And next as to the all-important question of atmosphere. In what precedes, the presence of an atmosphere has been assumed, and, fortunately, there is very convincing evidence, both visual and spectroscopic, that Venus is well and abundantly supplied with air, by which it is not meant that Venus's air is precisely like the mixture of oxygen and nitrogen, with a few other gases, which we breathe and call by that name. In fact, there are excellent reasons for thinking that the atmosphere of Venus differs from the earth's quite as much as some of her other characteristics differ from those of our planet. But, however it may vary from ours in constitution, the atmosphere of Venus contains water vapor, and is exceedingly abundant. Listen to Professor Young:
"Its [Venus's] atmosphere is probably from one and a half to two times as extensive and as dense as our own, and the spectroscope shows evidence of the presence of water vapor in it."
And Prof. William C. Pickering, basing his statement on the result of observations at the mountain observatory of Arequipa, says: "We may feel reasonably certain that at the planet's [Venus's] surface the density of its atmosphere is many times that of our own."
We do not have to depend upon the spectroscope for evidence that Venus has a dense atmosphere, for we can, in a manner, see her atmosphere, in consequence of its refractive action upon the sunlight that strikes into it near the edge of the planet's globe. This illumination of Venus's atmosphere is witnessed both when she is nearly between the sun and the earth, and when, being exactly between them, she appears in silhouette against the solar disk. During a transit of this kind, in 1882, many observers, and the present writer was one, saw a bright atmospheric bow edging a part of the circumference of Venus when the planet was moving upon the face of the sun—a most beautiful and impressive spectacle.
Even more curious is an observation made in 1866 by Prof. C.S. Lyman, of Yale College, who, when Venus was very near the sun, saw her atmosphere in the form of a luminous ring. A little fuller explanation of this appearance may be of interest.
When approaching inferior conjunction—i.e., passing between the earth and sun—Venus appears, with a telescope, in the shape of a very thin crescent. Professor Lyman watched this crescent, becoming narrower day after day as it approached the sun, and noticed that its extremities gradually extended themselves beyond the limits of a semicircle, bending to meet one another on the opposite side of the invisible disk of the planet, until, at length, they did meet, and he beheld a complete ring of silvery light, all that remained visible of the planet Venus! The ring was, of course, the illuminated atmosphere of the planet refracting the sunlight on all sides around the opaque globe.
In 1874 M. Flammarion witnessed the same phenomenon in similar circumstances. One may well envy those who have had the good fortune to behold this spectacle—to actually see, as it were, the air that the inhabitants of another world are breathing and making resonant with all the multitudinous sounds and voices that accompany intelligent life. But perhaps some readers will prefer to think that even though an atmosphere is there, there is no one to breathe it.
Venus's Atmosphere seen as a Ring of Light.