a few days; and, hence, would arise the denominations of the summer and winter solstices, which have been applied to the points of the heavens occupied by the sun on the days of midsummer and midwinter. The daily lengths of the shadows would increase from midsummer to midwinter in precisely the same order as that in which the diminutions took place; and a few repetitions of the observations of the days when the shadow was the longest and the shortest would, probably, suffice to enable the ancient astronomers to assign three hundred and sixty-five days to the length of the year, or period between two successive arrivals of the sun at the same solstice. The difficulty of ascertaining the precise days on which the shadow was the greatest and least would, no doubt, be immediately felt, but a mean of several observed times must have given the length of the year correctly, with respect to the number of complete days, and greater accuracy was probably for a long time disregarded. The near coincidence of the day of midsummer, or of the least shadow of the gnomon, with the first appearance of a rise in the waters of the Nile, on which the fertility of the soil of Egypt depends, appears to have induced the people of that country to consider the epoch of the summer solstice as the commencement of one of the parts into which they divided their agricultural year. The movements of the moon in the heavens would be found to resemble those of the sun, for she has, as we have already mentioned, one motion from the west towards the east, and another, in common with all the celestial bodies, in the contrary direction; again, she attains on different nights, different elevations above the horizon, when in the middle of her nocturnal course; and it must have been soon observed that, within a period equal to, but not always coincident with that which elapses between one change and the following, her elevation varies by a gradual increase from the least to the greatest, and then decreases as gradually to the least. CHAPTER IV. THE PRINCIPAL PHENOMENA OF THE SUN, The planets distinguished from the fixed stars.-Their names and symbols. -The visible movements of the inferior planets. Observed periods of their deviation from the sun. The superior planets are alternately in conjunction with, and in opposition to the sun.-Observed sidereal movements of the planets. Possibility that the modern planetary systems may have been imagined by the ancients.-The phenomena of eclipses observed.-Effects of eclipses on the minds of men.-The cause of solar eclipses early known.— Uncertainty of the ancients respecting the cause of lunar eclipses.—The most ancient eclipses recorded.-Probable nature of the early observations of eclipses. The apparent paths of the moon and planets, and the situation of the moon's apsides recognised.-The comets were noticed by the Chaldeans. THE practice of observing the heavens frequently in Egypt and Syria, countries which, as Plato observes in the dialogue Epinomis, enjoy, almost constantly, a serene and cloudless atmosphere, would in time permit the spectator to perceive that all the stars do not retain their relative distances unaltered: some of the principal stars would appear to be endowed with movements which resemble those of the sun and moon; for while they partake of the general motion of the celestial sphere from east to west they, at certain times, proceed with a different motion in the same, at other times in a contrary direction; and, again, at various times, when in the middle of their nocturnal course, they may be seen differently elevated above the horizon: lastly, there are certain periods when, for some days, they appear stationary in the heavens with relation to the other stars, though still partaking of the general motion from east to west. It might further be observed that two of these wandering stars, or planets as they were called, had this peculiarity, that they were never seen beyond a certain angular distance from the sun : one of them, to which the name of Mercury was given, would be visible only when near that luminary and for a short time after his setting or before his rising: the other, which was dis tinguished by the name of Venus, would exhibit similar phenomena, but would deviate further from the sun than Mercury; it would, consequently, rise at an earlier, or continue visible in the west to a later hour. The other planets, Mars, Jupiter and Saturn which, as well as those last mentioned, were known from the earliest period of astronomical history, would appear to have greater ranges of motion, being sometimes seen near the sun and sometimes at a distance from him equal to the whole diameter of the visible heavens; for they would frequently be seen rising in the east when that luminary was setting, or setting when he was rising. Jupiter and Venus from their superior brilliancy, which permits them to be visible when in the vicinity of the sun, were early distinguished by the epithets of Lucifer and Hesperus, and received the denomination of morning or evening stars according as they were seen with the sun at his rising or setting. The names of the planets may have been given from some resemblance imagined to exist between them and the attributes of the deities worshipped in the heathen world; thus one planet, from its swift motion and vicinity to the sun, received the name of the Messenger of the Gods; another, from the brilliancy of its light was denominated Venus, and a third, from its fiery aspect received the appellation of the God of War, and so on. While the reveries of a degrading superstition prevailed among men, a child which happened to be born at the time of the rising of any particular planet was considered as under the protection of the deity whose name it bore, and his disposition was supposed to resemble that of his patron-god. The Jewish and Arabian writers of the middle ages inform us that the Chaldeans gave to cach planet a certain symbol denoting its quality; as a hexagon to Saturn, a triangle to Jupiter, a square to the sun, and the like; and the symbol of the Deity, the First Cause, or the Eternal Mind was a circle which, as we know from other authorities, was considered as the most perfect figure. We are informed also that fire, or the sun, was considered as a symbol of the First Cause, and that all these symbols were worshipped as representations of their prototypes: and we learn from the same writers that the Chaldeans divided the heavens into what were called mansions; assigning to each planet a mansion and the care of some species of beings on the earth. According to Plutarch, Pythagoras ascribed the figures of what are called the regular bodies to the four elements and the universe; thus he is said to have made the cube represent the earth; the pyramid, fire; the octahedron, air; the icosahedron, water, and the dodecahedron, the whole of nature. M. Baillyb supposes these symbols to have originated in the idea that material substances are formed by crystallisation and that the regular bodies have, respectively, the forms of the primitive crystals in each of the elements; but it is more probable that the symbols, like those applied by the Chaldeans to the planets, were intended to represent the comparative excellences of the different elements themselves. The opinion of a divine agency in the stars, which was, first, attributed to them by the Chaldeans, probably spread from the country occupied by this people, as a centre, into other parts of the East, and to Egypt; from whence it was, in time, conveyed to Europe, where, during the long period which preceded the birth of the modern philosophy, it almost universally prevailed. There cannot be a doubt that the movements of the planets, as far as they could be ascertained by the unassisted eye were, from the first, attentively studied; but since the variations of their apparent forms could not, without the aid of telescopes, be distinguished, the principal fact on which a knowledge of the nature of their real movements depends was wanting; therefore if the ancients ever possessed that knowledge we are certain that they must have attained it by conjecture only. Yet enough could be observed, to have led them to the formation of an hypothesis concerning those motions similar to that which is now universally admitted; and sundry passages in the works of ancient authors have been adduced to prove that such an hypothesis had not escaped the ingenuity of the Egyptian and Greek philosophers. Whether this were the case or not will be for ever uncertain, but to enable us to form a judgment of its possibility De Placitis, Lib. II. Cap. 2. b Astron. Ancienne. it will be necessary to shew what were the phenomena which might have been observed before any instrument was invented, and before any means were in use for measuring time except those afforded by the succession of days and nights. Perhaps the most striking phenomena of the planetary motions would be the limited deviations of Mercury and Venus from the sun; repeated observations must have shewn that, from the respective times at which they first became visible near the rising sun, they recede from that luminary daily towards the south, to a certain extent; then returning eastward they again rejoin it and cease to be seen on that side of the horizon. After being for several days invisible, each planet may be seen to emerge from the light about the setting sun; from which time it again continues daily to recede from that luminary towards the south till the deviation is equal to that which was observed on the eastern side of the horizon; it is then found to return towards the west and, after being again concealed for some days in the solar light, it reappears before sunrise in the east, and the phenomena are repeated in the same order as before. The extent of the deviations of Venus must have been found to be nearly twice as great as that of the deviations of Mercury; and the former, if estimated by comparison with the circumference of a circle in the heavens, whose centre is at the eye of the spectator, would appear equal to about one-eighth of such circumference. At first, indeed, it might have been imagined that the star which appears near the sun in the morning is not the same as that which is, afterwards, seen near him in the evening; but the regularity of the phenomena we have described must soon have led to a conviction of their identity. It is asserted, however, by Pliny " that Pythagoras, who learned the fact in Egypt, was the first to make it known to the Greeks. The times in which the deviations are accomplished would be found to be variable, and they could not be estimated precisely because of the impossibility of observing the planets when very near the sun. But, as the interval between two consecutive appearances of Mercury in the east at sunrise might be found, Nat. Hist. Lib. II. |