VIII
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But to return—why does the bow not complete the full circle in its form, but appear as only a semicircle when stretched to the full extent of its greatest span? Some are of opinion that the reason is that the sun, being much higher than the clouds, strikes them only on the upper side. Hence their lower parts are not touched by his light. Receiving the sun only on one side, the clouds reproduce only one portion of him, and this is never more than a half. There is very little force in this contention. My reason for saying so? The sun, even though he is on the upper side, yet strikes, and therefore colours, the whole cloud. How could it be otherwise? His rays are wont to be transmitted through the clouds and to penetrate any density in them. Further, the proof they advance is flatly in opposition to their main proposition. For if the sun is higher than the clouds, and his beams, therefore, shed only on their upper side, the bow would never come down as far as the earth. Yet it does descend to the very ground. Besides, the bow is never seen except opposite to, not below, the sun. The fact is, the sun’s highness or lowness does not affect the matter: the side of the cloud that faces him is struck by him throughout its whole extent.
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Furthermore, sometimes even the setting sun produces a rainbow; surely at that time, being near30 the earth, he strikes the clouds on their lower side. And yet then, too, the bow is only a semicircle, though the clouds receive the sunlight on their lower and darker portions. The Stoics, who hold that the light is reflected in the cloud as in a mirror, make the cloud hollow like the section of a ball. Such a mirror, being but part of a circle, cannot, they think, reproduce a whole circle. I give my adherence to the proposition, but I cannot agree to the argument in its support. For, if the whole figure of a circle placed opposite a concave mirror is reproduced in it, then there can surely be nothing to prevent the whole of a ball being seen in a semicircular circular mirror. Besides, we have already shown that complete rings resembling a rainbow surround the sun and the moon at times. Why should the circle be complete in the halo, but never in the rainbow? And then again, why should the clouds that receive the sunlight be always hollow ones, and not sometimes flat or bulging?
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Aristotle says that rainbows are formed, after the autumnal equinox, at any hour of the day, but in summer only either in the early part of the day, or when the sun has begun to sink. The cause of this is obvious. In the first place, about midday the great heat of the sun dispels the clouds: he cannot be reflected in the clouds which he breaks up. But in the early morning and as he sinks toward the west, his rays have less power, and can thus be resisted and reflected by the clouds. In the second place, the sun is not wont to form a bow except when he faces the clouds in which it is formed. When the days are shortening in autumn, his rays are always slanting. Therefore, he has some clouds facing him that he can strike, at any part of the day, even at the hour at which he attains his meridian height. But in the summer season he sails right overhead. Therefore, in the great altitude of his midday course, he looks down on the earth too directly to encounter any clouds. He has them at that period all beneath him.