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Notwithstanding its high albedo 0·62, according to Zöllner proof is wanting that any of the light of Uranus is inherent. Mr. Albert Taylor announced, indeed, in 1889, his detection, with Common's giant reflector, of bright flutings in its spectrum; but Professor Keeler's examination proved them to be merely contrast effects.
Practical assurance was gained from them that the innermost planet has a rough rind of dusky rock, absorbing all but 17 per cent. of the light poured upon it by the fierce adjacent sun. Its "albedo," in other words, is 0·17, which is precisely that ascribed to the moon. The absence of any appreciable Mercurian atmosphere followed almost necessarily from these results.
This shows the extreme uncertainty of these estimates, while the fact that both Venus and Jupiter are wholly cloud-covered, while we are only half-covered, renders it almost certain that our albedo is far less than Mr. Lowell makes it. It is evident that mathematical calculations founded upon such uncertain data cannot yield trustworthy results.
The interesting query as to Jupiter's surface incandescence has been studied since Bond's time with the aid of all the appliances furnished to physical inquirers by modern inventiveness, yet without bringing to it a categorical reply. Zöllner in 1865, Müller in 1893, estimated his albedo at 0·62 and 0·75 respectively, that of fresh-fallen snow being 0·78, and of white paper 0·70.
The electron telescope could detect its larger planets, especially a gas-giant fifth-orbit world of high albedo. Calhoun had his four students estimate its distance again, pointing out the difference that could be made in breakout position if the Med Ship were mis-aimed by as much as one second of arc. "And now," he said briskly, "we'll have coffee. I'm going to graduate you as pilots.
We're already working on it." "Anything yet?" "No, sir. We'll have the albedo measurement in a few minutes. It'll take longer to figure the mass." The asteroid's efficiency in reflecting sunlight was its albedo. The efficiency depended on the material of which it was made. The albedo of pure metallic thorium was known. If the asteroid's albedo matched it, that would be one piece of evidence.
There can be little doubt, however, that its supply is about the minimum adequate to the needs of a living, and perhaps a life-nuturing planet. The climate of Mars seems to be unexpectedly mild. Its theoretical mean temperature, taking into account both distance from the sun and albedo, is 34° C. below freezing.
Professor Pickering, from determinations of light-intensity, assigned to Vesta a diameter of 319 miles, to Pallas 167, to Juno 94, down to twelve and fourteen for the smaller members of the group. An albedo equal to that of Mars was assumed as the basis of the calculation.
The first satellite sometimes looks dusky, but never absolutely black, in travelling over the disc of Jupiter. The second appears uniformly white a circumstance attributed by Dr. Spitta to its high albedo.
Finally, Professor Barnard, directly measuring with the Yerkes refractor the minute discs presented by the original quartette, obtained the following authentic data concerning them: Diameter of Ceres, 477 miles, albedo = 0·18; diameter of Pallas, 304 miles, albedo = 0·23; diameter of Vesta, 239 miles, albedo = 0·74; diameter of Juno, 120 miles, albedo = 0·45.
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