SECTION III.

CATALOGUES AND MAPS OF THE STARS,

MAGNITUDES, NUMBER, AND MODE OF DISTINGUISHING THEM.

242. By measuring the right ascension and declination of the stars with great accuracy, for which purpose observatories have been founded in different parts of the world, astronomers have been enabled to catalogue the stars, and to indicate distinctly the exact point occupied by each one in the heavens.

At a very early period, probably as remote as the time of the Argonautic expedition (B. C. 1200), the whole face of the heavens was mapped out into constellations, or groups of stars: these were fancifully supposed to have a resemblance to certain figures of men or animals, and were supplied with names accordingly. This division, originally founded on no scientific basis, the moderns have retained; while the first formers of a catalogue of the stars of the southern hemisphere advanced one step further in absurdity, by introducing such constellations as the "air-pump," the "sculptor's tools," the "chemist's furnace," the "sextant," and other mathematical instruments. The only advantage of the division into constellations is, the facility they afford for roughly pointing out the position of a planet, the moon, or a comet; for more accurate reference we must have recourse to the right ascension and declination of a heavenly body, which will enable us to fix upon its position with the greatest nicety.

243. The most ancient catalogue of the stars is that

made by Hipparchus (B.c. 120), who was induced to undertake the labour by remarking the sudden appearance of a new star: being laudably anxious to afford to posterity the means of judging of the heavens as they appeared in his day, he commenced and completed an account of the position and magnitude of 1026 fixed stars. This catalogue, enlarged and improved, may be found in the works of Ptolemy, the celebrated geographer of Alexandria, who flourished A.D. 170. Ptolemy's catalogue gives the latitude and longitude of 2000 stars, arranged in forty-eight constellations.

As may be supposed, the invention of the telescope has brought into view and assisted in defining the places of thousands of stars which were unknown a few centuries ago. The united labours of European astronomers have succeeded in bringing this branch of astronomical science to the highest point of perfection. In the catalogue published by the British Association, the right ascension and declination of 8377 stars are laid down with great accuracy.

244. The Celestial Globe exhibits the figures of the constellations, and such of the stars as are visible, without the aid of the telescope; and it is a most valuable auxiliary in obtaining a knowledge of the positions of the stars. Its place may be supplied, but very inefficiently, by maps of the stars; of which those published by the Useful Knowledge Society, with the "Companion," by Mr. Augustus de Morgan, are the most available. The disadvantage of these maps is, that some constellations are of necessity only to be traced by consulting two maps, part appearing in one and part appearing in the other; while from the projection adopted,

(the gnomonic,) the bordering constellations of each map are much enlarged and distorted. In making use of the celestial globe no such difficulty exists. Let the globe be duly rectified, by placing the brass meridian exactly north and south, so that it may coincide with the true meridian of the place: let the pole be elevated as many degrees above the horizon as are equal to the latitude of the place you may be in: bring the sun's place for the day of observation to the meridian, and, having set the index to XII. turn the globe westward till the index points to the time of the evening; then will you have an accurate representation of the heavens for that instant, each constellation being exactly above its representative on the celestial globe. By continuing to turn the globe westward as the time passes, the student of astronomy will soon become acquainted with the diurnal motion of the heavens-the silent and imperceptible advance of the constellations from the eastward till they culminate, and eventually set. Facing northward, he will observe the beautiful revolution of the circumpolar stars round the Pole Star, which will ever appear fixed, as the centre of the celestial movements. Again, turning southward, he will remark (and after a few evenings fully comprehend the cause) that the diurnal arcs of those stars which rise and set decrease in size as they recede from the pole; till in the far south a star will rise no higher than a few degrees above the horizon, and describe an arc of small duration. A few evenings thus employed will tend more to explain the principles of the celestial sphere, than months spent over the globes without actual inspection of the heavens themselves.

245. The stars are said to be of the first, second, or third magnitudes, according to the gradations of the light they emit. The brightest stars are of the first magnitude: after the sixth magnitude they are termed telescopic stars. Sir John Herschel estimates the light of the first six magnitudes to be in the proportions of 100, 25, 12, 6, 2, 1. On maps of the stars, or on the celestial globe, the magnitudes are distinguished by the different sizes or shapes of the stars.

The number of stars visible to the naked eye on a starry night will not be more than 1000 of the whole number of stars, 17 are of the first magnitude, 76 of the second, 223 of the third.

The letters of the Greek alphabet are used to designate the largest stars in any constellation: thus, a Canis majoris denotes the brightest star in the Great Dog; B Orionis, the second-sized star in Orion; y Leonis, the third in size of the constellation Leo. When the stars of any constellation are so numerous that the letters of the Greek alphabet are exhausted, those of the English are used; and if more stars remain to be designated, recourse is had to numerals: it will thus appear that the Greek letters always indicate the largest stars in a constellation.

SECTION IV.

DETERMINATION OF THE POSITIONS OF THE STARS BY THE METHOD OF ALIGNMENTS.

246. THE following method of tracing the relative position of the constellations will facilitate an acquaint

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ance with the principal stars, and will be found useful in connexion either with maps of the stars or the celestial globe.

We will suppose the day of observation to be January the 1st, at eight o'clock in the evening; though the directions given will apply equally well on any other day of the year, to such of the constellations as may at that time be above the horizon, if it be borne in mind that they will then be differently circumstanced with regard to that circle.

The most remarkable constellation in the northern hemisphere is the Great Bear. Occupying as it does a large space in the heavens, and in our latitude being always visible, it may be discovered without difficulty, by remarking its form on the globe; and by means of it, when once known, we may gain acquaintance with the other constellations.

Four of the seven principal stars composing the Great Bear are in the form of a trapezium. Two of them, a and ẞ, called the Pointers, point directly to a star in the Little Bear, which is so near the north pole, that it has obtained the name of Polaris, or the Pole Star. On the 1st of January, at eight o'clock, the Great Bear will be midway between the horizon and the north Polar Star, in the direction N.N.E. The Pole Star is in the extremity of the tail of the Little Bear, which constellation resembles the Great Bear in form, but is inferior to it both in the space it occupies in the heavens, and in the magnitude of its component stars.

Between the Great and Little Bear lies part of the tail of Draco; which constellation half surrounds the Little Bear, and returns into itself with a serpentine