Nadir n. : That point of the heavens, or lower hemisphere, directly opposite the zenith; the inferior pole of the horizon; the point of the celestial sphere directly under the place where we stand.

Nadir n. : The lowest point; the time of greatest depression.

Example:
What is the point directly Nadir in the sky?

Taxonomy
Nadir definition, free encyclopedia, Zenith, diagram, geometry, lowest point, 04-23, search, free dictionary, relationship, nathir, one of two, flat surface, scientists, sciences, the force, force of gravity, satellite.

Defination from the web:
nadir (from Arabic نظير nathir, "opposite") is the direction pointing directly below a particular location; that is, it is one of two ... Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms. ...