nd collectively they form the solar system.

It is comparatively easy to learn the proportions of this system, to
measure the relative distances of the planets from the sun, and even the
relative sizes of the planets themselves. Peculiar difficulties are,
however, experienced when we seek to ascertain the actual _size_ of the
system as well as its shape. It is this latter question which the
transit of Venus offers us a method of solving.

Look, for instance, at an ordinary map of Europe. We see the various
countries laid down with precision; we can tell the courses of the
rivers; we can say that France is larger than England, and Russia larger
than France; but no matter how perfectly the map be constructed,
something else is necessary before we can have a complete conception of
the dimensions of the country. We must know _the scale on which the map
is drawn_. The map contains a reference line with certain marks upon it.
This line is to give the scale of the map. Its duty is to tell us that
an inch on the map corresponds with so many miles on the actual surface.
Unless it be supplemented by the scale, the map would be quite useless
for many purposes. Suppose that we consulted it in order to choose a
route from London to Vienna, we can see at once the direction to be
taken and the various towns and countries to be traversed; but unless we
refer to the little scale in the corner, the map will not tell how many
miles long the journey is to be.

A map of the solar system can be readily constructed. We can draw on it
the orbits of some of the planets and of their satellites, and we can
include many of the comets. We can assign to the planets and to the
orbits their proper proportions. But to render the map quite efficient
something more is necessary. We must have the scale which is to tell us
how many millions of miles on the heavens correspond to one inch of the
map. It is at this point we encounter a difficulty. There are, however,
several ways of solving the problem, though they are all difficult and
laborious. The most celebrated method (though far from the best) is that
presented on an occasion of the transit of Venus. Herein, then, lies the
importance of this rare event. It is one of the best-known means of
finding the actual scale on which our system is constructed. Observe the
full importance of the problem. Once the scale has been determined, then
all is known. We know the size of the sun; we know his distance; we k