astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, the meridian is the great circle passing through the celestial poles, as well as the

zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...

and nadir of an observer's location. Consequently, it contains also the

north North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating Direction (geometry), direction or geography. Etymology T ...

and south points on the

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

, and it is

perpendicular In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...

to the celestial equator and horizon. Meridians, celestial and geographical, are determined by the pencil of planes passing through the Earth's rotation axis. For a location ''not'' on this axis, there is a unique meridian plane in this axial-pencil through that location. The intersection of this plane with Earth's surface defines two '' geographical meridians'' (either one east and one west of the prime meridian, or else the prime meridian itself and its anti-meridian), and the intersection of the plane with the

celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

is the celestial meridian for that location and time. There are several ways to divide the meridian into semicircles. In one approach, the observer's upper meridian extends from a celestial pole and passes through the zenith to contact the opposite pole, while the lower meridian passes through the nadir to contact both poles at the opposite ends. In another approach known as the

horizontal coordinate system 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 coord ...

, the meridian is divided into the local meridian, the semicircle that contains the observer's zenith and the north and south points of their horizon, and the opposite semicircle, which contains the nadir and the north and south points of their horizon. On any given (sidereal) day/night, a celestial object will appear to drift across, or transit, the observer's upper meridian as Earth rotates, since the meridian is fixed to the local horizon. At culmination, the object contacts the upper meridian and reaches its highest point in the sky. An object's

right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...

and the local sidereal time can be used to determine the time of its culmination (see hour angle). The term ''meridian'' comes from the Latin ''meridies'', which means both "midday" and "south", as the celestial equator appears to tilt southward from the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

References

*Millar, William (2006). ''The Amateur Astronomer's Introduction to the Celestial Sphere''. Cambridge University Press. *Local Horizon and Meridian. (''A Modern Almagest - An Updated Version of Ptolemy’s Model of the Solar System'' by Richard Fitzpatrick, Professor of Physics, The University of Texas at Austin) {{Authority control Astronomical coordinate systems Circles

See also

References