zenith

zenith in astronomy, the point in the sky directly overhead; more precisely, it is the point at which the celestial sphere is intersected by an upward extension of a plumb line from the observer's location. Its position in the sky thus depends on the direction of the earth's gravitational field at the observer's location. The zenith is a reference point in the altazimuth coordinate system ; its altitude above the celestial horizon is 90°. The angular distance from the zenith to a celestial body is called the zenith distance. The nadir, directly opposite the zenith, has a zenith distance of 180°; the celestial horizon has a zenith distance of 90°.

zenith In astronomy, point on the celestial sphere that is directly overhead. The zenith distance of a heavenly body is the angle it makes with the zenith. It is diametrically opposite the nadir.

zenith The point on the celestial sphere directly above an observer. A line to the zenith is at right angles to the plane of the horizon. The zenith is in the opposite direction from the nadir. Strictly speaking, this definition refers only to the astronomical zenith; two other, slightly different forms of zenith may also be defined. The geocentric zenith is the direction indicated by a line from the centre of the Earth through the observer. The geodetic zenith is at right angles to the geoid at the observer's location. All three forms of zenith differ slightly because of the non-spherical shape of the Earth. If used without qualification, ‘zenith’ means the astronomical zenith.