Meridian (astronomy)

The meridian, or more precisely the celestial meridian (Latin vicious meridianus " lunch group" ) is the one great circle on the celestial sphere, of the North and the south point of the horizon, the northern and southern celestial pole ( extensions of the Earth's rotation axis ) and the zenith ( point above the apex of the observer ) and the nadir ( point under the feet ) includes. The north celestial pole is approximately characterized by the polar star ( at the location of the south celestial pole is no conspicuous star ).

The meridian is perpendicular to the horizon of the observer - another great circle on the celestial sphere - to whom he is a vertical circle. Meridian and horizon, the reference circles in the astronomical coordinate system of the horizon from which the altitude or the azimuth angle of a celestial object is measured. It is with Meridian often only one quarter circle between the zenith and that meant the intersection with the horizon, from which the azimuth measurement is performed ( South Point on the northern hemisphere or northern point in the southern hemisphere ).

In Meridian reach all the fixed stars, the seemingly cross it daily from east to west, its greatest height (upper culmination); twelve hours sidereal time later they reach their lowest height (lower culmination), which is interesting only for circumpolar stars, since the other - like the sun - pass the north below the horizon. The path of the fixed stars is at right angles to the meridian. The generally accepted equation " meridian passage = culmination " but not ( "change the stars ") applies to the sun, moon and planets: Your declination (height above the celestial equator ) is not constant, so is their web is not exactly perpendicular to the meridian, and they rise or fall to a small extent in the meridian passage, so that the culmination slightly east or west of the meridian takes place. ( " Meridian passage is the culmination " then applies only to the moment of constant declination, or about the summer and winter solstice and Hoch-/Tiefstand of the Moon). For the planet that is largely negligible, for the sun is the maximum deviation of a few seconds for the moon but a few minutes. Therefore, the Meridian is not the line of the highest sun.

A change in the longitude of the observer's location, then change the course of the meridian sky, and thus the timing of the meridian passages ( culmination ) in sidereal time. A change in the latitude, the Kulminationshöhen change ( and the location of the celestial poles to the zenith -nadir and north - south).

• Astronomical coordinate system