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.