Meridian (astronomy)
The meridian or, more precisely, the celestial meridian ( Latin circulus meridianus 'midday circle ') is the great circle on the celestial sphere on which the south and north points are on the horizon , zenith and nadir and the two celestial poles .
The meridian is perpendicular to the horizon of the observer - another great circle on the celestial sphere - on the other hand it is a vertical circle . Meridian and horizon are the reference circles in the astronomical coordinate system of the horizon, from which the height or azimuth angle of a celestial object is measured. The meridian often only refers to the quarter circle between the zenith and the point of intersection with the horizon from which the azimuth measurement is made (south point on the northern hemisphere of the earth or north point on the southern hemisphere).
The meridian can also be defined as the connection of the points of the celestial sphere in which the stars, which seem to cross it daily from east to west, reach their greatest height (upper culmination ); twelve hours sidereal time later they reach their lowest height (lower culmination). This can only be observed for circumpolar stars , since other stars - like the sun - pass the north below the horizon. The orbit of the fixed stars is at right angles to the meridian.
The common equation "meridian passage = culmination" does not apply to the sun, moon and planets (" changing stars "): Their declination (height above the celestial equator) is not constant, so their orbit is not exactly at right angles on the meridian, and they rise or fall slightly when crossing the meridian, so that the culmination takes place a little east or west of the meridian. ("Meridian passage is culmination" then only applies to the moment of constant declination, i.e. about summer and winter solstice and high / low level of the moon). For the planets this is largely negligible, for the sun the maximum deviation is a few seconds, for the moon it is a few minutes. Therefore the meridian is not the line of the highest position of the sun.
If the longitude of the observation location changes, the course of the celestial meridian and thus the times of meridian crossings (culminations) in sidereal time also change . If the geographical latitude changes, the culmination heights change (and the position of the celestial poles to zenith – nadir and north – south).
literature
- Joachim Herrmann : dtv - Atlas for Astronomy , Munich 1973
- Hans-Ulrich Keller : Astrowissen , Stuttgart 1994