CHAP. IV. THE MILKY WAY. THE ZODIAC. 163 treat them lightly, or altogether disregard them*, except for briefly naming remarkable stars, as a Leonis, ß Scorpii, &c. &c., by letters of the Greek alphabet attached to them. The reader will find them on any celestial charts or globes, and may compare them with the heavens, and there learn for himself their position. (253.) There are not wanting, however, natural districts in the heavens, which offer great peculiarities of character, and strike every observer: such is the milky way, that great luminous band, which stretches, every evening, all across the sky, from horizon to horizon, and which, when traced with diligence, and mapped down, is found to form a zone completely encircling the whole sphere, almost in a great circle, which is neither an hour circle, nor coincident with any other of our astronomical grammata. It is divided in one part of its course, sending off a kind of branch, which unites again with the main body, after remaining distinct for about 150 degrees. This remarkable belt has maintained, from the earliest ages, the same relative situation among the stars; and, when examined through powerful telescopes, is found (wonderful to relate!) to consist entirely of stars scattered by millions, like glittering dust, on the black ground of the general heavens. (254.) Another remarkable region in the heavens is the zodiac, not from any thing peculiar in its own constitution, but from its being the area within which the apparent motions of the sun, moon, and all the greater planets are confined. To trace the path of any one of these, it is only necessary to ascertain, by continued observation, its places at successive epochs, and entering these upon our map or sphere in sufficient number to form a series, not too far disjoined, to connect them by lines from point to point, as we mark out the course of * This disregard is neither supercilious nor causeless. The constellations seem to have been almost purposely named and delineated to cause as much confusion and inconvenience as possible. Innumerable snakes twine through long and contorted areas of the heavens, where no memory can follow them; bears, lions and fishes, large and small, northern and southern, confuse all nomenclature, &c. A better system of constellations might have been a material help as an artificial memory. a vessel at sea by mapping down its place from day to day. Now when this is done, it is found, first, that the apparent path, or track, of the sun on the surface of the heavens, is no other than an exact great circle of the sphere which is called the ecliptic, and which is inclined to the equinoctial at an angle of about 23° 28', intersecting it at two opposite points, called the equinoctial points, or equinoxes, and which are distinguished from each other by the epithets vernal and autumnal; the vernal being that at which the sun crosses the equinoctial from south to north; the autumnal, when it quits the northern and enters the southern hemisphere. Secondly, that the moon and all the planets pursue paths which, in like manner, encircle the whole heavens, but are not, like that of the sun, great circles exactly returning into themselves and bisecting the sphere, but rather spiral curves of much complexity, and described with very unequal velocities in their different parts. They have all, however, this in common, that the general direction of their motions is the same with that of the sun, viz. from west to east, that is to say, the contrary to that in which both they and the stars appear to be carried by the diurnal motion of the heavens; and, moreover, that they never deviate far from the ecliptic on either side, crossing and recrossing it at regular and * equal intervals of time, and confining themselves within a zone, or belt (the zodiac already spoken of), extending 9° on either side of the ecliptic. (255.) It would manifestly be useless to map down on globes or charts the apparent paths of any of those bodies which never retrace the same course, and which, therefore, demonstrably, must occupy at some one moment or other of their history, every point in the area of that zone of the heavens within which they are circumscribed. The apparent complication of their movements arises (that of the moon excepted) from our viewing them from a station which is itself in motion, and would disappear, could we shift our point of view and observe them from the sun. On the other hand the apparent CHAP. IV. THE ECLIPTIC. SIDEREAL YEAR. 165 motion of the sun is presented to us under its least involved form, and is studied, from the station we occupy, to the greatest advantage. So that, independent of the importance of that luminary to us in other respects, it is by the investigation of the laws of its motions in the first instance that we must rise to a knowledge of those of all the other bodies of our system. (256.) The ecliptic, which is its apparent path among the stars, is traversed by it in the period called the sidereal year, which consists of 365d 6h 9m 95.6, reckoned in mean solar time, or 366d 6h 9m 95.6 reckoned in sidereal time. The reason of this difference (and it is this which constitutes the origin of the difference between solar and sidereal time) is, that as the sun's apparent annual motion among the stars is performed in a contrary direction to the apparent diurnal motion of both sun and stars, it comes to the same thing as if the diurnal motion of the sun were so much slower than that of the stars, or as if the sun lagged behind them in its daily course. Where this has gone on for a whole year, the sun will have fallen behind the stars by a whole circumference of the heavens or, in other words in a year, the sun will have made fewer diurnal revolutions, by one, than the stars. So that the same interval of time which is measured by 366d 6h, &c. of sidereal time, if reckoned in mean solar days, hours, &c. will be called 365d 6h, &c. Thus, then, is the proportion between the mean solar and sidereal day esta.. blished, which, reduced into a decimal fraction, is that of 1.00273791 to 1. The measurement of time by these different standards may be compared to that of space by the standard feet, or ells of two different nations; the proportion of which, once settled, can never become a source of error. (257.) The position of the ecliptic among the stars may, for our present purpose, be regarded as invariable. It is true that this is not strictly the case; and on comparing together its position at present with that which it held at the most distant epoch at which we possess observations, we find evidences of a small change, which theory accounts for, and whose nature will be hereafter explained; but that change is so excessively slow, that for a great many successive years, or even for whole centuries, this circle may be regarded as holding the same position in the sidereal heavens. (258.) The poles of the ecliptic, like those of any other great circle of the sphere, are opposite points on its surface, equidistant from the ecliptic in every direction. They are of course not coincident with those of the equinoctial, but removed from it by an angular interval equal to the inclination of the ecliptic to the equinoctial (23° 28'), which is called the obliquity of the ecliptic. In the annexed figure, if Pp represent the north and south poles (by which, when used without qualification we always mean the poles of the equinoctial), and EQAV the equinoctial, V SA W the ecliptic, and Kk, its poles the spherical angle QVS is the obliquity of the ecliptic, and is equal in angular measure to P K or SQ. If we suppose the sun's apparent motion to be in the direction V SA W, V will be the vernal and A the autumnal equinox. S and W, the two points at which the ecliptic is most distant from the equinoctial, are termed solstices, because, when arrived there, the sun ceases to recede from the equator, and (in that sense, so far as its motion in declination is concerned) to stand still in the heavens. S, the point where the sun has the greatest northern declination, is called the summer solstice, and W, that where it is farthest south, the winter. These epithets obviously have their origin in the dependence of the seasons on the sun's declination, which will be explained in the next chapter. The circle E K PQk p, which passes through the poles of the ecliptic and equinoctial, is called the solstitial colure; and a meridian drawn through the equinoxes, PV p A, the equinoctial colure. (259.) Since the ecliptic holds a determinate situation in the starry heavens, it may be employed, like the equinoctial, to refer the positions of the stars to, by circles CHAP. IV. CELESTIAL LONGITUDES AND LATITUDES. 167 drawn through them from its poles, and therefore perpendicular to it. Such circles are termed, in astronomy, circles of latitude the distance of a star from the ecliptic, reckoned on the circle of latitude passing through it, is called the latitude of the stars-and the arc of the ecliptic intercepted between the vernal equinox and this circle, its longitude. In the figure X is a star, PX R a circle of declination drawn through it, by which it is referred to the equinoctial, and K X T a circle of latitude referring it to the ecliptic then, as VR is the right ascension, and RX the declination, of X, so also is V T its longitude, and T X its latitude. The use of the terms longitude and latitude, in this sense, seems to have originated in considering the ecliptic as forming a kind of natural equator to the heavens, as the terrestrial equator does to the earth the former holding an invariable position with respect to the stars, as the latter does with respect to stations on the earth's surface. The force of this observation will presently become apparent. (260.) Knowing the right ascension and declination of an object, we may find its longitude and latitude, and vice versa. This is a problem of great use in physical astronomy the following is its solution: In our last figure, E K PQ, the solstitial colure is of |