when it has arrived at either, it turns, and goes toward the other. They, of course, bound those places where the sun is vertical.
Polar Circles. The Polar circles are two small circles, parallel to the tropics, described round the poles at the distance of 23° 28'; that around the north pole is called the arctic circle ; that around the south pole, the antarctic circle.
A direct or right sphere is that, where both the poles are in the horizon, and the sun, moon and stars ascend directly above, and descend directly below the horizon. This position is peculiar to those places, which are under the equator.
An oblique sphere is that, where all the diurnal motions are oblique to the horizon. This is common to all parts of the earth, except those under the poles and the equator. In an oblique sphere, one of the poles is elevated above, and the other depressed below, the horizon.
A parallel sphere is that where the equator and all its parallels are parallel to the horizon. This position is peculiar to those parts which lie directly under the poles.
greatest latitude is that of the poles, which are 90 degrees distant from the equator. If the place be situated between the equator and the north pole, it is said to be in north Jatitude ; if it lie between the equator and the south polé, it is in south latitude. A parallel of latitude is any small circle parallel with the equator.
The elevation of the pole above the horizon is always equal to the latitude of the place ; for to a person situated at the equator, both poles will rest in the horizon. If you travel one, two, or more degrees north, the north pole will rise one, two, or more degrees, and will keep pace with your distance from the equator.
The inhabitants of the earth are sometimes distinguished, according to the several meridians and parallels under which they live.
1. Those who live in the same latitude, and same hemisphere, but under opposite meridians,—Their seasons are the same, as also the length of their days and nights; but when it is mid-day with one, it is midnight with the other.
2. Those who live in the same latitude, and under the same meridian, but in opposite hemispheres. These have noon and midnight at the same time; but the longest day with the one is the shortest with the other; consequently, when it is midsummer with one, it is midwinter with the other.
3. Those who live in the same latitude, but in opposite hemispheres, and under opposite meridians. These are called Antipodes. When it is mid-day with one it is midnight with the other; the longest day with one is the shortest with the other ; and consequently, when it is midsummer with the one, it is midwinter with the other.
Longitude. Every place on the surface of the earth has its me. ridian. The longitude of a place is the distance of its meridian from some other fixed meridian, measured on the equator. Longitude is either east or west. All places east of the fixed or first meridian are in east longitude; all west, in west longitude.
Opposition. A body is in opposition with the sun, when the earth is directly between it and the sun.
Conjunction. A body is in conjunction with the sun, when they are both in a straight line with the earth, on the same side of it, If the body is between the earth and the sun, it is said to be in its inferior conjunction ; but when the sun is between it and the earth, the body is said to be in its superior conjunction.
Quadrature. A body is in quadrature, when a line drawn from the centre of the body to the centre of the earth, makes a right angle with a line, drawn from the centre of the earth to the centre of the sun.
Elongation. The greatest elongation of a heavenly body is its greatest apparent distance from the sun.
Eccentricity. The eccentricity of the orbit of a planet is the distance from the sun to the centre of the orbit; the sun not being in the centre, but in one of the foci.
Aphelion. A planet is in its aphelion, when it is farthest'from
Perihelion. The perihelion is that point in the orbit of a planet, which is nearest to the sun.
A Digit is a twelfth part of the diameter of the sun or moon.
Planets are bodies, which revolve about the sun in orbits nearly circular, whose planes make a very small angle with the plane of the ecliptic ; and with a motion according to the order of the signs of the ecliptic, or from west to east.
Satellites or inoons, are bodies revolving round the planets, which are called their primaries; and, in company with them, round the sun.
Asteroids are very small bodies, revolving round the sun, in orbits making larger angles with the plane of the ecliptic, and with motions either direct, i. e. from west to east; or retrograde, i. e. from east to west.
Coets are bodies revolving about the sun in extremely elliptical orbits; whose planes may make any angle with the ecliptic, and whose motions are either direct or retrograde.
THE SOLAR SYSTEM.
The system of heavenly bodies, to which the earth belongs, is composed of the Sun, the Planets, the Satellites, the Asteroids, and the Comets.
The Sun, the most glorious of the heavenly luminaries, is the source of light, and heat to all the bodies which revolve around it.
The number of Planets is seven ; the names of which according to their nearness to the sun, aré Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Herschel. The two first are called inferior planets ; the four last, superior.
The number of Satellites is eighteen. The earth has one ; Jupiter four; Saturn seven ; Herschel six. These revolve round their respective primaries, and accompany them in their annual revolntions round the sun.
The number of Asteroids at present known is four. Their or- bits lie between those of Mars and Jupiter, Their names, according to their nearness to the sun, are Ceres, Pallas, Juno, and Vesta.
The number of Comets belonging to our system is not yet as"certained.
Astronomers have, at different periods, supposed the principal bodies, which compose the solar sysťem, arranged in different orders. Such a supposed arrangement is called a System of the world. The most distinguished of these systems are the Ptolemaic, the Tychonic, and the Copernican.
The ProLEMAIC System is so called from Claudius Ptolemy, a celebrated astronomer of Pelusium in Egypt; not because he was the author of it, but because he adopted and endeavored to support it. According to this hypothesis, the earth is immoveably fixed in the centre of the universe, and all the other bodies revolve round it from east to west in the space of twenty-four hours, at distances, which increase in the order, in which they are here named,
viz. the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the fixed stars. The sun and planets were supposed to be firmly set in separate crystalline spheres, inclosed by a concave one, containing the fixed stars, which would of course be all equally distant from the earth. Above this starry sphere were imagined to be the two crystalline spheres, the primum mobile, communicating motion to all the interior spberes; and, finally, the empyrean heaven or heaven of heavens, to which a cubic form was attributed. Beside the above motion, performed in the course of twenty-four hours, the sun and planets were supposed to revolve about the earth in certain stated or periodical times, agreeably to their annual appearances.
The phenomena to be explained by this system are inconsistent with it, and show its absurdity in a very satisfactory manner.
The Tychonic or BRAHEAN SYSTEM was invented by Tycho Brahe, a nobleman of Denmark. With Ptolemy he supposed the earth to be at rest in the centre of the universe, and the moon, the sun, planets, and fixed stars, to revolve about it in twenty-four hours. He also supposed that these bodies had an annual motion around the earth; that the moon's orbit was nearest to the earth; then the sun's; and that Mercury, Venus, Mars, Jupiter, and Saturn, revolved about the sun as their centre, and accompanied it as their primary in its annual revolution round the earth. As he denied the earth's diurnal rotation on its axis, he was obliged to admit one of the most gross absurdities of the Ptolemaic hypothesis, that is, the revolution of the whole universe, to its farthest visible limits, about the earth's axis in the space of a day, produced by the primum mobile. Some of his followers, however, varied from his system so far as to ascribe this apparent diurnal motion of the heavens to a real rotation of the earth on its axis, and were therefore called Semi-Tychonics.
The COPERNICAN SYSTEM is so called from Copernicus, a native of Thorn in Royal Prussia, and is the true SOLAR SYSTEM. It had been taught by some of the Pythagorean philosophers, but was nearly lost, when Copernicus undertook to restore it, and published new and demonstrative arguments in its favour. It supposes the sun to be in the centre of the system, and all the planets to move round the sun in the order already mentioned. These, together with the satellites, asteroids, and comets form the constituent parts of the Solar System.
This supposition readily solves all the appearances observable in the motion of the planets, and also agrees with the strictest philosophical and mathematical reasoning.
All the planets are opaque and spherical bodies, and receive their light from the sun. Their orbits are not circular, but elliptical. Hence, in their revolutions, they are sometimes nearer to, and sometimes farther from, that luminary, The influence of the sun is the cause of the motions of the planets ; and this influence increases as their distance from the sun decreases. Hence also we see the reason why the planets move faster, as they approach wearer to the sun, and slower as they recede from it.