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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 celestial poles.
Diagram showing the relationship between the zenith, the nadir, and different types of horizon.Note that the zenith is opposite the nadir. The nadir [a] [b] is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface.
In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location. Consequently, it contains also the north and south points on the horizon, and it is perpendicular to the celestial equator and horizon.
Zenith, nadir: Altitude (a) or elevation Azimuth (A) North or south point of horizon Equatorial: Center of the Earth (geocentric), or Sun (heliocentric) Celestial equator: Celestial poles: Declination (δ) Right ascension (α) or hour angle (h) March equinox: Ecliptic: Ecliptic: Ecliptic poles: Ecliptic latitude (β) Ecliptic longitude (λ ...
The geometric center of the Earth, i.e. the arithmetic mean position of all points within the oblate spheroid that is the precise shape of the Earth. geocentric With reference to, or pertaining to, the geometric center of the Earth; [14] centered upon the Earth, e.g. a geocentric orbit. geocentric zenith
The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles of a spherical coordinate system: altitude and azimuth. Therefore, the horizontal coordinate system is sometimes called the az/el system, [1] the alt/az system, or the alt-azimuth system, among ...
Zenith: the (direction to the) intersection of the upwards-extending gravity vector at a point and the celestial sphere. Nadir: the (direction to the) antipodal point where the downward-extending gravity vector intersects the (obscured) celestial sphere. Celestial horizon: a plane perpendicular to the gravity vector at a point.
Elevation – along a curve from a point on the horizon to the zenith, directly overhead. Depression – along a curve from a point on the horizon to the nadir, directly below. Vertical – spanning the height of a body. Longitudinal – spanning the length of a body. Lateral – spanning the width of a body. The distinction between width and ...