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Let us now see how the brightness is produced that sometimes envelops the heavenly bodies. History has put on record that, on the day of the late Emperor Augustus’ entrance into Rome on his return from Apollonia, a parti-coloured circle, such as is wont to be seen in a rainbow, appeared round the sun. The Greeks call this a Halo; our most appropriate name for it is a Crown. Let me explain how it is formed. When a stone is thrown into a pond, the water is observed to part in numerous circles, which, very narrow at first, gradually widen out more and more until the impulse disappears, lost in the surface of the smooth water beyond. Let us suppose something of the same kind to occur in the atmosphere. When condensed it is capable of receiving an impact: the light of sun, moon, or any heavenly body encountering it forces it to recede in the form of circles. Moisture, be it observed, and air, and everything else that takes shape from a blow, is driven into the same form as that possessed by the object that strikes it. Now every kind of light is round. Therefore, the air when struck by light will assume this form. Accordingly the Greeks gave the name Threshing-floor (i.e. Halo) to a brightness of this kind, because spaces set apart for threshing corn were, as a rule, round.
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Be the better name threshing-floors, or be it crowns, there is no reason to suppose that they are formed in the neighbourhood of the heavenly bodies. They are a very long distance from them, though as seen from the earth they seem to touch and encircle them. In reality such an image is formed not very far from the earth, but the wonted frailty of human vision is deceptive, and we imagine the ring is formed close round the heavenly body itself. But no such thing could possibly occur in the neighbourhood of the sun and stars, as there is nothing but thin ether there. It is only when bodies have become rough and dense that shape can be impressed upon them. In subtle bodies there is no point on which form can lay hold or to which it can adhere. A phenomenon of the same nature as the halo may often be witnessed in baths, because the atmosphere is thick and dark: it is most frequent when the wind is in the south, when the air is heaviest and most dense.
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Halos sometimes are dissolved gradually and fade away, sometimes they are broken up on one side. In the latter case seafaring men look for wind in the direction in which the circle of the crown has been broken. If the parting is on the north, there will be a north wind, if on the west, zephyrs will follow. This is a proof that these crowns are formed in the region of the sky in which the winds are usually formed. The upper regions of air have no crowns because they have no winds either. An additional proof of the connection of winds and halos is afforded by the fact that the halo is never formed unless the atmosphere is at rest, and the wind, as it were, inactive. Under other circumstances it is not usually observed.
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The atmosphere when it is at rest may be fashioned to any pattern by being driven or drawn in any direction. But when it is in motion, light cannot even strike it. It takes no shape and offers no resistance, because the part first affected is always dissipated by the motion. Therefore it is that no heavenly body can ever be surrounded by a figure of the kind referred to unless when the atmosphere is dense and motionless, and so preserves the ray of round light that strikes upon it. Nor is it without good reason. Recollect the analogy mentioned a little ago. A pebble thrown into a pond or lake or any other circumscribed piece of water produces innumerable circles; but it has not the same effect if thrown into a river. And why so? Because in the latter case the water as it hurries on prevents the formation of any definite figure. So in the atmosphere the same thing happens; when it is stationary, it may receive a pattern; when it rushes in rapid motion, it evades all control, warding off every blow and every form as it approaches. When these crowns, of which I have spoken, have disappeared uniformly on all sides, and vanished in their own tracks, it is an indication of equilibrium in the atmosphere: there is perfect quietness and you may then look out for rain. When they break up at one side, it means wind in that quarter. If they burst at several points, a storm is brewing. The reason of this may be gathered from the explanations I have now given. If the ring fade all round, it is evident that the atmosphere is equable, and therefore calm. But if it is broken through on one side, evidently there must be an inclination of the air in that direction: hence that quarter will produce wind. But when the halo is rent and torn on all sides, plainly an attack is being made on it from several quarters at once, and a disquieted atmosphere is assailing it on this side and on that. So this disturbance of the heavens, the repeated effort and striving in all directions, betokens evidently that a storm is coming up with sudden shiftings of the wind.
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These crowns may be observed generally by night round the moon and other stars, but very seldom by day; in fact, so rarely in the latter case, that certain of the Greeks have denied that they appear at all by day. But history proves that they do. The cause of the infrequency of their appearance by day is that the sun’s light is stronger then, and the atmosphere itself when stirred and warmed by it is less dense. The moon’s power, on the other hand, is feebler, and is therefore more easily resisted by the surrounding air. The rest of the heavenly bodies are equally weak, and unable by their own force to burst through the atmosphere. So their shape is impressed and retained in the more solid and less yielding medium. For, in order to produce the phenomenon, the atmosphere must neither be so thick as to exclude or dissipate the light that streams in on it, nor yet so thin and rare as to furnish no hold to the rays that fall upon it. This particular consistency is obtained at night: the sluggish air is at that time struck with the faint light from moon or stars without violence or rudeness, and, being thicker than it is wont to be by day, is tinged thereby.