observation, that this concave frame of stars extends beneath us as well as above us; for it seems to revolve like a wheel upon its axis, and presents successive portions of its surface to our view. Observing this revolution more attentively, we find a remarkable difference in the motions of the stars according to the direction in which we look. Looking in one direction, towards the south, we observe some stars which just make their appearance above the horizon, attain a very small elevation, and then descend, and shortly disappear. A little further from the south, we find the stars describing a larger arc, and appearing for a longer period. But on turning our view to the north, we observe stars which are constantly visible in the absence of the sun, performing their circuits entirely above the horizon, in circles one within another, till we arrive at a star which, being itself stationary, is the centre of the revolutions of all the rest. This remarkable star must doubtless have attracted at a very early period the notice of mankind, and the discovery of it may be considered as the first step in the science of Astronomy.* It is not, strictly speaking, motionless; for it describes a very small circle round a point not marked exactly by any star. This point, the exact centre of the celestial motions, is called the Pole, and the star which nearly marks its situation, is called the Pole-star.

*The Phoenicians are said to have used this star as a guide to direct their course at sea.-See Bossut's History of Mathematics, by Bonnycastle, p. 90; Rutherford's Ancient History, vol. i. p. 162.

By observations made in travelling northward or southward, it is soon found that the pole is more or less elevated according to the part of the earth from which it is viewed. Thus, in the north of Scotland it appears considerably higher than at London; and in Lapland it is almost overhead. We see it more elevated than the inhabitants of Spain; and they see it more elevated than the inhabitants of Barbary. If we travelled continually southwards, we should see the pole-star gradually sinking lower and lower, and at length disappearing below the horizon. But then another motionless point or pole would be discovered in the opposite direction, (though not distinguished by any remarkable star,) and another hemisphere of stars would appear performing in like manner their circuits round it.

The general appearance, therefore, of the starry heavens, is that of a vast concave sphere turning round two fixed points diametrically opposite to one another, which points are called the Poles of the heavens, and distinguished by the names of North and South. It is represented by the artificial Celestial Globe, with which the learner should here be made acquainted. It is a sphere revolving on an axis, and containing on its surface a map or plan of the stars, according to their true relative situation; but differing from the real sphere of the heavens in this respect, that it represents the stars on a convex surface, which we look upon externally, whereas we are placed within the real sphere, and view the stars on its apparent concave surface. The

learner has only to imagine the celestial globe considerably enlarged in its dimensions, and made transparent, and to suppose his eye placed in the centre looking upwards; he will then have in idea an exact facsimile of the starry heavens. The wooden circle which surrounds the globe represents the Horizon,* which bounds our view of the heavens. That part of the globe which is above the circle represents the visible hemisphere; that part which is below, the invisible hemisphere. The central point of the former, or that point in the real heavens which is immediately above the head of the observer, is called the Zenith. The point directly opposite to this in the invisible hemisphere, is called the Nadir. If the pole of the globe be elevated in the same proportion as the real pole of the heavens in the place where the observer is situated, it will then represent exactly that aspect of the heavens which is there presented to him. And if the globe be made to revolve from the side marked East, towards that marked West, it will represent the motion of the celestial sphere.

The learner will observe that the surface of the celestial globe is covered with a variety of figures of men, women, animals, and other objects. This is a contrivance invented in very early times, for facilitating the knowledge of the stars. The author, time, and place of this invention, are lost in the darkness of

This word is derived from the Greek verb opiw, which signifies

to bound or limit.

antiquity; but there is little doubt that it first arose from the fancied resemblance of certain groups of stars to the figures from which they have derived their names. In the fine climate of Chaldæa, where Astronomy was probably first cultivated, and among pastoral tribes who had often no other roof above them than the canopy of heaven, we may well suppose it was a common and favourite employment to gaze on the cloudless firmament, and to exercise their fancy and ingenuity in tracing these resemblances, some of which are sufficiently obvious. Some knowledge of these groups or constellations is indispensable to the student of Astronomy, but is only to be acquired by a careful observation of the heavens themselves, assisted either by a teacher or by a celestial globe. The more remarkable constellations, and those which the student .will most easily discover, are Ursa Major, Ursa Minor, which includes the polar star, Orion, Taurus, including the two remarkable groups called the Hyades and the Pleiades, Gemini, Leo, Auriga, Bootes, Corona Borealis, Cassiopeia, &c.

Besides being distributed into constellations, the stars are further distinguished according to their different size or brilliance, there being seven magnitudes denoted on the celestial globe by their respective figures. Most of the stars of the first magnitude, together with some of the second, have received particular names, and with the names and situation of these, the student should by all means render himself familiar. The other stars are distinguished on the globe by the

letters of the Greek alphabet, which are called Bayer's characters, from the philosopher who first employed them. Their use is to designate particular stars, the name of the constellation pointing out the region of the sphere where any star is situated, and the letter specifying what particular star of that constellation is referred to; as a Arietis, y Draconis, B Lyra.

CHAPTER II.

CONTAINING DEFINITIONS OF SOME OF THE MORE REMARKABLE POINTS AND CIRCLES OF THE CELESTIAL SPHERE.

THE celestial globe, besides presenting this general plan of the stars, has marked upon it several points and circles, the use of which I now proceed to explain.

From observing the manner in which the heavenly sphere revolves, nothing was more natural and obvious than to notice those four remarkable points on the horizon, which are called the Cardinal Points. The direction of the poles having first pointed out the North and South points of the horizon, it was natural to distinguish the two intermediate points, the one on that side of the horizon from which the motion of the heavens proceeds, called the East; the other on that