They were again seen, however, by Derham about 1715, and six years later by Kirch, in Berlin, who has the following entry in his diary for Saturday, June 29, 1721:—“I found Venus in a region where the sky was not very clear. The planet was narrow, and I seemed to see its dark side, though this is almost incredible. The diameter of Venus was 65”, and its sickle seemed to tremble in the atmospheric vapors.” Again, on March 8th, 1726, he records a similar observation. “We observed Venus with the twenty-six foot telescope. I perceived her dark side, and its edge seemed to describe a smaller circle than that of the light side, as is the case of the moon.” This effect is due to irradiation, that is to say, to the glare from a bright surface, giving a deceptive enlargement to its apparent area. He again saw the dark side of the planet in October, 1759, as did Harding at Goettingen, with Herschel’s ten-foot reflector, on January 24th, 1806. This latter observer saw it on this occasion stand out against the background of the sky as of a pale ashen green, while on February 28th following, it seemed to him of a pale reddish gray, like the color of the eclipsed moon.
That the latter body should send to us from her nocturnal shadows sufficient light to be visible is easily explicable, since she is then flooded with earth-light reflected on her from a surface thirteen and one-half times greater than her own, and probably casting on her an illumination transcending our full moonlight in the same proportion. But the secondary light of Venus admits of no such explanation, since earth-light on her surface, diminished by 1/12000th part compared to what it is on that of the moon, would be quite insufficient to render her visible to our eyes. The phenomenon was therefore adduced as an argument for the habitability of the planets by Gruithuisen, of the Munich Observatory, who, writing early in this century, suggested that the ashen light of Venus might be due to general illuminations in celebration by her inhabitants of some periodically recurring festivity, The materials for a flare-up on so grand a scale would, he thought, exist in abundance, as he conjectured the vegetation of our planetary neighbor to be more luxuriant than that of our Brazilian forests. The phosphorescence of the Aphroditean oceans, warm and teeming with life, as they are held to be by Zollner, was advanced as an explanatory hypothesis, with scarcely more plausibility, by Professor Safarik, while others have resorted to the supposition of atmospheric or electrical luminosity producing on a large scale some such display as that of the aurora borealis.
Professor Vogel, of Berlin, who himself saw part of the night-side of Venus, in its semi-obscurity in November, 1871, ascribed its visibility to a twilight effect caused by a very extensive atmosphere. The light thus transmitted to us by aerial diffusion and giving the ashen light, is reflected sunlight, while that sent by the luminous arc on its edge is direct sunlight, refracted, or bent round to us, from behind the planet. The silver selvedge of the dawn edging the dark limb may consequently be the brightest part of the broken nimbus that then seems to surround her.