CELESTIAL DISTANCES
Up to this we have merely taken a general view of the solar system--a
bird's-eye view, so to speak, from space.
In the course of our inquiry we noted in a rough way the _relative_
distances at which the various planets move around the sun. But we have
not yet stated what these distances _actually_ are, and it were
therefore well now to turn our attention to this important matter.
Each of us has a fair idea of what a mile is. It is a quarter of an
hour's sharp walk, for instance; or yonder village or building, we know,
lies such and such a number of miles away.
The measurements which have already been given of the diameters of the
various bodies of the solar system appear very great to us, who find
that a walk of a few miles at a time taxes our strength; but they are a
mere nothing when we consider the distances from the sun at which the
various planets revolve in their orbits.
The following table gives these distances in round numbers. As here
stated they are what are called "mean" distances; for, as the orbits are
oval, the planets vary in their distances from the sun, and we are
therefore obliged to strike a kind of average for each case:--
Mercury about 36,000,000 miles.
Venus " 67,200,000 "
Earth " 92,900,000 "
Mars " 141,500,000 "
Jupiter " 483,300,000 "
Saturn " 886,000,000 "
Uranus " 1,781,900,000 "
Neptune " 2,791,600,000 "
From the above it will be seen at a glance that we have entered upon a
still greater scale of distance than in dealing with the diameters of
the various bodies of the system. In that case the distances were
limited to thousands of miles; in this, however, we have to deal with
millions. A million being ten hundred thousand, it will be noticed that
even the diameter of the huge sun is well under a million miles.
How indeed are we to get a grasp of such distances, when those to which
we are ordinarily accustomed--the few miles' walk, the little stretch of
sea or land which we gaze upon around us--are so utterly minute in
comparison? The fact is, that though men may think that they can picture
in their minds such immense distances, they actually can not. In matters
like these we unconsciously employ a kind of convention, and we estimate
a thing as being two or three or more times the size of another. More
than this we are unable to do. For instance, our ordinary experience of
a mile enables us to judge, in a way, of a stretch of several miles,
such as one can take in with a glance; but in our estimation of a
thousand miles, or even of one hundred, we are driven back upon a mental
trick, so to speak.
In our attempts to realise such immense distances as those in the solar
system we are obliged to have recourse to analogies; to comparisons with
other and simpler facts, though this is at the best a mere self-cheating
device. The analogy which seems most suited to our purpose here, and one
which has often been employed by writers, is borrowed from the rate at
which an express train travels.
Let us imagine, for instance, that we possess an express train which is
capable of running anywhere, never stops, never requires fuel, and
always goes along at sixty miles an hour. Suppose we commence by
employing it to gauge the size of our own planet, the earth. Let us send
it on a trip around the equator, the span of which is about 24,000
miles. At its sixty-miles-an-hour rate of going, this journey will take
nearly 17 days. Next let us send it from the earth to the moon. This
distance, 240,000 miles, being ten times as great as the last, will of
course take ten times as long to cover, namely, 170 days; that is to
say, nearly half a year. Again, let us send it still further afield, to
the sun, for example. Here, however, it enters upon a journey which is
not to be measured in thousands of miles, as the others were, but in
millions. The distance from the earth to the sun, as we have seen in the
foregoing table, is about 93 millions of miles. Our express train would
take about 178 _years_ to traverse this.
Having arrived at the sun, let us suppose that our train makes a tour
right round it. This will take more than five years.
Supposing, finally, that our train were started from the sun, and made
to run straight out to the known boundaries of the solar system, that is
to say, as far as the orbit of Neptune, it would take over 5000 years to
traverse the distance.
That sixty miles an hour is a very great speed any one, I think, will
admit who has stood upon the platform of a country station while one of
the great mail trains has dashed past. But are not the immensities of
space appalling to contemplate, when one realises that a body moving
incessantly at such a rate would take so long as 10,000 years to
traverse merely the breadth of our solar system? Ten thousand years!
Just try to conceive it. Why, it is only a little more than half that
time since the Pyramids were built, and they mark for us the Dawn of
History. And since then half-a-dozen mighty empires have come and gone!
Having thus concluded our general survey of the appearance and
dimensions of the solar system, let us next inquire into its position
and size in relation to what we call the Universe.
A mere glance at the night sky, when it is free from clouds, shows us
that in every direction there are stars; and this holds good, no matter
what portion of the globe we visit. The same is really true of the sky
by day, though in that case we cannot actually see the stars, for their
light is quite overpowered by the dazzling light of the sun.
We thus reach the conclusion that our earth, that our solar system in
fact, lies plunged within the midst of a great tangle of stars. What
position, by the way, do we occupy in this mighty maze? Are we at the
centre, or anywhere near the centre, or where?
It has been indeed amply proved by astronomical research that the stars
are bodies giving off a light of their own, just as our sun does; that
they are in fact suns, and that our sun is merely one, perhaps indeed a
very unimportant member, of this great universe of stars. Each of these
stars, or suns, besides, may be the centre of a system similar to what
we call our solar system, comprising planets and satellites, comets and
meteors;--or perchance indeed some further variety of attendant bodies
of which we have no example in our tiny corner of space. But as to
whether one is right in a conjecture of this kind, there is up to the
present no proof whatever. No telescope has yet shown a planet in
attendance upon one of these distant suns; for such bodies, even if they
do exist, are entirely out of the range of our mightiest instruments. On
what then can we ground such an assumption? Merely upon analogy; upon
the common-sense deduction that as the stars have characteristics
similar to our particular star, the sun, it would seem unlikely that
ours should be the only such body in the whole of space which is
attended by a planetary system.
"The Stars," using that expression in its most general sense, do not lie
at one fixed distance from us, set here and there upon a background of
sky. There is in fact no background at all. The brilliant orbs are all
around us in space, at different distances from us and from each other;
and we can gaze between them out into the blackness of the void which,
perhaps, continues to extend unceasingly long after the very outposts of
the stellar universe has been left behind. Shall we then start our
imaginary express train once more, and send it out towards the nearest
of the stars? This would, however, be a useless experiment. Our
express-train method of gauging space would fail miserably in the
attempt to bring home to us the mighty gulf by which we are now faced.
Let us therefore halt for a moment and look back upon the orders of
distance with which we have been dealing. First of all we dealt with
thousands of miles. Next we saw how they shrank into insignificance when
we embarked upon millions. We found, indeed, that our sixty-mile-an-hour
train, rushing along without ceasing, would consume nearly the whole of
historical time in a journey from the sun to Neptune.
In the spaces beyond the solar system we are faced, however, by a new
order of distance. From sun to planets is measured in millions of miles,
but from sun to sun is measured in billions. But does the mere stating
of this fact convey anything? I fear not. For the word "billion" runs as
glibly off the tongue as "million," and both are so wholly unrealisable
by us that the actual difference between them might easily pass
unnoticed.
Let us, however, make a careful comparison. What is a million? It is a
thousand thousands. But what is a billion? It is a million millions.
Consider for a moment! A million of millions. That means a million, each
unit of which is again a million. In fact every separate "1" in this
million is itself a million. Here is a way of trying to realise this
gigantic number. A million seconds make only eleven and a half days and
nights. But a billion seconds will make actually more than thirty
thousand years!
Having accepted this, let us try and probe with our express train even a
little of the new gulf which now lies before us. At our old rate of
going it took almost two years to cover a million miles. To cover a
billion miles--that is to say, a million times this distance--would thus
take of course nearly two million years. Alpha Centauri, the nearest
star to our earth, is some twenty-five billions of miles away. Our
express train would thus take about fifty millions of years to reach it!
This shows how useless our illustration, appropriate though it seemed
for interplanetary space, becomes when applied to the interstellar
spaces. It merely gives us millions in return for billions; and so the
mind, driven in upon itself, whirls round and round like a squirrel in
its revolving cage. There is, however, a useful illustration still left
us, and it is the one which astronomers usually employ in dealing with
the distances of the stars. The illustration in question is taken from
the velocity of light.
Light travels at the tremendous speed of about 186,000 miles a second.
It therefore takes only about a second and a quarter to come to us from
the moon. It traverses the 93,000,000 of miles which separate us from
the sun in about eight minutes. It travels from the sun out to Neptune
in about four hours, which means that it would cross the solar system
from end to end in eight. To pass, however, across the distance which
separates us from Alpha Centauri it would take so long as about four
and a quarter years!
Astronomers, therefore, agree in estimating the distances of the stars
from the point of view of the time which light would take to pass from
them to our earth. They speak of that distance which light takes a year
to traverse as a "light year." According to this notation, Alpha
Centauri is spoken of as being about four and a quarter light years
distant from us.
Now as the rays of light coming from Alpha Centauri to us are chasing
one another incessantly across the gulf of space, and as each ray left
that star some four years before it reaches us, our view of the star
itself must therefore be always some four years old. Were then this star
to be suddenly removed from the universe at any moment, we should
continue to see it still in its place in the sky for some four years
more, after which it would suddenly disappear. The rays which had
already started upon their journey towards our earth must indeed
continue travelling, and reaching us in their turn until the last one
had arrived; after which no more would come.
We have drawn attention to Alpha Centauri as the nearest of the stars.
The majority of the others indeed are ever so much farther. We can only
hazard a guess at the time it takes for the rays from many of them to
reach our globe. Suppose, for instance, we see a sudden change in the
light of any of these remote stars, we are inclined to ask ourselves
when that change did actually occur. Was it in the days of Queen
Elizabeth, or at the time of the Norman Conquest; or was it when Rome
was at the height of her glory, or perhaps ages before that when the
Pyramids of Egypt were being built? Even the last of these suppositions
cannot be treated lightly. We have indeed no real knowledge of the
distance from us of those stars which our giant telescopes have brought
into view out of the depths of the celestial spaces.