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Side Lights On Astronomy

S >> Simon Newcomb >> Side Lights On Astronomy




It is a scientific inference, based on facts so numerous as not to
admit of serious question, that during the history of our globe
there has been a continually improving development of life. As
ages upon ages pass, new forms are generated, higher in the scale
than those which preceded them, until at length reason appears and
asserts its sway. In a recent well-known work Alfred Russel
Wallace has argued that this development of life required the
presence of such a rare combination of conditions that there is no
reason to suppose that it prevailed anywhere except on our earth.
It is quite impossible in the present discussion to follow his
reasoning in detail; but it seems to me altogether inconclusive.
Not only does life, but intelligence, flourish on this globe under
a great variety of conditions as regards temperature and
surroundings, and no sound reason can be shown why under certain
conditions, which are frequent in the universe, intelligent beings
should not acquire the highest development.

Now let us look at the subject from the view of the mathematical
theory of probabilities. A fundamental tenet of this theory is
that no matter how improbable a result may be on a single trial,
supposing it at all possible, it is sure to occur after a
sufficient number of trials--and over and over again if the trials
are repeated often enough. For example, if a million grains of
corn, of which a single one was red, were all placed in a pile,
and a blindfolded person were required to grope in the pile,
select a grain, and then put it back again, the chances would be a
million to one against his drawing out the red grain. If drawing
it meant he should die, a sensible person would give himself no
concern at having to draw the grain. The probability of his death
would not be so great as the actual probability that he will
really die within the next twenty-four hours. And yet if the whole
human race were required to run this chance, it is certain that
about fifteen hundred, or one out of a million, of the whole human
family would draw the red grain and meet his death.

Now apply this principle to the universe. Let us suppose, to fix
the ideas, that there are a hundred million worlds, but that the
chances are one thousand to one against any one of these taken at
random being fitted for the highest development of life or for the
evolution of reason. The chances would still be that one hundred
thousand of them would be inhabited by rational beings whom we
call human. But where are we to look for these worlds? This no man
can tell. We only infer from the statistics of the stars--and this
inference is fairly well grounded--that the number of worlds
which, so far as we know, may be inhabited, are to be counted by
thousands, and perhaps by millions.

In a number of bodies so vast we should expect every variety of
conditions as regards temperature and surroundings. If we suppose
that the special conditions which prevail on our planet are
necessary to the highest forms of life, we still have reason to
believe that these same conditions prevail on thousands of other
worlds. The fact that we might find the conditions in millions of
other worlds unfavorable to life would not disprove the existence
of the latter on countless worlds differently situated.

Coming down now from the general question to the specific one, we
all know that the only worlds the conditions of which can be made
the subject of observation are the planets which revolve around
the sun, and their satellites. The question whether these bodies
are inhabited is one which, of course, completely transcends not
only our powers of observation at present, but every appliance of
research that we can conceive of men devising. If Mars is
inhabited, and if the people of that planet have equal powers with
ourselves, the problem of merely producing an illumination which
could be seen in our most powerful telescope would be beyond all
the ordinary efforts of an entire nation. An unbroken square mile
of flame would be invisible in our telescopes, but a hundred
square miles might be seen. We cannot, therefore, expect to see
any signs of the works of inhabitants even on Mars. All that we
can do is to ascertain with greater or less probability whether
the conditions necessary to life exist on the other planets of the
system.

The moon being much the nearest to us of all the heavenly bodies,
we can pronounce more definitely in its case than in any other. We
know that neither air nor water exists on the moon in quantities
sufficient to be perceived by the most delicate tests at our
command. It is certain that the moon's atmosphere, if any exists,
is less than the thousandth part of the density of that around us.
The vacuum is greater than any ordinary air-pump is capable of
producing. We can hardly suppose that so small a quantity of air
could be of any benefit whatever in sustaining life; an animal
that could get along on so little could get along on none at all.

But the proof of the absence of life is yet stronger when we
consider the results of actual telescopic observation. An object
such as an ordinary city block could be detected on the moon. If
anything like vegetation were present on its surface, we should
see the changes which it would undergo in the course of a month,
during one portion of which it would be exposed to the rays of the
unclouded sun, and during another to the intense cold of space. If
men built cities, or even separate buildings the size of the
larger ones on our earth, we might see some signs of them.

In recent times we not only observe the moon with the telescope,
but get still more definite information by photography. The whole
visible surface has been repeatedly photographed under the best
conditions. But no change has been established beyond question,
nor does the photograph show the slightest difference of structure
or shade which could be attributed to cities or other works of
man. To all appearances the whole surface of our satellite is as
completely devoid of life as the lava newly thrown from Vesuvius.
We next pass to the planets. Mercury, the nearest to the sun, is
in a position very unfavorable for observation from the earth,
because when nearest to us it is between us and the sun, so that
its dark hemisphere is presented to us. Nothing satisfactory has
yet been made out as to its condition. We cannot say with
certainty whether it has an atmosphere or not. What seems very
probable is that the temperature on its surface is higher than any
of our earthly animals could sustain. But this proves nothing.

We know that Venus has an atmosphere. This was very conclusively
shown during the transits of Venus in 1874 and 1882. But this
atmosphere is so filled with clouds or vapor that it does not seem
likely that we ever get a view of the solid body of the planet
through it. Some observers have thought they could see spots on
Venus day after day, while others have disputed this view. On the
whole, if intelligent inhabitants live there, it is not likely
that they ever see sun or stars. Instead of the sun they see only
an effulgence in the vapory sky which disappears and reappears at
regular intervals.

When we come to Mars, we have more definite knowledge, and there
seems to be greater possibilities for life there than in the case
of any other planet besides the earth. The main reason for denying
that life such as ours could exist there is that the atmosphere of
Mars is so rare that, in the light of the most recent researches,
we cannot be fully assured that it exists at all. The very careful
comparisons of the spectra of Mars and of the moon made by
Campbell at the Lick Observatory failed to show the slightest
difference in the two. If Mars had an atmosphere as dense as ours,
the result could be seen in the darkening of the lines of the
spectrum produced by the double passage of the light through it.
There were no lines in the spectrum of Mars that were not seen
with equal distinctness in that of the moon. But this does not
prove the entire absence of an atmosphere. It only shows a limit
to its density. It may be one-fifth or one-fourth the density of
that on the earth, but probably no more.

That there must be something in the nature of vapor at least seems
to be shown by the formation and disappearance of the white polar
caps of this planet. Every reader of astronomy at the present time
knows that, during the Martian winter, white caps form around the
pole of the planet which is turned away from the sun, and grow
larger and larger until the sun begins to shine upon them, when
they gradually grow smaller, and perhaps nearly disappear. It
seems, therefore, fairly well proved that, under the influence of
cold, some white substance forms around the polar regions of Mars
which evaporates under the influence of the sun's rays. It has
been supposed that this substance is snow, produced in the same
way that snow is produced on the earth, by the evaporation of
water.

But there are difficulties in the way of this explanation. The sun
sends less than half as much heat to Mars as to the earth, and it
does not seem likely that the polar regions can ever receive
enough of heat to melt any considerable quantity of snow. Nor does
it seem likely that any clouds from which snow could fall ever
obscure the surface of Mars.

But a very slight change in the explanation will make it tenable.
Quite possibly the white deposits may be due to something like
hoar-frost condensed from slightly moist air, without the actual
production of snow. This would produce the effect that we see.
Even this explanation implies that Mars has air and water, rare
though the former may be. It is quite possible that air as thin as
that of Mars would sustain life in some form. Life not totally
unlike that on the earth may therefore exist upon this planet for
anything that we know to the contrary. More than this we cannot
say.

In the case of the outer planets the answer to our question must
be in the negative. It now seems likely that Jupiter is a body
very much like our sun, only that the dark portion is too cool to
emit much, if any, light. It is doubtful whether Jupiter has
anything in the nature of a solid surface. Its interior is in all
likelihood a mass of molten matter far above a red heat, which is
surrounded by a comparatively cool, yet, to our measure, extremely
hot, vapor. The belt-like clouds which surround the planet are due
to this vapor combined with the rapid rotation. If there is any
solid surface below the atmosphere that we can see, it is swept by
winds such that nothing we have on earth could withstand them.
But, as we have said, the probabilities are very much against
there being anything like such a surface. At some great depth in
the fiery vapor there is a solid nucleus; that is all we can say.

The planet Saturn seems to be very much like that of Jupiter in
its composition. It receives so little heat from the sun that,
unless it is a mass of fiery vapor like Jupiter, the surface must
be far below the freezing-point.

We cannot speak with such certainty of Uranus and Neptune; yet the
probability seems to be that they are in much the same condition
as Saturn. They are known to have very dense atmospheres, which
are made known to us only by their absorbing some of the light of
the sun. But nothing is known of the composition of these
atmospheres.

To sum up our argument: the fact that, so far as we have yet been
able to learn, only a very small proportion of the visible worlds
scattered through space are fitted to be the abode of life does
not preclude the probability that among hundreds of millions of
such worlds a vast number are so fitted. Such being the case, all
the analogies of nature lead us to believe that, whatever the
process which led to life upon this earth--whether a special act
of creative power or a gradual course of development--through that
same process does life begin in every part of the universe fitted
to sustain it. The course of development involves a gradual
improvement in living forms, which by irregular steps rise higher
and higher in the scale of being. We have every reason to believe
that this is the case wherever life exists. It is, therefore,
perfectly reasonable to suppose that beings, not only animated,
but endowed with reason, inhabit countless worlds in space. It
would, indeed, be very inspiring could we learn by actual
observation what forms of society exist throughout space, and see
the members of such societies enjoying themselves by their warm
firesides. But this, so far as we can now see, is entirely beyond
the possible reach of our race, so long as it is confined to a
single world.





VIII

HOW THE PLANETS ARE WEIGHED


You ask me how the planets are weighed? I reply, on the same
principle by which a butcher weighs a ham in a spring-balance.
When he picks the ham up, he feels a pull of the ham towards the
earth. When he hangs it on the hook, this pull is transferred from
his hand to the spring of the balance. The stronger the pull, the
farther the spring is pulled down. What he reads on the scale is
the strength of the pull. You know that this pull is simply the
attraction of the earth on the ham. But, by a universal law of
force, the ham attracts the earth exactly as much as the earth
does the ham. So what the butcher really does is to find how much
or how strongly the ham attracts the earth, and he calls that pull
the weight of the ham. On the same principle, the astronomer finds
the weight of a body by finding how strong is its attractive pull
on some other body. If the butcher, with his spring-balance and a
ham, could fly to all the planets, one after the other, weigh the
ham on each, and come back to report the results to an astronomer,
the latter could immediately compute the weight of each planet of
known diameter, as compared with that of the earth. In applying
this principle to the heavenly bodies, we at once meet a
difficulty that looks insurmountable. You cannot get up to the
heavenly bodies to do your weighing; how then will you measure
their pull? I must begin the answer to this question by explaining
a nice point in exact science. Astronomers distinguish between the
weight of a body and its mass. The weight of objects is not the
same all over the world; a thing which weighs thirty pounds in New
York would weigh an ounce more than thirty pounds in a spring-
balance in Greenland, and nearly an ounce less at the equator.
This is because the earth is not a perfect sphere, but a little
flattened. Thus weight varies with the place. If a ham weighing
thirty pounds were taken up to the moon and weighed there, the
pull would only be five pounds, because the moon is so much
smaller and lighter than the earth. There would be another weight
of the ham for the planet Mars, and yet another on the sun, where
it would weigh some eight hundred pounds. Hence the astronomer
does not speak of the weight of a planet, because that would
depend on the place where it was weighed; but he speaks of the
mass of the planet, which means how much planet there is, no
matter where you might weigh it.

At the same time, we might, without any inexactness, agree that
the mass of a heavenly body should be fixed by the weight it would
have in New York. As we could not even imagine a planet at New
York, because it may be larger than the earth itself, what we are
to imagine is this: Suppose the planet could be divided into a
million million million equal parts, and one of these parts
brought to New York and weighed. We could easily find its weight
in pounds or tons. Then multiply this weight by a million million
million, and we shall have a weight of the planet. This would be
what the astronomers might take as the mass of the planet.

With these explanations, let us see how the weight of the earth is
found. The principle we apply is that round bodies of the same
specific gravity attract small objects on their surface with a
force proportional to the diameter of the attracting body. For
example, a body two feet in diameter attracts twice as strongly as
one of a foot, one of three feet three times as strongly, and so
on. Now, our earth is about 40,000,000 feet in diameter; that is
10,000,000 times four feet. It follows that if we made a little
model of the earth four feet in diameter, having the average
specific gravity of the earth, it would attract a particle with
one ten-millionth part of the attraction of the earth. The
attraction of such a model has actually been measured. Since we do
not know the average specific gravity of the earth--that being in
fact what we want to find out--we take a globe of lead, four feet
in diameter, let us suppose. By means of a balance of the most
exquisite construction it is found that such a globe does exert a
minute attraction on small bodies around it, and that this
attraction is a little more than the ten-millionth part of that of
the earth. This shows that the specific gravity of the lead is a
little greater than that of the average of the whole earth. All
the minute calculations made, it is found that the earth, in order
to attract with the force it does, must be about five and one-half
times as heavy as its bulk of water, or perhaps a little more.
Different experimenters find different results; the best between
5.5 and 5.6, so that 5.5 is, perhaps, as near the number as we can
now get. This is much more than the average specific gravity of
the materials which compose that part of the earth which we can
reach by digging mines. The difference arises from the fact that,
at the depth of many miles, the matter composing the earth is
compressed into a smaller space by the enormous weight of the
portions lying above it. Thus, at the depth of 1000 miles, the
pressure on every cubic inch is more than 2000 tons, a weight
which would greatly condense the hardest metal.

We come now to the planets. I have said that the mass or weight of
a heavenly body is determined by its attraction on some other
body. There are two ways in which the attraction of a planet may
be measured. One is by its attraction on the planets next to it.
If these bodies did not attract one another at all, but only moved
under the influence of the sun, they would move in orbits having
the form of ellipses. They are found to move very nearly in such
orbits, only the actual path deviates from an ellipse, now in one
direction and then in another, and it slowly changes its position
from year to year. These deviations are due to the pull of the
other planets, and by measuring the deviations we can determine
the amount of the pull, and hence the mass of the planet.

The reader will readily understand that the mathematical processes
necessary to get a result in this way must be very delicate and
complicated. A much simpler method can be used in the case of
those planets which have satellites revolving round them, because
the attraction of the planet can be determined by the motions of
the satellite. The first law of motion teaches us that a body in
motion, if acted on by no force, will move in a straight line.
Hence, if we see a body moving in a curve, we know that it is
acted on by a force in the direction towards which the motion
curves. A familiar example is that of a stone thrown from the
hand. If the stone were not attracted by the earth, it would go on
forever in the line of throw, and leave the earth entirely. But
under the attraction of the earth, it is drawn down and down, as
it travels onward, until finally it reaches the ground. The faster
the stone is thrown, of course, the farther it will go, and the
greater will be the sweep of the curve of its path. If it were a
cannon-ball, the first part of the curve would be nearly a right
line. If we could fire a cannon-ball horizontally from the top of
a high mountain with a velocity of five miles a second, and if it
were not resisted by the air, the curvature of the path would be
equal to that of the surface of our earth, and so the ball would
never reach the earth, but would revolve round it like a little
satellite in an orbit of its own. Could this be done, the
astronomer would be able, knowing the velocity of the ball, to
calculate the attraction of the earth as well as we determine it
by actually observing the motion of falling bodies around us.

Thus it is that when a planet, like Mars or Jupiter, has
satellites revolving round it, astronomers on the earth can
observe the attraction of the planet on its satellites and thus
determine its mass. The rule for doing this is very simple. The
cube of the distance between the planet and satellite is divided
by the square of the time of revolution of the satellite. The
quotient is a number which is proportional to the mass of the
planet. The rule applies to the motion of the moon round the earth
and of the planets round the sun. If we divide the cube of the
earth's distance from the sun, say 93,000,000 miles, by the square
of 365 1/4, the days in a year, we shall get a certain quotient.
Let us call this number the sun-quotient. Then, if we divide the
cube of the moon's distance from the earth by the square of its
time of revolution, we shall get another quotient, which we may
call the earth-quotient. The sun-quotient will come out about
330,000 times as large as the earth-quotient. Hence it is
concluded that the mass of the sun is 330,000 times that of the
earth; that it would take this number of earths to make a body as
heavy as the sun.

I give this calculation to illustrate the principle; it must not
be supposed that the astronomer proceeds exactly in this way and
has only this simple calculation to make. In the case of the moon
and earth, the motion and distance of the former vary in
consequence of the attraction of the sun, so that their actual
distance apart is a changing quantity. So what the astronomer
actually does is to find the attraction of the earth by observing
the length of a pendulum which beats seconds in various latitudes.
Then, by very delicate mathematical processes, he can find with
great exactness what would be the time of revolution of a small
satellite at any given distance from the earth, and thus can get
the earth-quotient.

But, as I have already pointed out, we must, in the case of the
planets, find the quotient in question by means of the satellites;
and it happens, fortunately, that the motions of these bodies are
much less changed by the attraction of the sun than is the motion
of the moon. Thus, when we make the computation for the outer
satellite of Mars, we find the quotient to be 1/3093500 that of
the sun-quotient. Hence we conclude that the mass of Mars is
1/3093500 that of the sun. By the corresponding quotient, the mass
of Jupiter is found to be about 1/1047 that of the sun, Saturn
1/3500, Uranus 1/22700, Neptune 1/19500.

We have set forth only the great principle on which the astronomer
has proceeded for the purpose in question. The law of gravitation
is at the bottom of all his work. The effects of this law require
mathematical processes which it has taken two hundred years to
bring to their present state, and which are still far from
perfect. The measurement of the distance of a satellite is not a
job to be done in an evening; it requires patient labor extending
through months and years, and then is not as exact as the
astronomer would wish. He does the best he can, and must be
satisfied with that.





IX

THE MARINER'S COMPASS


Among those provisions of Nature which seem to us as especially
designed for the use of man, none is more striking than the
seeming magnetism of the earth. What would our civilization have
been if the mariner's compass had never been known? That Columbus
could never have crossed the Atlantic is certain; in what
generation since his time our continent would have been discovered
is doubtful. Did the reader ever reflect what a problem the
captain of the finest ocean liner of our day would face if he had
to cross the ocean without this little instrument? With the aid of
a pilot he gets his ship outside of Sandy Hook without much
difficulty. Even later, so long as the sun is visible and the air
is clear, he will have some apparatus for sailing by the direction
of the sun. But after a few hours clouds cover the sky. From that
moment he has not the slightest idea of east, west, north, or
south, except so far as he may infer it from the direction in
which he notices the wind to blow. For a few hours he may be
guided by the wind, provided he is sure he is not going ashore on
Long Island. Thus, in time, he feels his way out into the open
sea. By day he has some idea of direction with the aid of the sun;
by night, when the sky is clear he can steer by the Great Bear, or
"Cynosure," the compass of his ancient predecessors on the
Mediterranean. But when it is cloudy, if he persists in steaming
ahead, he may be running towards the Azores or towards Greenland,
or he may be making his way back to New York without knowing it.
So, keeping up steam only when sun or star is visible, he at
length finds that he is approaching the coast of Ireland. Then he
has to grope along much like a blind man with his staff, feeling
his way along the edge of a precipice. He can determine the
latitude at noon if the sky is clear, and his longitude in the
morning or evening in the same conditions. In this way he will get
a general idea of his whereabouts. But if he ventures to make
headway in a fog, he may find himself on the rocks at any moment.
He reaches his haven only after many spells of patient waiting for
favoring skies.

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