The
nadir (from /
ALA-LC:
naẓīr, meaning "opposite") is the direction pointing directly
below a particular location; that is, it is one of two
vertical directions at a specified location,
orthogonal to a horizontal flat surface there. Since the concept of
being below is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in
astronomy,
geophysics and related sciences (e.g.,
meteorology), the nadir at a given point is the local vertical direction pointing in the direction of the force of
gravity at that location. The direction opposite of the nadir is the
zenith.