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meridian altitude
Meridian Altitude is an astronomical navigation method of calculating an observer's latitude.
== Principle ==
This is the simplest calculation of astronomical navigation and is when an observer determines his latitude by measuring the altitude of a heavenly object at the time of its meridian passage. Meridian passage is when the object passes the observer's meridian, i.e. passes through south or north. This is usually done with the sun for a noon latitude but can be done with any heavenly object. Noon is when the sun passes through the meridian.
Let us imagine that the sun is overhead (at the zenith) at a point on the equator (latitude 0°) and observer A is standing at this point, the geographical point of the sun. If he were to measure the height of the sun above the horizon with a sextant he would find that the altitude of the sun was 90°. By subtracting this figure from 90° he would find the zenith distance of the sun which is 0°, which is the same as his latitude. Observer B is standing at one of the poles (latitude 90°N or 90°S) he would see the sun on the horizon at an altitude of 0°. By subtracting this from 90° he would find that the zenith distance is 90°, which is ''his'' latitude. Observer C at the same time is at latitude 20°N on the same meridian, i.e. on the same longitude as observer A. His measured altitude would be 70° and by subtracting this from 90° gives a 20° zenith distance which in turn is ''his'' latitude. In short, the zenith distance of a heavenly object at meridian altitude is the difference in latitude between it and the observer.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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