589. The stars, from their apparently various magnitudes, have been distributed into several classes, or orders. Those which appear largest, are called stars of the first magnitude; the next to them in lustre, stars of the second magnitude; and so on to the sixth, which are the smallest that are visible to the bare eye. This distribution having been made long before the invention of telescopes, the stars which cannot be seen without the assistance of these instruments, are distinguished by the name of telescopic stars.

590. The ancients divided the starry sphere into particular constellations, or clusters of stars, according as they lay near one another, so as to occupy those spaces which the figures of different sorts of animals or things would take up, if they were there delineated. And those stars which could not be brought into any particular constellation, were called unformed stars.

591. By this division, the stars are so distinguished from one another, that any particular star may be readily found in the heavens, by means of a celestial globe; on which the constellations are so delineated, that the most remarkable stars are placed in such parts of the figures, as are most easily distinguished. See plates I, and II.

592. The number of the ancient constellations is forty-eight, and upon our present globes about Seventy. On Senex's globes are inserted Bayer's letters; the first in the Greek alphabet being put to the largest star in each constellation, the second to the next, and so on; by which means every star is as easily found as if a name were given to it. Thus if the star y in the constellation of the ram be mentioned, every astronomer knows as well what star is meant, as if it were pointed out to him in the heavens.

593. The starry heavens are also divided into three parts, viz. 1. The Zodiac, which extends quite round the heavens; is about 16° broad, so that it takes in the orbits of all the planets, as well as that of the moon; and along the middle of which is the ecliptic. 2. All that region of the heavens which is on the north side of the zodiac, containing twenty-one constellations; and, 3. That on the south side, containing fifteen.

594. The following tables exhibit the names of the ancient and modern constellations, and the number of stars observed in each of them by different astronomers: