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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.
In a scientific context, the zenith is the direction of reference for measuring the zenith angle (or zenith angular distance), the angle between a direction of interest (e.g. a star) and the local zenith - that is, the complement of the altitude angle (or elevation angle).
The Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between the Tropic of Cancer and the Tropic of Capricorn. The point where this occurs is known as the subsolar point. In Islamic astronomy, the passing of the Sun over the zenith of Mecca becomes the basis of the qibla observation by shadows twice a year on 27/28 May and 15/16 July.
At a given location during the course of a day, the Sun reaches not only its zenith but also its nadir, at the antipodal point of that location 12 hours from solar noon.
In astronomy, the altitude in the horizontal coordinate system and the zenith angle are complementary angles, with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as a circle on the celestial sphere that passes through the zenith, nadir, and the .
A zenith telescope is a type of telescope designed to point straight up at or near the zenith, and used for precision measurement of star positions, to simplify telescope construction, or both. The NASA Orbital Debris Observatory and the Large Zenith Telescope are both zenith telescopes, since the use of liquid mirrors meant these telescopes could only point straight up.
On the International Space Station, zenith and nadir are used instead of up and down, referring to directions within and around the station, relative to the earth.
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