as before, and will proceed to a focus at the
same precise point as before. As, however, the telescope has moved, it
will, of course, have carried with it the pair of cross wires; they will
no longer be at the same point as at first, and consequently the image
of the star will not now coincide with their intersection.

The movement of the telescope arises from its connection with the earth:
for as the earth hurries along at a speed of eighteen miles a second,
the telescope is necessarily displaced with this velocity. It might at
first be thought, that in the incredibly small fraction of time
necessary for light to pass from the object-glass to the eye-piece, the
change in the position of the telescope must be too minute to be
appreciable. Let us suppose, for instance, that the star is situated
near the pole of the ecliptic, then the telescope will be conveyed by
the earth's motion in a direction perpendicular to its length. If the
tube of the instrument be about twenty feet long, it can be readily
demonstrated that during the time the light travels down the tube the
movement of the earth will convey the telescope through a distance of
about one-fortieth of an inch.[42] This is a quantity very distinctly
measurable with the magnifying power of the eye-piece, and hence this
derangement of the star's place is very appreciable. It therefore
follows that if we wish the star to be shown at the centre of the
instrument, the telescope is not to be pointed directly at the star, as
it would have to be were the earth at rest, but the telescope must be
pointed a little in advance of the star's true position; and as we
determine the apparent place of the star by the direction in which the
telescope is pointed, it follows that the apparent place of the star is
altered by the motion of the earth.

Every circumstance of the change in the star's place admits of complete
explanation in this manner. Take, for instance, the small circular path
which each star appears to describe. We shall, for simplicity, refer
only to a star at the pole of the ecliptic. Suppose that the telescope
is pointed truly to the place of the star, then, as we have shown, the
image of the star will be at a distance of one-fortieth of an inch from
the cross wires. This distance will remain constant, but each night the
direction of the star from the cross wires will change, so that in the
course of the year it completes a circle, and returns to its original
positio