three miles.

Now, let us return for a moment to the consideration of the wonderful
variations in Mercury's distance from the sun, for we shall find that
their effects are absolutely startling, and that they alone suffice to
mark a wide difference between Mercury and the earth, considered as the
abodes of sentient creatures. The total change of distance amounts, as
already remarked, to 14,000,000 miles, which is almost half the entire
distance separating the planet from the sun at perihelion. This immense
variation of distance is emphasized by the rapidity with which it takes
place. Mercury's periodic time, i.e., the period required for it to make
a single revolution about the sun--or, in other words, the length of its
year--is eighty-eight of our days. In just one half of that time, or in
about six weeks, it passes from aphelion to perihelion; that is to say,
in six weeks the whole change in its distance from the sun takes place.
In six weeks Mercury falls 14,000,000 miles--for it _is_ a fall, though
in a curve instead of a straight line--falls 14,000,000 miles toward the
sun! And, as it falls, like any other falling body it gains in speed,
until, having reached the perihelion point, its terrific velocity
counteracts its approach and it begins to recede. At the end of the next
six weeks it once more attains its greatest distance, and turns again to
plunge sunward.

Of course it may be said of every planet having an elliptical orbit
that between aphelion and perihelion it is falling toward the sun, but
no other planet than Mercury travels in an orbit sufficiently eccentric,
and approaches sufficiently near to the sun, to give to the mind so
vivid an impression of an actual, stupendous fall!

Next let us consider the effects of this rapid fall, or approach, toward
the sun, which is so foreign to our terrestrial experience, and so
appalling to the imagination.

First, we must remember that the nearer a planet is to the sun the
greater is the amount of heat and light that it receives, the variation
being proportional to the inverse square of the distance. The earth's
distance from the sun being 93,000,000 miles, while Mercury's is only
36,000,000, it follows, to begin with, that Mercury gets, on the
average, more than six and a half times as much heat from the sun as the
earth does. That alone is enough to make it seem impossible that Mercury
can be the home of living forms resembling those of the earth, for
imagine th