[Footnote 5:  Many of the present difficulties about temperatures on the various planets would be beautifully disposed of if we could accept the theory urged by Mr. Cope Whitehouse, to the effect that the sun is not really a hot body at all, and that what we call solar light and heat are only local manifestations produced in our atmosphere by the transformation of some other form of energy transmitted from the sun; very much as the electric impulses carried by a wire from the transmitting to the receiving station on a telephone line are translated by the receiver into waves of sound.  According to this theory, which is here mentioned only as an ingenuity and because something of the kind so frequently turns up in one form or another in popular semi-scientific literature, the amount of heat and light on a planet would depend mainly upon local causes.]

To an extent which most of us, perhaps, do not fully appreciate, we are indebted for many of the pleasures and conveniences and some of the necessities of life on our planet to its faithful attendant, the moon.  Neither Mercury nor Venus has a moon, but Mars has two moons.  This statement, standing alone, might lead to the conclusion that, as far as the advantages a satellite can afford to the inhabitants of its master planet are concerned, the people of Mars are doubly fortunate.  So they would be, perhaps, if Mars’s moons were bodies comparable in size with our moon, but in fact they are hardly more than a pair of very entertaining astronomical toys.  The larger of the two, Phobos, is believed to be about seven miles in diameter; the smaller, Deimos, only five or six miles.  Their dimensions thus resemble those of the more minute of the asteroids, and the suggestion has even been made that they may be captured asteroids which have fallen under the gravitational control of Mars.

The diameters just mentioned are Professor Pickering’s estimates, based on the amount of light the little satellites reflect, for they are much too small to present measurable disks.  Deimos is 14,600 miles from the center of Mars and 12,500 miles from its surface.  Phobos is 5,800 miles from the center of the planet and only 3,700 from the surface.  Deimos completes a revolution about the planet in thirty hours and eighteen minutes, and Phobos in the astonishingly short period—­although, of course, it is in strict accord with the law of gravitation and in that sense not astonishing—­of seven hours and thirty-nine minutes.

Since Mars takes twenty-four hours and thirty-seven minutes for one rotation on its axis, it is evident that Phobos goes round the planet three times in the course of a single Martian day and night, rising, contrary to the general motion of the heavens, in the west, running in a few hours through all the phases that our moon exhibits in the course of a month, and setting, where the sun and all the stars rise, in the east.  Deimos, on the other hand, has a period of revolution five or six hours longer than that of