- published: 28 Aug 2014
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An (astronomical) meridian is the great circle passing through the celestial poles, the zenith, and the nadir of a particular location. Consequently, it contains also the horizon's north and south points, and it is perpendicular to the celestial equator and to the celestial horizon. A celestial meridian matches the projection, onto the celestial sphere, of a terrestrial meridian. Hence there are an infinite number of meridians.
The meridian is undefined when the observer is at the North Pole or South Pole, since at these two points, the zenith and nadir are on the celestial poles, and any great circle passing the celestial poles passes the zenith and nadir.
There are several ways in which the meridian can be divided into semicircles. In one way, it's divided into the local meridian and the antimeridian. The former semicircle contains the zenith and is terminated by the celestial poles; the latter semicircle contains the nadir. In the horizontal coordinate system the meridian is divided into halves terminated by the horizon's north and south points. The upper meridian passes through the zenith, and the lower meridian, through the nadir.
Meridian, or a meridian line may refer to
Astronomy, a natural science, is the study of celestial objects (such as stars, galaxies, planets, moons, asteroids, comets and nebulae) and processes (such as supernovae explosions, gamma ray bursts, and cosmic microwave background radiation), the physics, chemistry, and evolution of such objects and processes, and more generally all phenomena that originate outside the atmosphere of Earth. A related but distinct subject, physical cosmology, is concerned with studying the Universe as a whole.
Astronomy is one of the oldest sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, and Maya performed methodical observations of the night sky. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy and the making of calendars, but professional astronomy is nowadays often considered to be synonymous with astrophysics.
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