MERCURY, VENUS, AND MARS.
Once again we have to journey through the high-roads of the Solar
System, paying a brief visit to each in turn of our seven chief
brother-and-sister planets, and learning a few more leading facts about
them. Having gone the same way before, it will not now seem quite so
far.
Busy, hurrying Mercury! we must meet him first in his wild rush through
space. If he were to slacken speed for a single instant, he would begin
to fall with fearful rapidity towards the sun. And if Mercury were to
drop into one of those huge black chasms of rent furnace-flame on the
sun’s surface, there would be a speedy end to his life as a planet.
Mercury’s day is about the same length as our day, and his year is
about one quarter the length of our year. If Mercury has spring,
summer, autumn, and winter, each season must be extremely short; but
this depends upon whether Mercury’s axis slopes like the earth’s
axis--a matter difficult to find out. Mercury is always so near to the
sun, that it is by no means easy to observe him well.
We know more about his orbit than his axis. The earth’s orbit, as
before explained, is not a circle, but an ellipse or oval. Mercury’s
orbit is an ellipse also, and a much longer--or, as it is called, a
more eccentric--ellipse. The earth is three millions of miles nearer
to the sun at one time of the year, than six months before or after.
Mercury is no less than fifteen millions of miles nearer at one time
than another, which must make a marked difference in the amount of heat
received.
Even when the distance is greatest, the sun as seen from Mercury looks
four and a half times as large as the sun we see. What a blazing
splendor of light! It is not easy to imagine human beings living there,
in such heat and glare, and with either no changes of season at all, or
such very short seasons rapidly following one another. Mercury may, and
very likely does, abound with living creatures, as much as the earth
abounds with them; only one fancies they must be altogether a different
kind of living creatures from any ever seen on earth. And yet we do not
know. Man can so wonderfully adapt himself or be adapted to different
climates on earth, from extreme heat to extreme cold, that we can not
tell how far this adapting power may reach.
Both Mercury and Venus seem to be enfolded in dense, cloud-laden
atmospheres, rarely parting so as to allow us to get even a glimpse
of the real planets within the thick, light-reflecting covering. Some
have thought that a heavy, moist, protecting atmosphere may help to
ward off the intense heat, and to make Mercury a more habitable place.
Our earthly atmosphere is rather of a kind to store up heat, and to
make us warmer than we should be without it; but there might be vapors
differently constituted which might act in some other way. At all
events, we know how easily God can have adapted either the planet to
the creatures he meant to place there, or the creatures to the climate.
“All things are possible” to him. The how and the what are interesting
questions for us, but we must often be content to wait for an answer.
A thick, gray ring or belt has been noticed round the small, black disk
of Mercury, while it has passed between us and the sun. The edge of
Mercury, seen against the bright photosphere beyond, would, if there
were no atmosphere, be sharp and clear as the edge of the airless moon.
This surrounding haze seems to show that Mercury has an atmosphere. The
sunlight reflected from Mercury’s envelope of clouds shines at least as
brightly as if it were reflected from his solid body.
The small size of Mercury makes attraction on his surface much less
than on earth. A lump of iron weighing on earth one pound, would weigh
on Mercury only about seven ounces, or less than half as much. So a
man would be a very light leaper indeed there, and an elephant might
be quite a frolicsome animal. If there are star-gazers in Mercury, and
if the cloud-laden atmosphere allows many clear views of the sky, the
earth and Venus must both be beautiful to look upon. Each of the two
would shine far more brightly than Jupiter, as seen at his best from
earth.
Like Mercury, Venus, the next planet, has an orbit lying inside our
orbit. Mercury and Venus are always nearer to the sun than we are;
and if Mercury and Venus traveled round the sun in orbits, the planes
of which were exactly the same as the plane of the earth’s orbit, we
should very often see them creeping over the surface of the sun. Not
that they really “creep over” it; only, as they journey between the sun
and us, we can see them pass like little black dots across the sun’s
disk. This is the same thing as when the moon passes across the sun’s
disk and eclipses it. But Mercury and Venus are too far away from us to
cause any eclipse of the sun’s light.
Mercury has given more trouble to astronomers than any other member of
the system; for, owing to his proximity to the sun, he is usually lost
in the solar glory, and is never seen in a dark part of the heavens,
even at the time of his greatest distance. This circumstance, together
with a small mass and an immense velocity, renders it difficult to
catch and watch him. In high latitudes, where the twilight is strong
and lengthened, while mists often overhang the horizon, the planet can
seldom be seen with the naked eye. Hence an old astro-meteorologist
contemptuously describes him as “a squirting lacquey of the sun, who
seldom shows his head in these parts, as if he was in debt.” Copernicus
lamented that he had never been able to obtain a sight of Mercury; and
the French astronomer Delambre saw him not more than twice with the
naked eye.
The most favorable times for making observations are about an hour and
three-quarters before sunrise in autumn, and after sunset in spring;
but very clear weather and a good eye are required. At certain periods,
when between the earth and the sun, on a line joining the centers of
the two bodies, Mercury appears projected on the solar disk as a small,
round, dark spot. This is called a _transit_, and would occur during
every revolution if the plane of his orbit coincided with that of the
orbit of the earth. But as one-half of his orbit is a little above
that of the earth, and the other half a little below it, he passes
above or below the sun to the terrestrial spectator, except at those
intervals, when, being between us and the sun, he is also at one of the
two opposite points where the planes of the respective orbits intersect
each other. Then, stripped of all luster, the planet passes over
the face of the great luminary as a black circular speck, affording
evidence of his shining by reflected light, and of his spherical form.
The first transit of Mercury recorded in history was predicted by
Kepler, and witnessed by Gassendi, at Paris, on the morning of a
cloudy day, the 7th of November, 1631. It was toward nine o’clock
when he saw the planet; but owing to its extreme smallness, he was
at first inclined to think it was a minute solar spot. It was then
going off the sun; and made its final egress toward half-past ten. The
observed time of the transit was nearly five hours in advance of the
computed time. This was a very satisfactory accordance for that age
between theory and observation; but it was the effect of a fortuitous
combination of circumstances, rather than of computations founded
upon well-established data. “The crafty god,” wrote Gassendi, in the
peculiar style of his day, “had sought to deceive astronomers by
passing over the sun a little earlier than was expected, and had drawn
a veil of dark clouds over the earth in order to make his escape more
effectual. But Apollo, knowing his knavish tricks from his infancy,
would not allow him to pass altogether unnoticed. To be brief, I have
been more fortunate than those hunters after Mercury who have sought
the cunning god in the sun. I found him out, and saw him where no one
else had hitherto seen him.” Since Gassendi’s time a number of transits
have been observed.
These crossings of the sun’s face, or “transits,” as they are called,
have been important matters. The transit of Venus especially was once
eagerly looked for by astronomers, since, by close observations of
Venus’s movements and positions, the distance of the sun could at that
time be better calculated than in any other way. Other methods are now
coming into vogue.
The transits of Venus are rare. Two come near together, separated by
only eight years, and then for more than one hundred years the little
dark body of Venus is never seen from earth to glide over the sun’s
photosphere. There was a transit of Venus in the year 1761, and another
in the year 1769. There was a transit of Venus in 1874, and another in
1882. At the last transits it was found that the sun, instead of being
ninety-five millions of miles away, as astronomers thought, was only
ninety-three millions of miles away. The reason why these transits
happen so seldom, is that the orbits of Mercury and Venus lie in rather
a different plane or level from the earth’s orbit. So, like the moon,
though often passing between us and the sun, they generally go just a
little higher or just a little lower than his bright face.
Mercury and Venus show phases like the moon, although they do not
circle round the earth as the moon does. These “phases,” or changes of
shape, are probably never visible except through a telescope. It will
be easier to think about the phases of Venus alone, than to consider
both together. Her orbit lies within the earth’s orbit, and the earth
and Venus travel round the sun--as do all the planets--in the same
direction. But as Venus’s pathway is shorter than ours, and as her
speed is greater, she is much the quickest about her yearly journey,
and she overtakes us again and again at different points of our orbit
in turn.
[Illustration: PHASES OF VENUS.]
At one time she comes between us and the sun. That is her nearest
position to us, and she is then only about twenty-five millions of
miles distant. A beautiful sight she would be, but unfortunately her
bright side is entirely turned away, and only her dark side is turned
towards us. So then she is “new Venus,” and is invisible.
At another time she is completely beyond the sun, and at her farthest
position away from us. Her shining is quite lost in the sun’s rays
coming between. And though we get a good view of her as “full Venus,”
at a little to one side or the other, yet so great is her distance--as
much as one hundred and fifty-seven millions of miles--that her size
and brightness are very much lessened.
Between these two nearest and farthest points, she occupies two middle
distances, one on each side of the sun. Then, like the moon at her
“quarters,” she turns to us only half of her bright side. But this
is the best view of Venus that we have, as a brilliant, untwinkling,
starlike form,--the Evening Star of ancients and of poets. Between
these four leading positions Venus is always traveling gradually
from one to another--always either waxing or waning in size and in
brightness. Mercury passes through the same seeming changes.
Inhabitants of Venus must have a glorious view of the earth, with her
attendant moon. For just at the time when the two planets are nearest
together, and when she is only “new Venus” to us, a dark and invisible
body, the earth is “full earth” to Venus. The very best sight we ever
have of Venus can not come near that sight. But if Mercury and Venus
really are so often covered with heavy clouds as astronomers believe,
this must greatly interfere with any habits of star-gazing.
Venus and the earth have often been called twin-sister planets. There
are many points of likeness between them. In size they differ little,
and in length of day they are within an hour of being the same. Earth
certainly has a companion-moon, and Venus, it is believed, has not. At
one time several astronomers were pretty certain that they had caught
glimpses of a moon; but the supposed moon has of late quite vanished,
and nobody can say whether it ever really existed. Venus travels in
an ellipse which comes nearer to being a circle than the orbit of any
other planet.
Mountain-shadows have been watched through the telescope, in Venus,
as in the moon. Some astronomers have believed that they saw signs of
very lofty mountains--as much as twenty-eight miles, or four times the
height of our highest earthly mountains, but this requires confirmation.
There is a good deal of uncertainty about the climate of Venus. The
heat there must greatly surpass heat ever felt on earth--the sun being
about double the apparent size of our sun, and pouring out nearly
double the amount of light and heat that we receive.
This difference may be met, as already stated, by a sheltering, cloudy
atmosphere, or the inhabitants may have frames and eyesight suited to
the increased glare and warmth.
An atmosphere and water exist there as here. From what we have seen
above of the rapid and violent seasons of this planet, we might think
that the agitations of the winds, the rains, and the storms would
surpass everything which we see and experience here, and that its
atmosphere and its seas would be subject to a continual evaporation
and precipitation in torrential rains--an hypothesis confirmed by its
light, due, doubtless, to reflection from its upper clouds, and to the
multiplicity of the clouds themselves. To judge by our own impressions,
we should be much less pleased with this country than with our own, and
it is even very probable that our physical organization, accommodating
and complaisant as it is, could not become acclimatized to such
variations of temperature. But it is not necessary to conclude from
this that Venus is uninhabitable and uninhabited. We may even suppose,
without exaggeration, that its inhabitants, organized to live in the
midst of these conditions, find themselves at their ease, like a fish
in water, and think that our earth is too monotonous and too cold to
serve as an abode for active and intelligent beings.
Of what nature are the inhabitants of Venus? Do they resemble us in
physical form? Are they endowed with an intelligence analogous to
ours? Do they pass their life in pleasure, as Bernardin de St. Pierre
said; or, rather, are they so tormented by the inclemency of their
seasons that they have no delicate perception, and are incapable of
any scientific or artistic attention? These are interesting questions,
to which we have no reply. All that we can say is, that organized
life on Venus must be little different from terrestrial life, and
that this world is one of those which resemble ours most. It should,
then, be inhabited by vegetable, animal, and human races but little
different from those which people our planet. As to imagining it desert
or sterile--this is an hypothesis which could not arise in the brain
of any naturalist. The action of the divine sun must be there, as in
Mercury, still more fertile than his terrestrial work, already so
wonderful. We may add that Venus and Mercury, having been formed after
the earth, are relatively younger than our planet.
It is believed also that the axis of Venus, instead of being slanted
only as much as the earth’s axis, is tilted much more. Even if
the tilting is less than some have supposed, it is probably very
considerable. If Venus really does “lie over” in such a manner,
certain startling changes of climate on its surface--unpleasant
changes, according to our ideas--would take place.
Like earth, Venus would have her two arctic regions, where a burning
summer’s day would succeed a bitter winter’s night, each half a year in
length. She would have also her tropical region--only in that region
intense cold would alternate with intense heat, brief seasons of each
in turn. And between the tropics and the arctic regions would lie wide
belts, by turns entirely tropical and entirely arctic. The rapidity and
severity of these changes, following one another in a year about as
long as eight of our months, would seem to be too much for any human
frame to endure. But it all rests upon an _if_. And we may be quite
sure that _if_ there are any manner of human beings in Venus, their
frames are well suited to the climate of their world.
Though Mars is one of the inner group of four small planets, divided by
the zone of asteroids from the outer group of four great planets, yet
he belongs to the outside set of Superior Planets. His orbit surrounds
ours, being at all points farther off from the sun. Very slight
“phases” have been seen in Mars. He turns to us, from time to time,
just enough of his dark side to prove that he _has_ a dark side, and
that he does not shine like a star by his own light. But the phases on
that planet are by no means marked as they are with Venus.
Until lately it was believed that Mars possessed no moons. Two very
small ones have, however, been lately found circling round him.[2]
They have been named Deimos and Phobos, after the “sons of Mars” in
Greek mythology. Deimos travels round Mars in thirty-nine hours, while
Phobos performs the same journey in the astonishingly short period of
seven hours and a half!
[2] The discovery of the two moons is due to the agency of a woman.
Professor Asaph Hall, of Washington City, was searching by the aid of
the most powerful telescope which had yet been directed to Mars, and at
the very moment when the planet was in the most favorable condition for
observation. Having searched in vain during several evenings in August,
1877, he was about to give up, when Mrs. Hall begged him to search a
little more. He did so, and on the night of the 11th he discovered the
first of the satellites, and then on the 17th the second.
[Illustration: MARS AND THE PATH OF ITS SATELLITES.]
We have here, then, a system very different from that of the earth
and moon. But the most curious point is the rapidity with which the
inner satellite of Mars revolves round its planet. This revolution
is performed in seven hours, thirty-nine minutes, fifteen seconds,
although the world of Mars rotates on itself in twenty-four hours,
thirty-seven minutes--that is to say, this moon turns much more
quickly than the planet itself. This fact is inconsistent with all the
ideas we have had up to the present on the law of formation of the
celestial bodies. Thus, while the sun appears to revolve in the Martian
sky in a slow journey of more than twenty-four hours, the inner moon
performs its entire revolution in a third of a day. It follows that
_it rises in the west_ and _it sets in the east_! It passes the second
moon, eclipses it from time to time, and goes through all its phases
in eleven hours, each quarter not lasting even three hours. What a
singular world!
These satellites are quite small--they are the smallest celestial
bodies we know. The brightness of the planet prevents us from measuring
them exactly. It seems, however, that the nearer is the larger, and
shows the brightness of a star of the tenth magnitude, and that the
second shines as a star of the twelfth magnitude. According to the
most trustworthy photometric measures, the first satellite may have a
diameter of 7.45 miles, and the second a diameter of 6.2 miles. _The
larger of these two worlds is scarcely larger than Paris._ Should we
honor them with the title of worlds? They are not even terrestrial
continents, nor empires, nor kingdoms, nor provinces, nor departments.
Alexander, Cæsar, Charlemagne, or Napoleon, might care but little to
receive the scepter of such worlds. Gulliver might juggle with them.
Who knows, however? The vanity of men being generally in the direct
ratio of their mediocrity, the microscopical reasoning mites which
doubtless swarm on their surface have also, perhaps, permanent armies,
which mutilate each other for the possession of a grain of sand.
These two little moons received from their discoverer the names of
_Deimos_ (Terror) and _Phobos_ (Flight), suggested by the two verses of
Homer’s “Iliad” which represent Mars descending on the earth to avenge
the death of his son Ascalaphus:
“He ordered Terror and Flight to yoke his steeds,
And he himself put on his glittering arms.”
_Phobos_ is the name of the nearer satellite; _Deimos_, that of the
more distant.
The existence of these little globes had already been suspected from
analogy, and thinkers had frequently suggested that, since the earth
has one satellite, Mars should have two, Jupiter four, Saturn eight;
and this is indeed the fact--though as Jupiter is now found to have
_five_ satellites, this arithmetical progression is upset. But as we
experience too often in practice the insufficiency of these reasonings
of purely human logic, we can not give them more value than they
really possess. We might suppose in the same way now that Uranus has
sixteen satellites, and Neptune thirty-two. This is possible; but
we know nothing of them, and have not even the right to consider
this proportion as probable. It is not the less curious to read the
following passage, written by Voltaire in 1750 in his masterpiece, the
“Micromégas:”
“On leaving Jupiter, our travelers crossed a space of about a hundred
millions of leagues, and reached the planet Mars. They saw _two
moons_, which wait on this planet, and which have escaped the gaze
of astronomers. I know well that Father Castel wrote against the
existence of these two moons; but I agree with those who reason from
analogy. These good philosophers know how difficult it would be for
Mars, which is so far from the sun, to get on with less than two moons.
However this may be, our people found it so small that they feared they
might not find anything to lie upon, and went on their way.”
Here we have unquestionably a very clear prophecy, a rare quality
in this kind of writing. The astronomico-philosophical romance of
“Micromégas” has been considered as an imitation of Gulliver. Let us
open the masterpiece of Swift himself, composed about 1720, and we
read, word for word, in Chapter III of the “Voyage to Laputa:”
“Certain astronomers ... spend the greatest part of their lives in
observing the celestial bodies, which they do by the assistance of
glasses far excelling ours in goodness. For this advantage hath enabled
them to extend the discoveries much farther than our astronomers in
Europe; for they have made a catalogue of ten thousand fixed stars,
whereas the largest of ours do not contain above one-third part of that
number. They have likewise discovered two lesser stars, or satellites,
which revolve about Mars, whereof the innermost is distant from the
center of the primary planet exactly three of his diameters, and the
outermost five. The former revolves in the space of ten hours, and
the latter in twenty-one and a half; so that the squares of their
periodical times are very near in the same proportion with the cubes of
their distance from the center of Mars, which evidently shows them to
be governed by the same law of gravitation that influences the other
heavenly bodies.”
What are we to think of this double prediction of the two satellites of
Mars? Indeed, the prophecies which have been made so much of in certain
doctrinal arguments have not always been as clear, nor the coincidences
so striking. However, it is evident that no one had ever seen these
satellites before 1877, and that there was in this hit merely the
capricious work of chance. We may even remark that both the English and
French authors have only spoken ironically against the mathematicians,
and that in 1610, Kepler, on receiving the news of the discovery of the
satellites of Jupiter, wrote to his friend Wachenfals that “not only
the existence of these satellites appeared to him probable, but that
doubtless there might yet be found two to Mars, six or eight to Saturn,
and perhaps one to Venus and Mercury.” We can not, assuredly, help
noticing that reasoning from analogy is here found on the right road.
However this may be, this discovery truly constitutes one of the most
interesting facts of contemporary astronomy.
Mars is not only much smaller than the earth, but a good deal less
dense in his “make.” His material is only about three-quarters as heavy
as an equal amount of the earth’s material. A very heavy man on earth
would be a most light and active individual on Mars. Gold taken from
earth to Mars would weigh there no more than tin weighs upon earth.
Mars has, it seems, an atmosphere, even as earth has. Of all the
planets Mars, is the only one whose actual surface is discernible in
the telescope. Mercury and Venus are so hidden by dense envelopes of
clouds that the real planets within are only now and then to be dimly
caught sight of. Jupiter and Saturn are so completely enwrapped in
mighty masses of vapor, that we can not even be certain whether there
are any solid bodies at all inside.
But Mars can be studied. Here and there, it is true, clouds sweep over
the landscape, hiding from view for a little while one continent or
another, one sea or another, growing, changing, melting away, as do
the clouds of earth. Still, though these clouds come and go, there are
other markings on the surface of Mars which do not change. Or, rather,
they only change so much as the continents and oceans of earth would
seem to vary, if watched from another planet, as the daily movement of
earth carried them from west to east, or as they might be hidden for
a while by cloud-layers coming between. It has been curiously noted
that these clouds over Mars form often in the morning and evening, and
are afterwards dispersed by the heat of midday. Also there seems every
reason to believe that rainfalls take place in Mars as upon earth.
The red color of Mars is well known. This does not vanish in the
telescope, but it is found that parts only have the red or orange
hue, while other parts are dark and greenish. These are the markings
which remain always the same, and they have been so closely examined
that more is known about the geography of Mars than of any other world
outside our own.
Mars, at his nearest point, does not draw closer to us than forty
millions of miles. At such a distance one must not speak too
confidently. There are, however, many reasons for believing that the
red portions are continents and that the green portions are oceans.
The spectroscope has lately shown us that water does really exist in
the atmosphere of Mars--unlike the dreary, waterless moon. So we no
longer doubt that the cloudlike appearances are clouds, and that rain
sometimes falls on Mars. If there is rain, and if there are clouds and
vapor, there are probably oceans also.
Two singular white spots are to be seen at the north and south poles,
which we believe to be polar ice and snow. Somebody looking at our
earth in like manner from a distance, would doubtless perceive two
such white snow-spots. These two polar caps are seen to vary with the
seasons. When the north pole of Mars is turned towards the sun, the
white spot there grows smaller; and at the same time, the south pole of
Mars being turned away from the sun, the white spot there grows larger.
Again, when the south pole is towards the sun, and the north pole away
from the sun, the white spot at the south is seen to be the smallest,
and the white spot at the north is seen to be the largest. This is
exactly what takes place in the summers and winters of our north and
south poles.
The markings of Mars have been so carefully studied, that at last a map
has been made of the planet--a map of a world, never less than forty
millions of miles away! Names have been given to the continents and
oceans--such as Dawes Continent, Herschel Continent, De La Rue Ocean,
Airy Sea, Huggins Inlet, and so on.
Land and water seem to be very differently arranged on Mars from what
they are on earth. Here we have about three times as much water as
land, and to get from one continent to another without crossing the
sea is in some cases impossible. But a traveler there might go most
conveniently to and fro, hither and thither, to all parts of his world,
either on land or on water, without any change. If he preferred water,
he would never need to set foot on land; and if he preferred land, he
would never need to enter a boat. The two are so curiously mingled
together, narrow necks of land running side by side with long, narrow
sea-inlets, that Atlantic and Pacific Oceans are unknown.
Some have wondered whether the reddish color of the land may be caused
by grass and trees being red instead of green. Very strange if so it
were. But in that case, no doubt the inhabitants of Mars would find
green just as trying to their eyesight, as we should find red trying to
ours.