We may approximately compute, in general terms, the proportion of oxygen in the atmospheres of the other planets in the same way. Neptune, it is true, is so far distant from the sun that the solar orb only "appears about the same magnitude as Venus when at its greatest brilliancy, as viewed from the earth," but we must not forget that "the intensity of the sun's light would be more than ten thousand times greater than that of Venus" (Professor Dunkin, in "The Midnight Sky"). Unless the moon gathers a portion of the earth's oxygen (the planetary satellites, like Saturn's rings, thus constituting in their rotations a constituent part of the planets themselves), the percentage of this gas in her atmosphere must be exceedingly small, for her axial rotation has a period of a whole lunar month, being the same as that of her revolution around the earth as a center. The absence of apparent atmosphere and moisture from the visible lunar surface has already been mentioned and explained. The means by which this fact has been approximately determined are described by Professor Dunkin, in "The Midnight Sky," as follows: "Among the many proofs of the non-existence of a lunar atmosphere, it may be mentioned that no water can be seen; at least there is not a sufficient quantity in any one spot so as to be visible from the earth. Again, there are no clouds; for if there were, we should immediately discover them by the variable light and shade which they would produce. But one great proof of the absence of any large amount of vapor being suspended over the lunar surface is the sudden extinction of a star when occulted by the moon. The author has been a constant observer of these phenomena, and, though his experience is of long standing, he has never observed an occultation of a star or planet, especially at the unilluminated edge of a young moon, without having his conviction confirmed that there is no appreciable lunar atmosphere.... Professor Challis has subjected the results of a large number of these observations to a severe mathematical test, but he has not been able to discover the slightest trace of any effect produced by a lunar atmosphere."