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[5] [6] At a given location during the course of a day, the Sun reaches not only its zenith but also its nadir , at the antipode 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.
Azimuth is measured eastward from the north point (sometimes from the south point) of the horizon; altitude is the angle above the horizon. 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.
The rate of decrease of temperature with elevation is known as the adiabatic lapse rate, which is approximately 9.8 °C per kilometer (or 5.4 °F [3.0 °C] per 1000 feet) of altitude. [ 12 ] The presence of water in the atmosphere complicates the process of convection.
The angle of a celestial body with the zenith is the zenith angle (in astronomy, commonly referred to as the zenith distance).A body's angular position can also be given in terms of altitude, the angle above the geometric horizon; the altitude and the zenith angle are thus related by =.
ICAO further defines: elevation: "the vertical distance of a point or a level, on or affixed to the surface of the earth, measured from mean sea level." [2] I.e., elevation would be the altitude of the ground or a building.
It is the complement to the solar altitude or solar elevation, which is the altitude angle or elevation angle between the sun’s rays and a horizontal plane. [1] [2] At solar noon, the zenith angle is at a minimum and is equal to latitude minus solar declination angle. This is the basis by which ancient mariners navigated the oceans. [3]
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Even with these restrictions, if the polar angle (inclination) is 0° or 180°—elevation is −90° or +90°—then the azimuth angle is arbitrary; and if r is zero, both azimuth and polar angles are arbitrary. To define the coordinates as unique, the user can assert the convention that (in these cases) the arbitrary coordinates are set to zero.