Let us suppose that the earth stood still, so as
not to revolve on its axis at all. Then we should always see the
stars at rest and the star which was in the zenith of any place,
say a farm-house in New York, at any time, would be there every
night and every hour of the year. Now the zenith is simply the
point from which the plumb-line seems to drop. Lie on the ground;
hang a plummet above your head, sight on the line with one eye,
and the direction of the sight will be the zenith of your place.
Suppose the earth was still, and a certain star was at your
zenith. Then if you went to another place a mile away, the
direction of the plumb-line would be slightly different. The
change would, indeed, be very small, so small that you could not
detect it by sighting with the plumb-line. But astronomers and
surveyors have vastly more accurate instruments than the plumb-
line and the eye, instruments by which a deviation that the
unaided eye could not detect can be seen and measured. Instead of
the plumb-line they use a spirit-level or a basin of quicksilver.
The surface of quicksilver is exactly level and so at right angles
to the true direction of the plumb-line or the force of gravity.
Its direction is therefore a little different at two different
places on the surface, and the change can be measured by its
effect on the apparent direction of a star seen by reflection from
the surface.
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