observer (as through a centre) and
directed towards a certain point in the dome of the heavens. That point,
then, is one whose direction must not only be ascertained, but must be
in some way or other indicated. Whatever the nature of an astronomer's
instruments or observatory, whether he have but a few simple
contrivances in a structure of insignificant proportions, or the most
perfect instruments in a noble edifice of most exquisite construction
and of the utmost attainable stability, he must in every case have the
position of the pole of the heavens clearly indicated in some way or
other. Now, the pole of the heavens is a point lying due north, at a
certain definite elevation above the horizon. Thus the first
consideration to be attended to by the builder of any sort of
astronomical observatory, is the determination of the direction of the
true north (or the laying down of a true north-and-south line), while
the second is the determination, and in some way or other the indication
of the angle of elevation above the north point, at which the true pole
of the heavens may lie.

To get the true north-and-south line, however, the astronomer would be
apt at first, perhaps, rather to make mid-day observations than to
observe the stars at night. It would have been the observation of these
which first called his attention to the existence of a definite point
round which all the stars seem to be carried in parallel circles; but he
would very quickly notice that the sun and the moon, and also the five
planets, are carried round the same polar axis, only differing from the
stars in this: that, besides being thus carried round with the celestial
sphere, they also move upon that sphere, though with a motion which is
very slow compared with that which they derive from the seeming motion
of the sphere itself. Now, among these bodies the sun and moon possess a
distinct advantage over the stars. A body illuminated by either the sun
or the moon throws a shadow, and thus if we place an upright pointed rod
in sunlight or moonlight, and note where the shadow of the point lies,
we know that a straight line from the point to the shadow of the point
is directed exactly towards the sun or the moon, as the case may be.
Leaving the moon aside as in other respects unsuitable, for she only
shines with suitable lustre in one part of each month, we have in the
sun's motions a means of getting the north-and-south line by thus noting
the position of