[Illustration: FIG. 8.--The nearer you are, the bigger the Globe looks.]
[Illustration: FIG. 9.--The Globe is so far off that it lies beyond the
Picture. The dotted lines show how small it seems.]
I ought here to explain a principle which those who are learning
about the stars must always bear in mind. The principle asserts that
the further a body is, the smaller it looks. Perhaps this will be
understood from the adjoining little sketch (Fig. 8). It represents a
great globe, on which oceans and continents are shown, and you see
a little boy and a little girl are looking at the globe. The girl
stands quite close to it, and I have drawn two dotted lines from her
eye, one to the top of the globe, and the other to the under surface.
If she wants to examine the entire side of the globe which is visible
to her, she must first look along the upper dotted line, and then she
must turn her glance downwards until she comes to the lower line, and
having to turn her eyes thus up and down she will think the globe is
very big, and she will be quite right. The boy is, as you see, on the
other side of the globe, but I have put him much further off than the
girl. I have also drawn two dotted lines from his eye to the globe, and
it is plain that he will not have to turn his head much up and down to
see the whole globe. He can take it all in at a glance, and to him,
therefore, the globe will appear to be comparatively small, because he
is sufficiently far from it. The more distant he is, the smaller it
will appear. You can easily imagine that, if the globe were far enough,
the two lines that would include the whole would be like those shown
(Fig. 9), in which the globe is so distant that it cannot be seen in
the picture. The apparent size of the globe, which is really measured
by the angle between these two lines, would always be smaller and
smaller according as the distance was greater. Now you can understand
why an object seems smaller the further away it is; indeed, when
sufficiently far, the object ceases to be visible at all.
I could give many illustrations of the diminution of size by distance,
and so, doubtless, could you. Every boy knows that his kite looks
smaller and smaller the greater the length of string that he lets
out. I have seen in the West of Ireland a bird that seemed like a
little speck high up near the clouds, but from its flight and other
circumstances I knew that the speck was not a little bird. It was,
indeed, a great eagle, which was dwarfed by the elevation to which it
had soared.
It is in astronomy that we have the best illustrations of this
principle. Enormous objects seem to be small because they are so very
far off. You must therefore always remember that although an object may
appear to be small, this appearance may be only a delusion. It may be
that the object is very big, but very distant. In astronomy, this is
almost always the case, there is so much room above us, around us, on
all sides in space. Look up at the ceiling. It certainly does not bound
space, for there is another side to it; and then there is the roof of
the house. But the roof is not a boundary, for, of course, there is the
air above it, and then, higher up still, there are the clouds, and so
we can carry our imagination on and on through and beyond the air up
to where the stars are, and still on and on. And as there is unlimited
room, the celestial bodies take advantage of it, and are, generally
speaking, at distances so gigantic that, no matter how small they may
appear, their smallness is merely deceptive.
Let us try to illustrate in another way the exceeding remoteness of the
sun. So please imagine that you were on the sun, and that you took a
view of our earth from that distance. To find out what we must expect
to see, let us think of a balloon voyage. If you were to go up in a
balloon, you would at first see only the houses, or objects immediately
about you, but as you rose the view would become wider and wider. You
would see that London was surrounded by the country, and then, as you
still soared up and up, the sea would become visible, and you would
be able to trace out the coasts, east and west and south. If, in some
way, you could soar higher than any balloon could carry you, the whole
of the British Islands would presently lie spread like a map beneath.
Still on and on, and then the continent of Europe would be gradually
opened out, until the great oceans, and even other continents, would
at last be caught sight of, and then you would perceive that our whole
earth was indeed a globe. The higher you went, the less distinctly
would you be able to see the details on the surface. At last the
outlines of the continents and oceans would fade, and you would begin
to lose any perception of the shape of the earth itself. Long ere you
had reached the distance of the sun, the earth would look merely as the
planet Venus now does to us. It is instructive to consider how small
our earth would seem if it were possible to view it from the sun. Think
of that very familiar little globe, a lawn-tennis ball, which is two
and three-quarter inches in diameter. But suppose a tennis ball were
at the opposite side of the street, or still further away; suppose,
for example, that it were half a mile away, what could you expect to
see of it? And yet the earth, as seen from the sun, would appear to be
no larger than a tennis ball would look when viewed from a distance of
half a mile.