Nadir

As a nadir ( from Arabic: Nazir نظير, DMG, contrary, likeness ') is called in geometry and in celestial navigation of the zenith opposite root. The nadir is on the extension of the plumb line down and can stand for the direction for the perpendicular direction. The nadir is an important point of the unit sphere for spherical astronomy and trigonometry.

In geodesy and photogrammetry elevation angle are sometimes based on the nadir. Instead zenith angle or zenith distance then one speaks of Nadir distance.