The same author tells us of an eclipse of the moon that was predicted by one Gallus, a tribune of the second legion, on the eve of the battle of Pydna—a prediction which was duly fulfilled on the following night. The fact of its having been foretold quieted the superstitious fears of the soldiers, and gave them a very high opinion of Gallus. Other authors, among them Cicero, do not give so flattering a story, but state that Gallus's part consisted only in explaining the cause of the eclipse after it had happened. The date of this eclipse was the 3rd of September, 168 B.C.
Ennius, writing towards the end of the second century B.C., describes an eclipse which was said to have happened nearly two hundred years before (404, B.C.), in the following remarkable words:—"On the nones of July the moon passed over the sun, and there was night." Aristarchus, three centuries before Christ, understood and explained the nature of eclipses; but the chief of the ancient authors upon this subject was Hipparchus. He and his disciples were able to predict eclipses with considerable accuracy, both as to their time and duration. Geminus and Cleomedes were two other writers, somewhat later, who explained and predicted eclipses. In later times regular tables were drawn up, showing when the eclipses would happen. One that Ptolemy was the author of was founded on data derived from ancient observers—Callipus, Democritus, Eudoxus, Hipparchus—aided by his own calculations. After the days of Ptolemy the knowledge of the eclipses advanced pari passu with the advance of astronomy generally. So long as astronomy itself was empirical, the time of the return of an eclipse was only reckoned by the intervals that had elapsed during the same portion of previous cycles; but after the discovery of elliptic orbits and the force of gravitation the whole motion of the moon could be calculated with as great accuracy as any other astronomical phenomenon.
In point of fact, if the new moon is in the plane of the ecliptic there must be an eclipse of the sun; if the full moon is there, there must be an eclipse of the moon; and if it should in these cases be only partially in that plane, the eclipses also will be partial. The cycle of changes that the position of the moon can undergo when new and full occupies a period of eighteen years and eleven days, in which period there are forty-one eclipses of the sun and twenty-nine of the moon. Each year there are at most seven and at least two eclipses; if only two, they are eclipses of the sun. Although more numerous in reality for the whole earth, eclipses of the sun are more rarely observed in any particular place, because they are not seen everywhere, but only where the shadow of the moon passes; while all that part of the earth that sees the moon at all at the time sees it eclipsed.
We now come to comets.
The ancients divided comets into different classes, the chief points of distinction being derived from the shape, length, and brilliancy of the tails. Pliny distinguished twelve kinds, which he thus characterised:—"Some frighten us by their blood-coloured mane; their bristling hair rises towards the heaven. The bearded ones let their long hair fall down like a majestic beard. The javelin-shaped ones seem to be projected forwards like a dart, as they rapidly attain their shape after their first appearance; if the tail is shorter, and terminates in a point, it is called a sword; this is the palest of all the comets; it has the appearance of a bright sword without any diverging rays. The plate or disc derives its name from its shape, its colour is that of amber, it gives out some diverging rays from its sides, but not in large quantity. The cask has really the form of a cask, which one might suppose to be staved in smoke enveloped in light. The retort imitates the figure of a horn, and the lamp that of a burning flame. The horse-comet represents the mane of a horse which is violently agitated, as by a circular, or rather cylindrical, motion. Such a comet appears also of singular whiteness, with hair of a silver hue; it is so bright that one can scarcely look at it. There are bristling comets, they are like the skins of beasts with their hair on, and are surrounded by a nebulosity. Lastly, the hair of the comet sometimes takes the form of a lance."
Pingré, a celebrated historian of comets, tells us that one of the first comets noticed in history is that which appeared over Rome forty years before Christ, and in which the Roman people imagined they saw the soul of Cæsar endowed with divine honours. Next comes that which threw its light on Jerusalem when it was being besieged and remained for a whole year above the city, according to the account of Josephus. It was of this kind that Pliny said it "is of so great a whiteness that one can scarcely look at it, and one may see in it the image of God in human form."
Diodorus tells us that, a little after the subversion of the towns of Helix and Bura, there were seen, for several nights in succession, a brilliant light, which was called a beam of fire, but which Aristotle says was a true comet.
Plutarch, in his life of Timoleon, says a burning flame preceded the fleet of this general until his arrival at Sicily, and that during the consulate of Caius Servilius a bright shield was seen suspended in the heavens.
The historians Sazoncenas and Socrates relate that in the year 400 A.D. a comet in the form of a sword shone over Constantinople, and appeared to touch the town just at the time when great misfortunes were impending through the treachery of Gainas.
The same phenomenon appeared over Rome previous to the arrival of Alaric.