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.

(302.) 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, within which it suffers an interruption in its continuity. 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. It will be described more particularly in the subsequent portion of this work.

(303.) 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 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 (with certain exceptions among the smaller planets) not further than 8° or 9° on either side of the ecliptic.

(304.) 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 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.

(305.) 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 9.68, reckoned in mean solar time or 366d 6h 9m 9.68 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. When 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 366a 6h, &c. of sidereal time, will be called 365 days, 6 hours, &c., if reckoned in mean solar time. Thus, then, is the proportion between the mean solar and sidereal day established, 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 and borne in mind, can never become a source of error.

(306.) 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, for most ordinary purposes, as holding the same position in the sidereal heavens.

(307.) 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 next figure, if P p represent the north and south poles (by which when used without qualification we always mean the poles of the equinoctial), and EA QV the equinoctial, V SAW the ecliptic, and K k, its poles - the spherical angle Q VS is the obliquity of the ecliptic, and is equal in angular measure to P K or S Q. If we suppose the sun's apparent motion to be in the direction V SAW, 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, and W, that where it is farthest south, the winter solstice. 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 EKPQkp, 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.

(308.) 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 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 latitudepassing 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, P XR a circle of declination drawn

S

X

Z

W

R

through it, by which it is referred to the equinoctial, and KXT a circle of latitude referring it to the eclipticthen, as VR is the right ascension, and R X 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.

-

(309.) Knowing the right ascension and declination of an object, we may find its longitude and latitude, and vice versâ This is a problem of great use in physical astronomy-the following is its solution:- In our last figure, E K P Q, the solstitial colure is of course 90° distant from V, the vernal equinox, which is one of its poles so that V R (the right ascension) being given, and also V E, the arc E R, and its measure, the spherical angle E PR, or K P X, is known. In the spherical triangle K P X, then, we have given, 1st, The side P K, which, being the distance of the poles of the ecliptic and equinoctial, is equal to the obliquity of the ecliptic; 2d, The side P X, the polar distance, or the complement of the declination R X; and, 3d, the included angle KPX; and therefore, by spherical trigonometry, it is easy to find the other side K X, and the remaining angles. Now K X is the complement of the required latitude X T, and the angle P K X being known, and PK V being a right angle (because SV is 90°), the angle X KV becomes known. Now this is no other than the measure of the longitude V T of the object. The inverse problem is resolved by the same triangle, and by a process exactly similar.

(310.) It is often of use to know the situation of the ecliptic in the visible heavens at any instant; that is to say, the points where it cuts the horizon, and the altitude of its highest point, or, as it is sometimes called, the nonagesimal point of the ecliptic, as well as the longitude of this point on the ecliptic itself from the equinox. These, and all questions