An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

DOCTRINE OF THE SPHERE. 11
surface, or celestial vault. The great circles of the globe, extended every way to meet the concave surface of the heavens,
become circles of the celestial sphere.
24. The fforizon is the great circle which divides the earth
into upper and lower hemispheres, and separates the visible
heavens from the invisible. Thifs is the rationaal horizon. The
sensible horizon is a circle touching the earth at the place of the
spectator, and is bounded by the line in which the earth and
skies seem to meet. The sensible horizon is parallel to the
rational, but is distant from it by the semi-diameter of the earth,
or nearly 4,000 miles. Still, so vast is the distance of the starry
sphere, that both these planes appear to cut that sphere in the
same line; so that we see the same hemisphere of stars that we
should see if the upper half of the earth were removed, and we
stood on the rational horizon.
25. The poles of the horizon are the zenith and nadir. The
Zenith is the point directly over our head, and the Hladir that
directly under our feet. The plumb line is in the axis of the
horizon, and consequently directed towards its poles.
Every place on the surface of the earth has its own horizon;
and the traveller has a new horizon at every step, always extending 90 degrees from his zenith in all directions.
26. Verticcal circles are those which pass through the poles
of the horizon, perpendicular to it.
The.leridian is that vertical circle which passes through
the north and south points.
The Primze Vertical is that vertical circle which passes
through the east and west points.
27. As in geometry we determine the position of any point
by means of rectangular co-ordinates, or perpendiculars drawn
from the poinlt to planes at right angles to each other, so in
astronomy we ascertain the place of a body, as a fixed star, by
taking its angular distance from two great circles, one of which
is perpendicular to the other. Thus the horizon and the
meridian, or the horizon and the prime vertical, are co-ordinate
circles used for such measurements.