سرسو

thumb|upright=1.25|right|Diagram showing the relationship between the zenith, the nadir, and different types of [[horizon]] The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" point on the celestial sphere. The direction opposite of the zenith is the nadir.