55. When latitude and longitude are applied to places on the earth, they properly belong to geography. But as the method of finding them is purely astronomical, it is proper to treat of them as used both to designate the situation of places on the earth, and of the heavenly bodies. Before any thing can be understood of latitude and longitude, definite ideas must be obtained of the poles, the equator, parallels of latitude, and meridians. The earth turns round on an imaginary line passing through its centre, called its axis ; the extremities of this axis are, as before stated, called poles ; one north pole, the other south. If the axis be supposed to extend both ways to the starry heavens, its places or points among the stars are the celestial poles, one north, and the other south, directly over or beyond the poles of the earth of the same name. The northcelestial pole is very near a particular star, which on that account is called the pole star. 56. The equator is a circle surrounding the earth from west to east, at equal distance from the poles. Hence the equator divides the earth's surface into two equal parts, called hemispheres. If the plane of the equator were extended every way to the starry heavens, the circle it would make among the stars is called the celestial equator. It is from the equator that latitude on the earth is reckoned. All places between the equator and the north pole are in north latitude, and all places between the equator and the south pole are in south latitude. The latitude is greater, as the place is farther from the equator and nearer the poles. All circles, passing round the earth from west to east between the equator and the poles, are called parallels of latitude ; and when two places, as Boston and Philadelphia, differ in latitude, they are said to be on different parallels. There may be as many parallels as there are places not equally distant from the equator. 57. A line passing over the earth from the north to the south pole, and crossing the equator at right angles, is called a meridian. Every place on the earth's surface may be supposed to have such a line or circle passing through it; consequently, when a place lies more easterly or westerly than another, it is said to have a different meridian. Hence there may be as many as meridians, as there are places lying eastwardly and westwardly of each other. When places are on different meridians, they are said to be in different longitude. Celestial meridians are lines passing among the stars from one celestial pole to the other, crossing the celestial equator at right angles. When it is noon at any place, the sun is in the celestial meridian directly over the meridian of that place. Let the instructer explain right angles. 58. To illustrate what has been said, let Pl. III. fig. 1. represent the earth. The line NS is its axis ; the extremities of which, N and S, are the north and south poles of the earth. EQ shows the equator. The lines 10 10, 20 20, 30 30, &c. are parallels of latitude ; and the lines NAS, NBS, &c. are meridians. If each of these meridians be supposed to extend quite round the earth, (as they do on the artificial globe,) each would divide it into an eastern and western hemisphere ; just as the equator divides it into northern and southern. Much of what is said in this chapter may be illustrated with a terrestrial and celestial globe, if at hand, far better than by any figure. 59. Latitude and longitude are expressed in degrees and minutes. The latitude of a place on the globe is estimated by the number of degrees on its meridian between the equator and that place. For example, the place x is in latitude 40° north, because 40° of its meridian lie between the equator and it. The longitude of one place from another is determined by the number of degrees there are on the equator, between the meridian of one and the meridian of the other. For example, the place v is 20° west longitude from X, and x is 20° east longitude from v, because 20° of the equator lie between the meridians of v and x; as may be seen by the figures under the equator. 60. Of all the lines or circles passing round the earth from west to east, it is obvious that the equator is the only one which constitutes a great circle, that is, divides the earth's surface into two hemispheres. All the rest are less circles, that is, divide the earth's surface into two unequal parts; and more unequal as the circles are farther from the equator and nearer the poles. On this account it is much more natural to reckon latitude from the equator than from any other line or circle. But all the meridians are great circles, each dividing the earth's surface into two hemispheres. Hence there is no natural reason why longitude should be reckoned from one meridian rather than from another. Hence it was customary, till very lately, for writers of different nations to estimate longitude from different meridians, each selecting that of the capital of his own country as the first or prime meridian, and reckoning the longitude of all other places from this. Thus French writers estimated longitude from the meridian of Paris ; British from that of London ; American from that of Philadelphia, and afterwards of Washington. The obvious confusion and inconvenience of this practice at length induced writers in Europe and America to fix upon one prime meridian ; and for this purpose selected that of the Royal Observatory at Greenwich, near London. Hence, on most maps and charts recently pablished, longitude is laid down from the meridian of London or Greenwich. I 61. The equator being once assumed as the circle from which to reckon latitude, the poles become natural limits, beyond which it cannot be reckoned. For if latitude be reckoned beyond the poles on one side, the equator is approached on the other. Hence no place can have latitude exceeding 90°, the distance from the equator to the poles. Having agreed upon a certain meridian from which to reckon longitude both east and west, the opposite part of that meridian, continued round the earth, becomes the limit of longitude, which is obviously half a circle or 180° from the prime meridian. Hence no place on the earth's surface can have more than 180° longitude, and if a place has 180° longitude, it may be either east or west. 62. But the latitude and longitude of heavenly bodies are estimated somewhat differently from those of places on the earth's surface. It has been stated that the circle among the stars which the plane of the equator, extended every way to the starry heavens, would describe, is called the celestial equator. Now this celestial equator does not coincide with the ecliptic, but makes an angle with it of 231°, that is, the earth’s axis is not perpendicular to the plane of the ecliptic, but is inclined 231. (See Pl. 5. fig. 1.) Thus we have two great circles, the ecliptic and equator, passing through the heavens eastwardly and westwardly, from which the latitude of the heavenly bodies might be estimated. Astronomers have selected the ecliptic for this purpose, and have supposed lines or circles to cross it at right angles, as the meridians do the equator ; which lines or circles are called secondaries to the ecliptic. The points where all the secondaries to the ecliptic meet, |