measure not only exceeding any terrestrial standard, but even any
distance in the solar system. For purely astronomical purposes the
most convenient unit is the distance corresponding to a parallax
of 1", which is a little more than 200,000 times the sun's
distance. But for the purposes of all but the professional
astronomer the most convenient unit will be the light-year--that
is, the distance through which light would travel in one year.
This is equal to the product of 186,000 miles, the distance
travelled in one second, by 31,558,000, the number of seconds in a
year. The reader who chooses to do so may perform the
multiplication for himself. The product will amount to about
63,000 times the distance of the sun.

[Illustration with caption: A Typical Star Cluster--Centauri]

The nearest star whose distance we know, Alpha Centauri, is
distant from us more than four light-years. In all likelihood
this is really the nearest star, and it is not at all probable
that any other star lies within six light-years. Moreover, if we
were transported to this star the probability seems to be that the
sun would now be the nearest star to us. Flying to any other of
the stars whose parallax has been measured, we should probably
find that the average of the six or eight nearest stars around us
ranges somewhere between five and seven light-years. We may, in a
certain sense, call eight light-years a star-distance, meaning by
this term the average of the nearest distances from one star to
the surrounding ones.

To put the result of measures of parallax into another form, let
us suppose, described around our sun as a centre, a system of