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many have imagined them to be commissioned to announce to mortals, war, pestilence, or some other dreadful cala. mity: better observations however and wiser judgments have at last restored them to their due station in the universe, and comets are now acknowledged to be opaque bodies of a spherical form, similar to this earth or any other of the planets, possessing no light but what is produced by the
Comets to the eye unassisted by glasses, appear like a star shining through a cloud, or in a fog; but by ineans of a telescope, present the appearance of a solid globe, with an atınosphere surrounding it to a great distance, in proportion to the diameter of the globe: this central body is called the nucleus or kernel of the comet. It has been observed, that in proportion as comets approach the Sun, the atmosphere inclosing them is diminished, while the tail is inereased, and on the other hand, that as they retire from the Sun, the tail is diminished and the atmosphere increased; hence it is evident that the tail or luminous substance, accompanying the comet, is in one way or other produced by the action of the sun, on the body of the atmosphere of the comet. Many theories have been suggested to explain the nature and causes of the tail of a comet, but nothing beyond conjecture can as yet be offered on the subject. From the various appearances assumed by comets in difierent parts of their orbiis, it becomes extremely difficult to ascertain, whether on their reappearance they are the same or different from those formerly observed; besides from the position of the earth, with respect to the comet and the sun it may frequently happen that a comet may turn round the sun, and even pass near the earth in the day time, so as not to be visible to us, and consequently the period of its revolution being unnoticed, must remain unascertained.
FIXED STARS.—The stars are readily distinguished from the planets, not only by their constantly remaining in the
same position relatively to one another, but by the bright twinkling light they emit. This light occasions them to appear much larger than when observed through a telescope, which excludes all rays but those proceeding from the star itself; and the brightest star seen from the earth, presents a diameter, scarcely susceptible of measurement. On account however of their various apparent sizes, the stars are arranged in different classes, those of the greatest brilliancy and apparent size being called stars of the first magnitude, those neat in brilliancy and apparent size are called stars of the second magnitude, and so on to the fifth and sixth magnitudes, which are the smallest perceptible to the naked eye in clear weather: smaller stars only discoverable by glasses are termed telescopic stars.
In the earliest times, astronorers for the better knowledge of the stars, distributed them into a variety of figures, termed clusters of stars, asteri-ms or constellations, named after different persons, animals, or other objects of celebrity in those days; and by means of these constellations, the position of any siar can be as readily ascertained as that of any town on a map of the globe of the earih : of these constellations the iwelve signs of the Zodiac already mentioned are a portion, the antients dividing the whole heavens into ahoui forty-eight constellations, to which, in later times, about twenty-four have been added.
Of the nature of the stars we can judge only from analogy, supposing each to be a sun immoveable in its place, possessing intrinsic power of affording heat and light to a number of opaque globes revoking round it, as the earth and the other solar planets do round the sun : and if we suppose the stars to be all of the same magnitude with the sun, and with each other, then their different apparent magnitudes inust be occasioned by their different distances from our system : the nearest star to us, as for example, Sirius or the Dog star, one of the brightest, is imagined to be 2 1 2
not less than four hundred thousand times farther from the sun than we are. To form some notion of the vast distance even of Sirius it is suficient to consider, that if his position with respect to other stars be observed when the earth is in any fixed point of her orbit, and also when after six months she has arrived in a point diametrically opposite to the formver, no sensible change in the relative position of Sirius and the adjoining stars can be perceived, although the earth has changed her position in the heavens the whole diameter of her orbit, or twice the distance from her to the sun.
Milky way. Besides the brilliant appearance of the stars the heavens are begirt by a broad swathe or belt of whitish light, divided in some parts into two: this from its colour is called the galaxy or milky way, and is produced by the accumulated light of an innumerable multitude of stars 100 small or too remole to be distinguished by the naked eye. Whatever may have been the original structure of the universe at its first coming into existence, it seems now evident that the stars themselves are not placed or fixed at equal distances the one from the other in every direction, but that they are distributed into layers or strata of vast extent in length and breadth, but of more confined thickness or depth. If this be the case it will naturally follow, that the stars of one of these strata will, to an observer looking at them laterally, appear much more closely situated than 10 one who looks at the stratuin on its broad surface. Since then by examining the milky way we discover not only a great accumulation of light but also a prodigious assemblage of stars, and this enclosing us like a circle extended in all directions, it is but rational to conclude that we ourselves are situated within the limits of the stratum of stars termed the milky way, and that our sun is only one of the slars of that stratum.
In various parts of the heavens are likewise discovered bright spots entirely similar in appearance to the Galaxy,
which when examined by glasses present in the same manner clusters of stars at a great distance, whose assembled light is the cause of the brightness we perceive : these spots are from their cloudy look cailed nebule.
The antient astronomers have mentioned stars in certain parts of the heavens, which are now no longer to be seen: and even in later times, stars have been observed gradually to increase in brilliancy and magnitude to a certain point, and then gradually to diminish to another point, whence they returned to their original brightness: others again have entirely disappeared for a long series of years, and periodically resumed their place in the firmament. Repeated and accurate observations have ascertained that such variations in the appearances of the stars do rarely happen, but the causes of these variations are hitherto beyond our reach.
Although the stars have for the purpose of the more readily ascertaining their positions been distributed into sigus and constellations, still a more precise determination of their several places has been requisite; and this is obtained by means of referring their situations to certain imaginary points and lines in the heavens, in a way corresponding to the method of determining the situation of places on the face of this earth, with respect to latitude and longitude.
Let us suppose the axis of the earth to be produced indefinitely to the heavens, and the plane of the earth's equator likewise indefinitely extended, if then a circle be imagined to pass through the two poles of that axis, and through the centre of any star or other heavenly body, the space intercepted between this body and the heavenly equator will measure how much the body falls off, or declines from the equalor, or it will express the declination of that body, and the circle passing through the body and the poles is termed a circle of declination: the declination being either north
or south, accordingly as the heavenly body is situated on the north or the south side of the equator.
Having thus obtained the position of a star, &c. in northing and southing, it is requisite also to determine its position in easting or westing, and for this purpose, if a circle of declination be supposed to pass through the poles and to intersert the equator at the points where it crosses the plane of the ecliptic or of the earth's arnual path at the time of the vernal and autummal equinoxes, this circle may be considered as a first meridian, from which the star's distance, east or west, is to be determined, by measuring the distance on the equatur, intercepted between this first meridian at the points of the vernal equinox, and the circle of declination passing through the given star. This space on the arch of the equator between the point of the vernal equinox, and the circle of declination of any heavenly, is called the right ascension of that body.
Again, the position of a star may be determined with respect to the ecliptic in latitude and longitude; the latitude being measured on a great circle passing through the poles of the ecliptic and the star, and the longitude being the arch of the ecliptic, intercepted between that circle and the point of intersection of the eclip:ic and the equator at the vernal equinox, or from the first point of the sign Aries, and both the longitude and the right ascension are calculaied according to the order of the signs, formerly pointed out, from o° 10 360°.
It had long been observed that although the stars seemed to retain their position with respect to latitude or their perpendicular distance from the ecliptic, yet, that their declination and right ascension underwent a certain degree of alteration. This could only be accounted for, by supposing that the first point of the sign áries, or that were at the vernal equinox the plane of the equator intersected the plane of the ecliptic, changed its place with respect to