Of all the planets Mercury is nearest the sun. His average distance
is thirty-six million miles. He is nearly eighty times nearer than
Neptune, the outermost planet, and more than two and one-half times
nearer than we are. But his orbit departs so far from being a circle
that his distance from the sun varies as much as fifteen million miles.
When he is nearest the sun, or in perihelion, he is only twenty-eight
million miles from it; when he is farthest, or in aphelion, his
distance is forty-three million miles. There is even greater variation
in his distance from us. The difference between his least possible
and his greatest possible distance from us is as much as eighty-nine
millions of miles. For the earth has an elliptical orbit as well as
Mercury, and when we are at perihelion, which occurs in the winter,
we are three millions of miles nearer to the sun than we are in
mid-summer. If Mercury chances to be then at his greatest distance
from the sun, and also at inferior conjunction, or between us and the
sun, he is only forty-seven millions of miles from us. If, when we are
farthest from the sun, he also is at his greatest distance from it,
and is in superior conjunction, or on the other side of the sun from
us, he is one hundred and thirty-six millions of miles from us.
These changes in distance from the earth have much to do with Mercury’s
changes in apparent brightness to us. At his brightest, when he appears
at greatest elongation and we can see him without a telescope, he is
brighter than Arcturus, the brilliant first-magnitude star in Boötes,
that swings over us nightly from early spring to late autumn. When seen
with the naked eye, he is also red in color, somewhat like Arcturus;
but through a telescope he is dull silver, like the moon, or even more
ashy in his paleness. As he goes farther and farther from us he becomes
dimmer and dimmer and can be followed only with a telescope until, even
with this aid to vision, he is lost in the rays of the sun at superior
conjunction. His apparent diameter as mathematically measured varies
from five seconds, when he is farthest away, to thirteen seconds, when
he is nearest.
When he is at his nearest possible distance from us, light travels from
Mercury to us in a little more than four minutes. At his greatest
possible distance we could not receive the waves of light that he sends
out in less than twelve minutes. As a matter of fact, we do not receive
them at all, for, as we have seen, he is invisible when at his greatest
possible distance from us, being then on the far side of the sun.
Another cause of Mercury’s apparent change in brightness is due to the
fact that, in common with Venus, he goes through phases from crescent
to full like the moon. This is, as we have seen, a result of his
shining only by reflected light and of his orbit’s being between ours
and the sun. If he shone by his own light, he would be at his nearest
approach to us a very brilliant body indeed. As it is, his dark side
is turned toward us when he is nearest, and when his full face is
illuminated he is on the far side of the sun. We see half of his face
when he is at greatest elongation; but he is brightest when we see
less than half, because he is then nearer to us, and the difference in
distance more than compensates for the difference in illumination.
These phases cannot be seen with the naked eye, but it requires only
a small telescope to show them, and a very charming little moon-like
body Mercury is when we see them. His horns point toward the east when
he is coming toward us and nearing inferior conjunction, and when he
is backing away from us and going toward greatest western elongation
they point toward the west. It was through the blunting of one of these
horns when the planet was in certain positions that a mountainous
surface was suspected, so great is the significance of small details in
observations.
As a mere place from which to view the other bright bodies Mercury
would be far superior to the earth. He not only has the sun nearly
seven times larger in appearance at its mean distance than we see it,
but, being himself nearest the sun, all the other planets are outer
planets in relation to him, and all have their discs fully illuminated.
The earth and the moon, as seen from Mercury, would show as a splendid
pair of stars circling about each other, the earth more brilliant than
any first-magnitude star, and the moon of the third magnitude, or about
as bright as Phecda, the star at the bottom of the bowl of the Big
Dipper, just under the beginning of the handle. The earth would show a
disc of about twenty seconds, and the moon one of about eight seconds,
with a distance between them of about 871 seconds. Some idea of what
this distance is may be had if one knows Mizar, the star at the bend
of the handle of the Dipper, and its tiny shining attendant, Alcor.
These two stars are 708 seconds apart. The distance between them is
about equal to one-third of the diameter of the moon as measured from
the earth. It does not appear to be nearly so much as that, and some
persons have difficulty in separating the two stars; but the moon is
not only inconstant but deceptive, and owing to its brilliancy seems
always proportionately larger than it really measures.
Venus would appear from Mercury as much as four times as large as
she seems to us--a veritable little moon, and always full, her size
varying slightly as Mercury speeded back and forth from the farthest to
the nearest point in his orbit, changing the extreme of the distance
between them from one hundred and ten million to less than twenty-four
million miles. If Mercury needed a moon, he could well find some
consolation for his lack of it in the presence of the lovely Venus in
his sky.