An imaginary sphere of infinite radius centered on the Earth and poles aligned with the poles of the Earth.
A point on the celestial sphere is really only a direction in space. Parallel lines meet the celestial sphere at the same point.
The point on the celestial sphere 180° from the sun.
The position on the celestial sphere at which a heavenly body would be seen from the centre of the earth at a particular time.
The position of a heavenly body (or space vehicle) on the celestial sphere corrected for aberration but not for planetary aberration.
See also: Nadir.