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.
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