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.
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