The parallel between Venus and
"That orbed maiden with fire laden,
Whom mortals call the moon,"
is carried a stage further. Most of us are familiar with the spectacle in which the Ancient Egyptians saw symbolized Horus on the lap of Isis, but which we more prosaically term "the old moon in the new moon's arms." The strongly illuminated half circle next the sun is then seen embracing with its horns a dusky sphere, contrasting with it as tarnished silver does with the newly burnished metal. The same phenomenon is occasionally, though very rarely, exhibited by Venus, while close to the sun at inferior injunction, when the shadowy form of the full orb is seen to shine dimly within her crescent with what is termed "the ashen light." More wonderful still, this "glimmering sphere" is then crowned, as with a silver halo, by a thin luminous arch, forming a secondary sickle facing the one nearest the sun, and doubtless due to the refraction of his rays round the globe of the planet, through the upper regions of her twilight atmosphere. This spectacle was first observed by the Jesuit Ricciolo, an opponent of the Copernican theory, on January 9th, 1643. He describes the planet as ruddy near the sun, yellowish in the middle, and of greenish blue on the side remote from the sun; while he also noted the bow of light limiting the dark hemisphere. Scarcely daring to trust his own eyesight, he ascribed these appearances, although he recorded them, to illusory reflection in the telescope.
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 Göttingen, 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.
A similar appearance is observed during transits of Venus, when she passes directly between us and the actual solar disk. A silver thread is then seen encircling that side of the planet which has not yet entered on the face of the sun or "a shadowy nebulous ring," as it was described by Mr. Macdonnell at Eden, surrounds the whole planetary disk when two-thirds of it have passed the solar edge. As it moves off it, the same aureole again becomes visible, testifying to the existence of an atmosphere of considerable extent exterior to the sharply outlined surface ordinarily visible. The shimmering haze of reflected sunlight which perpetually enfolds her is only made apparent to us under exceptional circumstances which cut off some portion of her more immediate light, just as we see the motes in the air illuminated by a candle if we hide the actual flame from our eyes. The perennial twilight which seems to reign over the nocturnal hemisphere of Venus may compensate, perhaps, for the want of a satellite to modify its darkness.
The great prolongation at other times of the horns of her crescent, so as to embrace almost her entire circumference with a tenuous ring of light, is doubtless due to the same cause, as their visibility should otherwise be limited to a half segment of a circle. The regions thus shining to us are obviously those on which the sun has not yet set, his appearance above the horizon being prolonged, as in our own case, by refraction, though to a much larger extent. The magnitude of the sun's disk as seen from Venus, a third larger than it appears to us, is also adducted by Mr. Proctor in his posthumous work, "The Old and the New Astronomy," edited and completed by Mr. A.C. Ranyard, as an element in extending the illumination of Venus to more than a hemisphere of her surface. As his diameter there is 44-1/4°, a zone of more than 22° wide outside the sunward hemisphere is he thinks illuminated by direct though partial sunlight, the orb being throughout this tract still partially above the horizon.