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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 circle, parallel to the former, which divides the earth into uprer 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 tropica/ Capricorn. The sun never parses these circles; but, when it ha* arrived at either, it turns, and goes toward the other. They, of course, bound those place* 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 pcculinr to those places, which are under the equator.
An oblicpte sphere is that, where all the diurnal motions are oblique to the horizon. This is common to nil 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 arc parallel to the horizon. This position is peculiar to those part?1 which lie directly under the pole*.
Zonrs. Zones are the divisions of the earth's surface, formed by the tropics and polar .circles. There are five zones; one torrid, tn u temperate*, and t-xn frigid zones.
The torrid zone is that part of the enrlh's surface included between the two tropics. The equator passes through the middle of this zone. The.temperate zones arc included between the tropics and the polar circles ; and the frigid zones, between the polar circles and the poles.
In every part of the torrid zone the sun is vertical, or directly over the heads of the inhabitants, twice every year, and the days and night* are always nearly equal.
. In the temperate zones the sun is never vertical, but rises and »eh every 2i hours. The days and nights are unequal, and their inequality increases as you approach the poles.
In,the frigid zones, the sun never sets for a certain number of days in summer, and never rises for an equal number in winter. At the poles, the sun is 6 months above, and G months below the horizon; of cour*e he rises only once in a year.
The inhabitants of the different zones may be distinguished by the direction in which their shadows fall at noon.—-Those who inhabit the torrid zone, have their hIiadow% one part of the year north, and the rest of the year soutli of them at noon day; but when the sun is vertical, which is twice every year, they have no shadow at noon.
In the temperate zones the shadows at noon always fall one way; in the northern temperate zone they always fall towards the north, and in the southern always towards the south.
At the poles, the sun for six months moves round without setting, and the shadows are in every 24 hours of that period, successively r.-ist towards every point ofthe horizon.
Climates. The word climate has two significations, one geographical and the other astronomical. Irt common language, the word is used to denote the difference in the seasons and the temperature of the air. When two places differ in these respects, they are said to be in different climates.
In an astronomical sense, a climate is a tract of the earth's surface, included between the equator and a parallel of latitude, or !>etween two parallels, of such a breadth, that the length of the day in one is half an hour longer than in, the other. Within the polar circles, however, the breadth of a climate is ^uch, that the length of the longest day, or the longest time of the sun's continuance above the horizon without setting, is a month longer in one parallel, as you proceed towards the elevated pole, than in the other.
There are 30 climates between the equator and either pole. In the first 24, between the equator and either polar circle, the period of increase for every climate is half an hour. In the other *ix, between either polar circle and its pole, the period of increase for each climate is a month.
Latitude. The latitude of a place is its distance from the equator, reckoned in degrees, kc. north or south, on tho meridian. The
greatest latitude is that of the polos, which are 90 decree
The elevation of the pole-above the horizon i-j always
The inhabitant" of the earth are sometimes dialing cording to the several meridtas and parallels under wj live.
1. Those who live in the same latitude, and same hemisphere but under opposite meridians.—Their seasons are the same, I so the length of their days and nights ; hut when it is mid-day one, k i< midnight with the other.
2. Those who live in the same latitude, and under the s meridian, but in opposite hemispheres. These have noon and mid night at the same lime; but the longest day with the on> shortest with the othe&; consequently, when it is midsummer with one, it is midwinter with the other. ,.
3. Those who live in the same latitude, but in opposite 1 sphere*, and under opposite meridians. These are called Antip odes. When it is mid-day with one it is midnight with the other ;' the longest day with one fc the shortest with the other; and conJ sequently, when it is midsummer with the one, il is midwinter w ah the other.
Longitude. Every place on the surface of the earth has it- meridian. The longitude of a place is the distance of iN no from some other lixed meridian, measured on the equator. J^H gitudc is either east orVest: All places east of the fixed or fiHI meridian are in east longitude; all west, in west longitude.
Opposition. A body is in opposition with the sun, when ths earth is directly between il and the sun.
Conjunction. A body is in conjunction with the sun, when It arc both in a straight line with the earth, on the same sit] If the body is between the earth and the sun, it is said to inferior conjunction ; hoj when the sun is between it and t the body is $iid to he 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, make- 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 tfttt greatest apparent distance from the sun.
Eccentricity. The eccentricity of the orbit of u 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 the sun.
Pcrt&tiion, The perihelion is that point in the orbit of a planet. which is nearest to the sun.
A Dig-it is a twelfth part of (lie diameter of the sun or moon.
Planets are bodies, which rc\ <>lve about the sun in orbits nearly circular, frbose planes make a very small angle with the plane of the ecliplic ; and with a motion according to the order of the signs of the ecliptic, or from west to east.
Satellites or moons, are bodies revolving round the planets, which are called their primaries; and, in company with them, round the
Asteroids are very small bodies, revolving round the sun, in orbit* 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.
Cometraro 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 tkt fJotneis.
The Sun, the most glorious of the heavenly luminaries, is the foarce of light, and heat to all the bodies which revolve around iL
The number of Planets is seven; the names of which according to their nearness to the sun, are Mercury, Venus, the Earth, Mart, Jupiter, Saturn, Herschel. The two first are called inferior planets; the (our last, superior.
Thenumber 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 revolutions round the sun.
The number of Asteroids at present known is four. Their orbits lie between those of Mars and Jupiter. Their names, according- lo their nearness to the sun, are Ceres, Pallas, Juno, and Vesta.
The number of Comets belonging to our system is not yet ascertained.
Astronomers have, at different periods, supposed the principal bodies, which compose the solar sysfem, arranged in different orders. Soch a supposed arrangement is called a System of the worltl. The most distinguished of these systems are the Ptolemaic, the Tifchonic, and tlfe Copernican.
The 1'tolemaic 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 if. According to this hypothesis, the earth is immoveably fired 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 l'rom the' earth. Above this starry sphere were imagined lo be the two crystalline spheres, the primum mobile, communicatingmotion to all the interior spheres; 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 then annoal appearances.
The phenomena to be explained by this system are inconsistent with it, and show its absurdity in a very satisfactory manner.
The TYCHONicor Brahean System was invented by Tycho Brahe, a nobleman of. Denmark. With Ptolemy he snpposed 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-font hours. He also supposed that these bodies had an annual motiM around the earth; that the moon's orbit was nearest to the earth; then the sun's; and that Mercury, Venus, Mars, Jupiter, and Salurn, revolved about the sun as their centre, and accompanied it a« their primary in its annual revolution round the earth. Ashe 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 farther 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 syst»m so far as to ascribe this apparent diurnal motion of the heavens to a real rotation of the earth on its axis, and werr therefore called Semi-Tijchonics.
The Copersicajj System is so railed from Copernicus, a n of Thorn in Royal Prussia, and is the Tm-e Solar System. It had been taught by some of the Pythagorean philosophers, but wa* nearly lost, when Copernicus undertook to restore it, and published new and demonstrative arguments in its favour. It suppose 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 part- of the Solar System.
This supposition readily solves all the appearances observable in the motion of the planets, and also agrees with the stn philosophical and mathematical reasoning.
All the planets arc opaque and spheiical 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 th. sun is the cause of the motions of the planets; and this infill increases as their distance from the sun decreases. Hence also we see the reason why the planets move faster, as they approach /roarer to the sun, and slower as they recede from it.