ing. 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 separat