HORIZON: The
horizontal circle which separates the part of the sky visible to you and the part of the sky hidden by the
earth.
ZENITH: The point on
the sky directly overhead.
MERIDIAN: The circle
which starts on the northern horizon, runs through the zenith, continuing on to the southern
horizon. It separates the eastern half of the sky from the western half.
CELESTIAL
POLES: The points where the extension of the
rotation axis of the earth
would intersect the celestial sphere.
CELESTIAL
EQUATOR: The circle around the sky which would be a projection of the earths equator.