The horizontal circle which separates the part of the sky
visible to you and the part of the sky hidden by the earth.
The point on the sky directly overhead.
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.
The points where the extension of the rotation axis
of the earth would intersect the celestial sphere.
The circle around the sky which would be a
projection of the earths equator.