It is probable that Mercury has no alternations of light and darkness,
causing day and night such as we know them. That is, the planet does
not rotate on its axis in such a way as to turn first one side and then
the other toward the sun as the earth does. In this, as in some other
things, Mercury must accept the fate that overtakes many other small
bodies which revolve around large ones--that of our moon, for instance,
and the satellites of some of the other planets. Working under the law
of gravitation, which gives such power to the large bodies, the sun
has so retarded the rotation of Mercury that the planet now makes but
one rotation on its axis during one circuit around that central body,
and so keeps always the same face toward the sun. Some astronomers
do not regard this as having been wholly proved; but all the later
observations of Mercury strongly indicate that it is the fact, and it
is coming to be more and more regarded as established.
But, even if this is the predicament into which Mercury has come, the
planet is probably not in so bad a plight as many another body to
which the same sort of thing has happened. The extreme eccentricity
of his orbit, which has given him the true mercurial temperament,
resulting in sprightliness, agility, and changeableness, is accountable
for some mitigating circumstances. The sun may hold him so that he
cannot turn his face away from that luminary; but it cannot keep him
from rotating on his axis at a uniform rate of speed, and from this,
combined with the vagaries caused by his eccentric orbit, come some
interesting things.
Since Mercury is less than two-thirds as far from the sun at perihelion
as he is at aphelion, there is a corresponding variation in his rate of
speed. When he is nearest the sun, at perihelion, he darts along at the
rate of thirty-five miles a second; at aphelion, when he is farthest
from the sun, he travels only twenty-three miles a second. Twenty-three
miles in one second is not exactly a snail’s pace, terrestrially
considered, and it is faster than the earth moves at any time; but the
planet was named Mercury because of his swiftness, and we would not
expect much lagging even when he is moving at his slowest gait. This
difference in speed in different parts of his orbit causes what is
called the librations of Mercury. When he is traveling at his swiftest
pace he gets a little ahead of his rotation, the speed of which is
uniform, and thus throws the sunlight somewhat farther around on one
side. When his speed decreases, he falls behind his time of rotation,
and thus gets a little more sunlight on the other side. Thus, during
each revolution he juggles the sunlight a little farther around him
than he could if he were a more steady-going planet.
These librations result in there being two strips on the surface of
Mercury--one on each side--which undoubtedly have a day and night,
varying in length in the different parts of the strips. The part that
lies nearest the illuminated side of the planet has alternate periods
of sunlight and darkness, each of considerable duration, while that
part nearest the dark side has merely a glimmer of sunlight every
eighty-eight days, which is Mercury’s sidereal year, or the time
required for him to make one revolution around the sun. These two
strips on which the light varies comprise about one-eighth of the
surface of Mercury. One half of his entire surface is always light, and
of the other three-eighths are always dark. It is this dark, cold side
that is turned toward us when Mercury is nearest to us.
It is possible that on those parts of Mercury where the sunlight and
darkness are unstable there may be something resembling a tolerable
temperature. They are something more than a thousand miles in breadth,
and perhaps near the center of them the sun may give heat sufficient
to enliven and yet not burn. More than likely, they are alternately
scorched and frozen. For it takes more than the mere presence of
sunlight to make a climate tolerable. Atmosphere is what is necessary,
and we have seen that Mercury has probably lost practically all his
atmosphere long, long ago. An atmosphere absorbs much of the radiant
energy that comes from the sun before it reaches the more solid parts
of a planet, and it also acts as a blanket in preventing the too rapid
escape of such heat as the planet may have acquired. Thus it has the
doubly beneficent office of tempering the rays that would otherwise
be scorching and of hindering a radiation that would leave the planet
stiffened and frozen.
Stiffened and frozen is what the dark side of Mercury undoubtedly is.
The sun has never shone upon it since Mercury became a solid body. All
the inherent heat it had has long since passed off into space, and its
temperature must be somewhere near the absolute zero. The absolute zero
is the point in temperature where all known substances become solid. It
is more than 450° below the Fahrenheit zero, or more than 350° lower
than any temperature recorded in our arctic regions--a degree of cold
unthinkable to any but the scientist.
On the other side of Mercury the heat is beyond anything we have any
notion of. With an equal atmosphere it would receive from the sun six
thousand times as much light and heat as Neptune on an equal space,
and, on an average, seven times as much as the earth. At Mercury’s
distance from the sun his hot side would be more than 300° above
zero, if there were absolutely no atmospheric protection. Even though
tempered by a thin atmosphere, as it may be, the heat on this side is
still probably enough to boil away any water that might be there and to
change some other substances from what we regard as their normal state.
Stability, at least, is a quality of the hot and the cold side of
Mercury. Scorched and seared and desolate of life, as we know it, the
one side lies under a blazing, dazzling sun. Cold and hard and bleak,
and no less desolate, the other side turns its face toward the darkness
of space. Thus they will remain until the end of time. And let us hope
that, when the final catastrophe occurs and a new nebula is formed, the
matter composing Mercury may find a place in a larger mass, and in its
new incarnation have a fuller and larger life.
It is the atmosphere also which causes twilight, as well as the gradual
changing from heat to cold. With no atmosphere, we would drop from full
daylight to the darkness of starlight at the setting of the sun. So,
with the thin air that Mercury probably has (if he has any), the two
zones which are alternately light and dark, and hot and cold, are not
much better off than the parts which are permanently either light or
dark. They are plunged alternately from the temperature and light of
the hot side of Mercury to the temperature of the cold side, with few
gradations to prepare them for such extremes. Thus the only part of
the planet that might be expected to have any variations of seasons
fulfils the expectation with little satisfaction.
The only changes in climate which may have an appreciable effect are
mainly those caused by the eccentricity of Mercury’s orbit, which
carries him so near the sun at certain times and so comparatively far
away at others. When he is nearest the sun he receives more than twice
as much heat and light as when he is farthest away. At aphelion he
receives four times as much heat and light as the earth. At perihelion
the amount of heat and light is increased to more than nine times that
of the earth. Since it takes Mercury a little more than twelve weeks
to make one revolution around the sun, he passes from nearest distance
to farthest, or the reverse, every six weeks. And thus, as viewed
from the planet, the sun expands gradually for six weeks until it has
increased its diameter two and one-half times, and the next six weeks
it diminishes in the same proportion. At such times, of course, the
amount of heat is more or less according to the planet’s distance from
the sun; but all the time it is very great.
Moreover, it is believed that the axis on which Mercury rotates stands
perpendicular to his orbit. This being the case, there would be on
Mercury no change of seasons such as the earth has. The earth’s axis
is inclined a little more than twenty-three degrees to its orbit, and
from this we get the sun’s rays in a great variety of directions and
different degrees of obliquity, causing the seasons, as we know them,
in grateful variation. With the axis perpendicular, as it probably is
in the case of Mercury, the sun’s rays fall on the face of the planet
always with the same degree of directness, the only relief from their
greatest heat being when the planet backs away from the sun every six
weeks, and when in his librations he turns first one sun-burned cheek
and then the other toward the coolness of space.
Thus we must regard the smallest of our family of planets, Mercury,
as always the dwarf among us, with never a fair chance to develop a
rich and luscious life according to our ideas of such a life. Beaten
by the sun’s hard rays, and with no sufficient atmospheric protection;
pulling always at his tether, but held firmly with his face to the
center; circling at times with mercurial swiftness and thus cheating
the sun into sending its rays farther toward the dark, cold side of
him than it otherwise would, and with all his defects from a human
point of view, we may still regard him as a right merry, roguish little
planet, after all. He may be prematurely aged, he may have missed many
experiences that the larger planets are having, he may have a long time
to wait for the final change that will reunite us all; but he is not
lying in sluggish inactivity until it comes.
In view of the fact that he is the only planet that twinkles, may it
not suggest, when we see his ruddy face peering through the thick
atmospheric mists near our horizon, that the impish little body is
winking at us, and that it may be with planets as it is with people:
they may not always be in an unfortunate plight because their fate is
different from ours?