ed hopeless, for here we are
tied to the earth and we cannot get away into space. But the astronomer
was nothing daunted. He knew that in its journey round the sun the earth
travels in an orbit which measures about one hundred and eighty-five
millions of miles across, so he resolved to take observations of the
stars when the earth was at one side of this great circle, and again,
six months later, when she had travelled to the other side. Then indeed
he would have a magnificent base line, one of one hundred and
eighty-five millions of miles in length. What was the result? Even with
this mighty line the stars are found to be so distant that many do not
move at all, not even when measured with the finest instruments, and
others move, it may be, the breadth of a hair at a distance of several
feet! But even this delicate measure, a hair's-breadth, tells its own
tale; it lays down a limit of twenty-five billion miles within which no
star can lie!

This system which I have explained to you is called finding the star's
parallax, and perhaps it is easier to understand when we put it the
other way round and say that the hair's-breadth is what the whole orbit
of the earth would appear to have shrunk to if it were seen from the
distance of these stars!

Many, many stars have now been examined, and of them all our nearest
neighbour seems to be a bright star seen in the Southern Hemisphere. It
is in the constellation or star group called Centaurus, and is the
brightest star in it. In order to designate the stars when it is
necessary to refer to them, astronomers have invented a system. To only
the very brightest are proper names attached; others are noted according
to the degree of their brightness, and called after the letters of the
Greek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet'
comes, you know, from the first two letters of this Greek series. As
this particular star is the brightest in the constellation Centaurus, it
is called Alpha Centauri; and if ever you travel into the Southern
Hemisphere and see it, you may greet it as our nearest neighbour in the
starry universe, so far as we know at present.

CHAPTER XI

THE CONSTELLATIONS

From the very earliest times men have watched the stars, felt their
mysterious influence, tried to discover what they were, and noted their
rising and setting. They classified them into groups, called
constellations, and gave such groups the names of figures and an