The Probert Encyclopaedia of Science & Technology

NADIR

The nadir is the point on the celestial sphere directly below an observer and diametrically opposite the zenith. It is 90 degrees away from the horizon and is approximately the direction in which a plumb-bob hangs. Because the Earth is not spherical the direction of the plumb-bob nadir would not in general pass through the Earth's centre. Furthermore, the plumb-bob can be deviated slightly by large local masses such as mountains so that for precise astronomical measurements corrections have to be made to clarify which nadir is being used.
Research Nadir