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 deg., a zone of more than 22 deg. 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.
[Illustration: GEOGRAPHICAL ASPECT OF VENUS.]
BY SIR ROBERT S. BALL.
The group of bodies which cluster around our sun forms a little island, so to speak, in the extent of infinite space. We may illustrate this by a map in which we shall endeavor to show the stars placed at their proper relative distances. We first open the compasses one inch, and thus draw a little circle to represent the path of the earth. We are not going to put in all the planets. We take Neptune, the outermost, at once. To draw its path I open the compasses to thirty inches, and draw a circle with that radius. That will do for our solar system, though the comets no doubt will roam beyond these limits. To complete our map we ought of course to put in some stars.