e us from
the sun in about eight minutes. It travels from the sun out to Neptune
in about four hours, which means that it would cross the solar system
from end to end in eight. To pass, however, across the distance which
separates us from Alpha Centauri it would take so long as about four
and a quarter years!

Astronomers, therefore, agree in estimating the distances of the stars
from the point of view of the time which light would take to pass from
them to our earth. They speak of that distance which light takes a year
to traverse as a "light year." According to this notation, Alpha
Centauri is spoken of as being about four and a quarter light years
distant from us.

Now as the rays of light coming from Alpha Centauri to us are chasing
one another incessantly across the gulf of space, and as each ray left
that star some four years before it reaches us, our view of the star
itself must therefore be always some four years old. Were then this star
to be suddenly removed from the universe at any moment, we should
continue to see it still in its place in the sky for some four years
more, after which it would suddenly disappear. The rays which had
already started upon their journey towards our earth must indeed
continue travelling, and reaching us in their turn until the last one
had arrived; after which no more would come.

We have drawn attention to Alpha Centauri as the nearest of the stars.
The majority of the others indeed are ever so much farther. We can only
hazard a guess at the time it takes for the rays from many of them to
reach our globe. Suppose, for instance, we see a sudden change in the
light of any of these remote stars, we are inclined to ask ourselves
when that change did actually occur. Was it in the days of Queen
Elizabeth, or at the time of the Norman Conquest; or was it when Rome
was at the height of her glory, or perhaps ages before that when the
Pyramids of Egypt were being built? Even the last of these suppositions
cannot be treated lightly. We have indeed no real knowledge of the
distance from us of those stars which our giant telescopes have brought
into view out of the depths of the celestial spaces.

CHAPTER VI

CELESTIAL MEASUREMENT

Had the telescope never been invented our knowledge of astronomy would
be trifling indeed.

Prior to the year 1610, when Galileo first turned the new instrument
upon the sky, all that men knew of the starry realms was gathered from
observation wit