FOUR SMALL WORLDS
What must the sun appear to Mercury, who is so much nearer to him than
we are? To understand that we should have to imagine our sun increased
to eight or nine times his apparent size, and pouring out far greater
heat and light than anything that we have here, even in the tropics. It
was at first supposed that Mercury must have an extra thick covering of
clouds to protect him from this tremendous glare; but recent
observations tend to prove that, far from this, he is singularly free
from cloud. As this is so, no life as we know it could possibly exist on
Mercury.
His year--the time he takes to go round the sun and come back to the
same place again--is eighty-eight days, or about one-quarter of ours. As
his orbit is much more like an ellipse than a circle, it follows that he
is much nearer to the sun at one time than at another--in fact, when he
is nearest, the size of the sun must seem three and a half times
greater than when he is furthest away from it! Even at the best Mercury
is very difficult to observe, and what we can learn about him is not
much; but, as we have heard, his axis is supposed to be upright. If so
his seasons cannot depend on the bend toward or away from the sun, but
must be influenced solely by the changes in his distance from the sun,
which are much greater than in our own ease. There is some reason to
believe, too, that Mercury's day and year are the same length. This
means that as the planet circles round the sun he turns once. If this is
so the sun will shine on one half of the planet, producing an
accumulated heat terrific to think of; while the other side is plunged
in blackness. The side which faces the sun must be heated to a pitch
inconceivable to us during the nearer half of the orbit--a pitch at
which every substance must be at boiling-point, and which no life as we
know it could possibly endure. Seen from our point of view, Mercury goes
through all the phases of the moon, as he shines by the reflected light
of the sun; but this point we shall consider more particularly in regard
to Venus, as Venus is nearer to us and easier to study. For a long time
astronomers had a fancy that there might be another planet even nearer
to the sun than Mercury, perhaps hidden from us by the great glare of
the sun. They even named this imaginary planet Vulcan, and some thought
they had seen it, but it is tolerably certain that Vulcan existed only
in imagination. Mercury is the nearest planet to the sun, and also the
smallest, of course excepting the asteroids. It is about three thousand
miles in diameter, and as our moon is two thousand miles, it is not so
much bigger than that. So far as we are concerned, it is improbable we
shall ever know very much more about this little planet.
But next we come to Venus, our beautiful bright neighbour, who
approaches nearer to us than any other heavenly body except the moon.
Alas! when she is nearest, she like Mercury, turns her dark side toward
us, coming in between us and the sun, so that we cannot observe her at
all.
Everyone must have noticed Venus, however carelessly they have looked at
the sky; but it is likely that far more people have seen her as an
evening than a morning star, for most people are in bed when the sun
rises, and it is only before sunrise or after sunset we can see Venus
well. She is at her best from our point of view when she seems to us to
be furthest from the sun, for then we can study her best, and at these
times she appears like a half or three-quarter moon, as we only see a
part of the side from which the sunlight is reflected. She shines like a
little silver lamp, excelling every other planet, even Jupiter, the
largest of all. If we look at her even with the naked eye, we can see
that she is elongated or drawn out, but her brilliance prevents us from
seeing her shape exactly; to do this we must use a telescope.
[Illustration: DIFFERENT PHASES OF VENUS.]
It is a curious fact that some planets shine much more brightly than
others, without regard to their size--that is to say, the surface on
which the sun's rays strike is of greater reflecting power in some than
in others. One of the brightest things in Nature that we can imagine is
a bank of snow in sunlight; it is so dazzling that we have to look away
or wink hard at the sight; and the reflective power of the surface of
Venus is as dazzling as if she were made of snow. This is probably
because the light strikes on the upper surface of the clouds which
surround her. In great contrast to this is the surface of Mercury, which
reflects as dully as a mass of lead. Our own moon has not a high
reflecting power, as will be easily understood if we imagine what the
world would be if condemned to perpetual moonlight only. It would,
indeed, be a sad deprivation if the mournful cold light of the moon,
welcome enough as a change from sunlight, were to take the place of
sunlight in the daytime.
For a very long time astronomers could not discover what time Venus took
in rotating on her own axis--that is to say, what the length of her day
was. She is difficult to observe, and in order to find out the rotation
it is necessary to note some fixed object on the surface which turns
round with the planet and comes back to the same place again, so that
the time it takes in its journey can be measured. But the surface of
Venus is always changing, so that it is impossible to judge at all
certainly. Opinions differ greatly, some astronomers holding that
Venus's day is not much longer than an earthly day, while others believe
that the planet's day is equal to her year, just as in the case of
Mercury. Venus's year is 225 days, or about seven and a half of our
months, and if, indeed, her day and year are the same length, very
peculiar effects would follow. For instance, terrible heat would be
absorbed by the side of the planet facing the sun in the perpetual
summer; and the cold which would be felt in the dreary winter's night
would far exceed our bitterest Arctic climate. We cannot but fancy that
any beings who might live on a planet of this kind must be different
altogether from ourselves. Then, there is another point: even here on
earth very strong winds are caused by the heating of the tropics; the
hot air, being lighter than the cold air, rises, and the colder air from
the poles rushes in to supply its place. This causes wind, but the winds
which would be raised on Venus by the rush of air from the icy side of
the planet to the hot one would be tornadoes such as we could but
faintly dream of. It is, of course, useless to speculate when we know so
little, but in a subject so intensely interesting we cannot help
guessing a little.
Venus is only slightly smaller than the earth, and her density is not
very unlike ours; therefore the pull of gravity must be pretty much
there what it is here--that is to say, things will weigh at her surface
about the same as they do here. Her orbit is nearly a circle, so that
her distance from the sun does not vary much, and the heat will not be
much greater from this cause at one time of the year than another.
As her orbit is tilted up a little she does not pass between us and the
sun at each revolution, but occasionally she does so, and this passing
is called a transit. Many important facts have been learned by watching
these transits. Mercury also has transits across the sun, but as she is
so much smaller than Venus they are not of such great importance. It was
by the close observation of Venus during her transits that the distance
from the earth to the sun was first measured. Not until the year 2004
will another transit of Venus occur.
It is not difficult to imagine that the earth must appear a splendid
spectacle from Venus, whence she is seen to great advantage. When
nearest to us she must see us like a little moon, with markings as the
continents and seas rotate, and these will change as they are obscured
by the clouds rolling over them. At the North and South Poles will be
glittering ice-caps, growing larger and smaller as they turn toward or
away from the sun. A brilliant spectacle!
[Illustration: ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY.]
We might say with a sigh, 'If only we could see such a world!' Well, we
can see a world--not indeed, so large as Venus, yet a world that comes
almost as near to us as Venus does, and which, unlike her, is outside us
in order from the sun, so that when it is nearest to us the full
sunlight is on it. This is Mars, our neighbour on the other side, and of
all the fascinating objects in the sky Mars is the most fascinating, for
there, if anywhere, should we be likely to discover beings like
ourselves!
Mars takes rather more than half an hour longer to rotate than we do,
and as he is so much smaller than the earth, this means that he moves
round more slowly. His axis is bent at nearly the same angle as ours is.
Mars is much smaller than the earth, his diameter is about twice that of
the moon, and his density is about three-quarters that of the earth, so
that altogether, with his smaller size and less density, anything
weighing a hundred pounds here would only weigh some forty pounds on
Mars; and if, by some miraculous agency, you were suddenly transported
there, you would find yourself so light that you could jump enormous
distances with little effort, and skip and hop as if you were on
springs.
[Illustration: _Memoirs of the British Astronomical Association._
MAP OF MARS.]
Look at the map of Mars, in which the surface appears to be cut up into
land and water, continents and oceans. The men who first observed Mars
with accuracy saw that some parts were of a reddish colour and others
greenish, and arguing from our own world, they called the greenish parts
seas and the reddish land. For a long while no one doubted that we
actually looked on a world like our own, more especially as there was
supposed to be a covering of atmosphere. The so-called land and water
are much more cut up and mixed together than ours, it is true. Here and
there is a large sea, like that marked 'Mare Australe,' but otherwise
the water and the land are strangely intermingled. The red colour of the
part they named land puzzled astronomers a good deal, for our land seen
at the same distance would not appear so red, and they came at last to
the conclusion that vegetation on Mars must be red instead of green! But
after a while another disturbing fact turned up to upset their theories,
and that was that they saw canals, or what they called canals, on Mars.
These were long, straight, dark markings, such as you see on the map.
It is true that some people never saw these markings at all, and
disbelieved in their existence; but others saw them clearly, and watched
them change--first go fainter and then darker again. And quite recently
a photograph has been obtained which shows them plainly, so they must
have an existence, and cannot be only in the eye of the observer, as the
most sceptical people were wont to suggest. But further than this, one
astronomer announced that some of these lines appeared to be double, yet
when he looked at them again they had grown single. It was like a
conjuring trick. Great excitement was aroused by this, for if the canals
were altered so greatly it really did look as if there were intelligent
beings on Mars capable of working at them. In any case, if these are
really canals, to make them would be a stupendous feat, and if they are
artificial--that is, made by beings and not natural--they show a very
high power of engineering. Imagine anyone on earth making a canal many
miles wide and two thousand miles long! It is inconceivable, but that is
the feat attributed to the Martians. The supposed doubling of the
canals, as I say, caused a great deal of talk, and very few people could
see that they were double at all. Even now the fact is doubted, yet
there seems every reason to believe it is true. They do not all appear
to be double, and those that do are always the same ones, while others
undoubtedly remain single all the time. But the canals do not exhaust
the wonders of Mars. At each pole there is an ice-cap resembling those
found at our own poles, and this tells us pretty plainly something about
the climate of Mars, and that there is water there.
This ice-cap melts when the pole which it surrounds is directed toward
the sun, and sometimes in a hot summer it dwindles down almost to
nothing, in a way that the ice-caps at the poles of the earth never do.
A curious appearance has been noticed when it is melting: a dark shadow
seems to grow underneath the edge of it and extends gradually, and as it
extends the canals near it appear much darker and clearer than they did
before, and then the canals further south undergo the same change. This
looks as if the melting of the snow filled up the canals with water, and
was a means of watering the planet by a system totally different from
anything we know here, where our poles are surrounded by oceans, and the
ice-caps do not in the least affect our water-supply. But, then, another
strange fact had to be taken into consideration. These straight lines
called canals ran out over the seas occasionally, and it was impossible
to believe that if they were canals they could do that. Other things
began to be discussed, such as the fact that the green parts of Mars did
not always remain green. In what is the springtime of Mars they are so,
but afterwards they become yellow, and still later in the season parts
near the pole turn brown. Thus the idea that the greenish parts are seas
had to be quite given up, though it appeared so attractive. The idea now
generally believed is that the greenish parts are vegetation--trees and
bushes and so on, and that the red parts are deserts of reddish sand,
which require irrigation--that is to say, watering--before anything can
be grown on them. The apparent doubling of the canals may be due to the
green vegetation springing up along the banks. This might form two broad
lines, while the canal itself would not be seen, and when the vegetation
dies down, we should see only the trench of the canal, which would
possibly appear faint and single. Therefore the arrangements on Mars
appear to be a rich and a barren season on each hemisphere, the growth
being caused by the melting of the polar ice-cap, which sends floods
down even beyond the Equator. If we could imagine the same thing on
earth we should have to think of pieces of land lying drear and dry and
dead in winter between straight canal-like ditches of vast size. A
little water might remain in these ditches possibly, but not enough to
water the surrounding land. Then, as summer progressed, we should hear,
'The floods are coming,' and each deep, huge canal would be filled up
with a tide of water, penetrating further and further. The water drawn
up into the air would fall in dew or rain. Vegetation would spring up,
especially near the canal banks, and instead of dreary wastes rich
growths would cover the land, gradually dying down again in the winter.
So far Mars seems in some important respects very different from the
earth. He is also less favourably placed than we are, for being so much
further from the sun, he receives very much less heat and light. His
years are 687 of our days, or one year and ten and a half months, and
his atmosphere is not so dense as ours. With this greater distance from
the sun and less air we might suppose the temperature would be very cold
indeed, and that the surface would be frost-bound, not only at the
poles, but far down towards the Equator. Instead of this being so, as we
have seen, the polar caps melt more than those on the earth. We can
only surmise there must be some compensation we do not know of that
softens down the rigour of the seasons, and makes them milder than we
should suppose possible.
Of course, the one absorbing question is, Are there people on Mars? To
this it is at present impossible to reply. We can only say the planet
seems in every way fitted to support life, even if it is a little
different from our earth. It is most certainly a living world, not a
dead one like the moon, and as our knowledge increases we may some day
be able to answer the question which so thrills us.
Our opportunities for the observation of Mars vary very greatly, for as
the earth's orbit lies inside that of Mars, we can best see him when we
are between him and the sun. Of course, it must be remembered that the
earth and the other planets are so infinitely small in regard to the
space between them that there is no possibility of any one of them
getting in such a position that it would throw a shadow on any other or
eclipse it. The planets are like specks in space, and could not
interfere with one another in this way. When Mars, therefore, is in a
line with us and the sun we can see him best, but some of these times
are better than others, for this reason--the earth's orbit is nearly a
circle, and that of Mars more of an ellipse.
[Illustration: ORBITS OF THE EARTH AND MARS.]
Look at the illustration and remember that Mars' year is not quite two
of ours--that is to say, every time we swing round our orbit we catch
him up in a different place, for he will have progressed less than half
his orbit while we go right round ours.
Sometimes when we overtake him he may be at that part which is furthest
away from us, or he may be at that part which is nearest to us, and if
he is in the latter position we can see him best. Now at these, the most
favourable times of all, he is still more than thirty-five millions of
miles away--that is to say, one hundred and forty times as far as the
moon, yet comparatively we can see him very well. He is coming nearer
and nearer to us, and very soon will be nearer than he has been since
1892, or fifteen years ago. Then many telescopes will be directed on
him, and much may be learned about him.
For a long time it was supposed that Mars had no moons, and when Dean
Swift wrote 'Gulliver's Travels' he wanted to make the Laputans do
something very clever, so he described their discovery of two moons
attending Mars, and to make it quite absurd he said that when they
observed these moons they found that one of them went round the planet
in about ten hours. Now, as Mars takes more than twenty-four hours to
rotate, this was considered ridiculous, for no moon known then took
less time to go round its primary world than the primary world took to
turn on its own axis. Our own moon, of course, takes thirty times as
long--that is a month contains thirty days. Then one hundred and fifty
years later this jest of Dean Swift's came true, for two moons were
really discovered revolving round Mars, and one of them does actually
take less time to complete its orbit than the planet does to
rotate--namely, a little more than seven hours! So the absurdity in
'Gulliver's Travels' was a kind of prophecy!
These two moons are very small, the outer one perhaps five or six miles
in diameter, and the inner one about seven; therefore from Mars the
outer one, Deimos, cannot look much more than a brilliant star, and the
inner one would be but a fifth part the apparent width of our own moon.
So Mars is not very well off, after all. Still, there is great variety,
for it must be odd to see the same moon appearing three times in the
day, showing all the different phases as it goes from new to full, even
though it is small!
Such wonderful discoveries have already been made that it is not too
much to say that perhaps some day we may be able to establish some sort
of communication with Mars, and if it be inhabited by any intelligent
beings, we may be able to signal to them; but it is almost impossible
that any contrivance could bridge the gulf of airless space that
separates us, and it is not likely that holiday trips to Mars will ever
become fashionable!