ON THE APPARENT MOTIONS OF THE VENLY BOL I ES.
Having given a very brief sketch of the history of this science, than which, few, if any, have higher claims to our venera tion and regard, we proceed to consider the science itself, intending to present the reader with a popular outline of the stu dy, freed as much as possible from ma thematical rrinciples, upon which it de pends, but for which few, in comparison, can be supposed, from previous studies, to haN e any taste.
When we cast our eyes towards the heavens, we perceive a vast concave he misphere, at an unknown distance, of which the eye seems to constitute the centre. The earth stretches at our feet like an immense plain, and appears to meet, and to bound, the heavenly hemis phere. The circle around, where the earth and heavens seem to meet and touch each other, is called the horizon. It is natural to imagine, that, besides the hemisphere which we perceive, there is another, exactly similar, concealed from our view by the earth, and that the earth therefore is suspended in the middle of this heavenly sphere, with all its inhabi tants. A little observation turns this sus picion into certainty; for in a clear even ing the heavenly hemisphere is seen Studded with stars, and its appearance is changing every instant. New stars are continually rising in the east, while others are setting in the west. Those stars, that early in the evening are seen just above the eastern horizon,will at midnight be seen in the middle of the starry hemisphere, and may be traced moving gradually towards the west, till at length they sink below the horizon. If we look to the north, we perceive that many stars in that quarter never set at all, but move round and round, describing a complete circle in 24 hours : these describe their circles round a fixed point in the heavens,and the circles diminish more and more the nearer the star is to that point. This fixed point is called the north pole. There must be .a similar fixed point in the southern hemis phere, called the south pole. In this way theheavenly sphere appears to turn round two fixed points, called the -poles, once in every 24 hours. The imaginary line which joints the points is called the axis of the world. We shall endeavour to il lustrate this by means of a figure.
Let II 0 (fig 1. Plate II.) represent the circle of the horizon, seen edgeways, when it will appear as a straight line : let H P E R Q represent the complete sphere of the heavens, of which H MO Is supposed to be the visible hemisphere, and H N 0 the invisible hemisphere: then P will be the pole or fixed point among the stars, visible to us, round which they all appear to turn, and will be the oppo site pole; the line P It will be the axis of the sphere.
To obtain precise views of the motions of the heavenly bodies, it is necessary to be able to assign precisely the place in which they are. This is done by means of several imaginary lines or circles, sup posed to be described upon the surface of the sphere. These circles are divided into degrees, minutes, and seconds. The great circle of the sphere, Q B, which is perpendicular to the axis of the world, and of course 90° distant from either pole, is called the equator. The smaller cir cles which the stars describe, in conse quence of their apparent diurnal motions, are called parallels, because they are pa rallel to the equator. The equator di
vides the heavenly sphere into two equal parts, the north and south ; but, to be able to assign the position of the stars, it is necessary to have another circle pass ing through the poles, and cutting the equator perpendicularly ; this is called the meridian, which is supposed to pass through the poles, and also directly over the head of the observer, M, and the point, N, exactly opposite to that. The first of these points is called the zenith, and the second is denominated the nadir. The meridian divides the circles describ ed by the stars into two equal parts, and when they reach it, they are either at their greatest height above the horizon, or they are at their least height. The si tuation of the pole is readily found, it be ing precisely halfway between the great est and least height of those stars that never set. Since H M 0, the visible part of the heavens, contains 130°, and it is 90°, between the pole, P, and the equator, E Q ; if, therefore, we take away P E from the semicircle HM 0, there remains 90° for the other two arcs, P H and E 0, that is, the elevation of the pole and the equator are, together, equal to 90°, so that the one being known and subtracted from 90°, the other also is found. Hence it is known, that " the elevation of the pole at any place is the complement of the elevation of the equator," or what that elevation wants of 90°. Hence also "the elevation of the equator is equal to the distance from the pole, P, to the zenith, M ;" for the elevation of the equa tor is the difference between that of the pole and 90°.
When we travel towards the north, we perceive that the north pole does not re main stationary, hut rises towards the ze nith, in proportion to the space that we pass over. On the contrary, it sinks just as much when we travel towards the south : from which we infer that the sur face of the earth is not plane, as would ap pear to a superficial observer, but curved. The heavenly bodies appear to describe a complete circle round the earth every 24 hours ; but, besides these motions, which are common to them all, there are several which possess motions peculiar to themselves. The sun is farther towards the south during winter than during sum mer; he does nottherefore keep the same station in the heavens, nor describe the same circle everyday. The moon not only changes her form, diminishes and increases, but, if she is observed in rela tion to certain fixed stars, it will be found that she proceeds to the eastward, mak ing, progress every clay, till, in about a month, she makes a complete tour of the heavens. There are eight other stars, which are continually changing their place ; sometimes they seem to be mov ing to the westward, sometimes to the eastward, and sometimes they appear sta tionary for a considerable time : these called planets. There are other bodies which appear only occasionally, move for some time with very great velocity, and afterwards advance beyond the regions visible to us : these are comets, of which one is now (November, 1807,) apparent. The greater number of the heavenly bo dies always retain the same, or nearly the same, relative distance from each other, and !are, that account, called stars.