outside of us with regard to the Sun, we never see him less than half illumined, but we do see him with a disk that lacks of being round,—about what the Moon shows us when two days off from full. It is when he is in quadrature—that is, a quarter way round the celestial circle from the Sun—that he shows thus, and we see him then with the telescope at closer range than we ever see the Moon without it. So observed we notice at once that his terminator, or inner edge, presents a very different appearance from the lunar one. Instead of looking like a saw, it looks comparatively smooth, like a knife. From this we know that, relatively to his size, he has no elevations or depressions upon his surface comparable to the lunar peaks and craters.
His terminator, however, is not absolutely perfect. Irregularities are to be detected in it, although much less pronounced than those of the Moon. His irregularities are of two kinds. The first, and by all odds the commonest phenomenon, consists in showing himself on occasions surprisingly flat; not in this case an inferable flatness, but a perfectly apparent one. In other words, his terminator does not show as a semi-ellipse, but as an irregular polygon. It looks as if in places the rind had been pared off. The peel thus taken from him, so to speak, is from twenty to forty degrees wide, according to the particular part of his surface that shows upon the terminator at the time.