CHAP. 21. (23.)—OF THE DIMENSIONS OF THE WORLD.
The stadium is equal to 125 of our Roman paces, or 625 feet[261]. Posidonius[262] supposes that there is a space of not less than 40 stadia around the earth, whence mists[263], winds and clouds[263] proceed; beyond this he supposes that the air is pure and liquid, consisting of uninterrupted light; from the clouded region to the moon there is a space of 2,000,000 of stadia, and thence to the sun of 500,000,000[264]. It is in consequence of this space that the sun, notwithstanding his immense magnitude, does not burn the earth. Many persons have imagined that the clouds rise to the height of 900 stadia. These points are not completely made out, and are difficult to explain; but we have given the best account of them that has been published, and if we may be allowed, in any degree, to pursue these investigations, there is one infallible geometrical principle, which we cannot reject. Not that we can ascertain the exact dimensions (for to profess to do this would be almost the act of a madman), but that the mind may have some estimate to direct its conjectures. Now it is evident that the orbit through which the sun passes consists of nearly 366 degrees, and that the diameter is always the third part and a little less than the seventh of the circumference[265]. Then taking the half of this (for the earth is placed in the centre) it will follow, that nearly one-sixth part of the immense space, which the mind conceives as constituting the orbit of the sun round the earth, will compose his altitude. That of the moon will be one-twelfth part, since her course is so much shorter than that of the sun; she is therefore carried along midway between the sun and the earth[266]. It is astonishing to what an extent the weakness of the mind will proceed, urged on by a little success, as in the above-mentioned instance, to give full scope to its impudence! Thus, having ventured to guess at the space between the sun and the earth, we do the same with respect to the heavens, because he is situated midway between them; so that we may come to know the measure of the whole world in inches. For if the diameter consist of seven parts, there will be twenty-two of the same parts in the circumference; as if we could measure the heavens by a plumb-line!
The Egyptian calculation, which was made out by Petosiris and Necepsos, supposes that each degree of the lunar orbit (which, as I have said, is the least) consists of little more than 33 stadia; in the very large orbit of Saturn the number is double; in that of the sun, which, as we have said, is in the middle[267], we have the half of the sum of these numbers. And this is indeed a very modest calculation[268], since if we add to the orbit of Saturn the distance from him to the zodiac, we shall have an infinite number of degrees[269].
CHAP. 22. (24.)—OF THE STARS WHICH APPEAR SUDDENLY, OR OF COMETS[270].
A few things still remain to be said concerning the world; for stars are suddenly formed in the heavens themselves; of these there are various kinds.
(25.) The Greeks name these stars comets[271]; we name them Crinitæ, as if shaggy with bloody locks, and surrounded with bristles like hair. Those stars, which have a mane hanging down from their lower part, like a long beard, are named Pogoniæ[272]. Those that are named Acontiæ[273] vibrate like a dart with a very quick motion. It was one of this kind which the Emperor Titus described in his very excellent poem, as having been seen in his fifth consulship; and this was the last of these bodies which has been observed. When they are short and pointed they are named Xiphiæ[274]; these are the pale kind; they shine like a sword and are without any rays; while we name those Discei[275], which, being of an amber colour, in conformity with their name, emit a few rays from their margin only. A kind named Pitheus[276] exhibits the figure of a cask, appearing convex and emitting a smoky light. The kind named Cerastias[277] has the appearance of a horn; it is like the one which was visible when the Greeks fought at Salamis. Lampadias[278] is like a burning torch; Hippias[279] is like a horse’s mane; it has a very rapid motion, like a circle revolving on itself. There is also a white comet, with silver hair, so brilliant that it can scarcely be looked at, exhibiting, as it were, the aspect of the Deity in a human form. There are some also that are shaggy, having the appearance of a fleece, surrounded by a kind of crown. There was one, where the appearance of a mane was changed into that of a spear; it happened in the 109th olympiad, in the 398th year of the City[280]. The shortest time during which any one of them has been observed to be visible is 7 days, the longest 180 days.