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.