Other Worlds | Page 8

is compelled to
move in its orbit, and Mercury, being the nearest to the sun of all the
planets, is by far the swiftest footed among them. But its velocity is
subject to remarkable variation, owing to the peculiar form of the orbit
in which the planet travels. This is more eccentric than the orbit of any
other planet, except some of the asteroids. The sun being situated in
one focus of the elliptical orbit, when Mercury is at perihelion, or
nearest to the sun, its distance from that body is 28,500,000 miles, but
when it is at aphelion, or farthest from the sun, its distance is
43,500,000 miles. The difference is no less than 14,000,000 miles!
When nearest the sun Mercury darts forward in its orbit at the rate of
twenty-nine miles in a second, while when farthest from the sun the
speed is reduced to twenty-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 the heat of the sun in the middle of a summer's day increased
six or seven fold! If there were no mitigating influences, the face of the
earth would shrivel as in the blast of a furnace, the very stones would
become incandescent, and the oceans would turn into steam.
Still, notwithstanding the tremendous heat poured upon Mercury as
compared with that which our planet receives, we can possibly, and for
the sake of a clearer understanding of the effects of the varying distance,
which is the object of our present inquiry, find a loophole to admit the
chance that yet there may be living beings there. We might, for instance,
suppose that, owing to the rarity of its atmosphere, the excessive heat
was quickly radiated away, or that there was something in the
constitution of the atmosphere that greatly modified the effective
temperature of the sun's rays. But, having satisfied our imagination on
this point, and placed our supposititious inhabitants in the hot world of
Mercury, how are we going to meet the conditions imposed by the
rapid changes of distance--the swift fall of the planet toward the sun,
followed by the equally swift rush away from it? For change of
distance implies change of heat and temperature.
It is true that we have a slight effect of this kind on the earth. Between
midsummer (of the