The group of bodies which cluster around our sun forms a little island,
so to speak, in the extent of infinite space. We may illustrate this
by a map in which we shall endeavor to show the stars placed at their
proper relative distances. We first open the compasses one inch, and
thus draw a little circle to represent the path of the earth. We are
not going to put in all the planets. We take Neptune, the outermost, at
once. To draw its path I open the compasses to thirty inches, and draw
a circle with that radius. That will do for our solar system, though
the comets no doubt will roam beyond these limits. To complete our map
we ought of course to put in some stars. There are a hundred million to
choose from, and we shall begin with the brightest. It is often called
the Dog Star, but astronomers know it better as Sirius. Let us see
where it is to be placed on our map. Sirius is beyond Neptune, so it
must be outside somewhere. Indeed, it is a good deal further off than
Neptune; so I try at the edge of the drawing-board; I have got a method
of making a little calculation that I do not intend to trouble you
with, but I can assure you that the results it leads me to are quite
correct; they show me that this board is not big enough. But could a
board which was big enough fit into this lecture theatre? Here, again,
I make my little calculations, and I find that there would not be room
for a board sufficiently great; in fact, if I put the sun here at one
end, with its planets around it, Sirius would be too near on the same
scale if it were at the further corner. The board would have to go out
through the wall of the theatre, out through London. Indeed, big as
London is, it would not be large enough to contain the drawing-board
that I should require. It would have to stretch about twenty miles from
where we are now assembled. We may therefore dismiss any hope of making
a practical map of our system on this scale if Sirius is to have its
proper place. Let us, then, take some other star. We shall naturally
try with the nearest of all. It is one that we do not know in this
part of the world, but those that live in the southern hemisphere are
well acquainted with it. The name of this star is Alpha Centauri. Even
for this star, we should require a drawing three or four miles long
if the distance from the earth to the sun is to be taken as one inch.
You see what an isolated position our sun and his planets occupy. The
members of the family are all close together, and the nearest neighbors
are situated at enormous distances. There is a good reason for this
separation. The stars are very pretty and perfectly harmless to us
where they are at present situated. They might be very troublesome
neighbors if they were very much closer to our system. It is therefore
well they are so far off; they would be constantly making disturbance
in the sun’s family if they were near at hand. Sometimes they would be
dragging us into unpleasantly great heat by bringing us too close to
the sun, or producing a coolness by pulling us away from the sun, which
would be quite as disagreeable.