Meridian

A great circle on the celestial sphere that passes through the zenith and the north celestial pole. It intersects with the horizon due north and due south. In the old days, north was the direction of the Rat and south was the direction of the Horse, so it is called the meridian. Similarly, there is a great circle called the Rabbit Line that passes due east, due west and the zenith. Due to the diurnal motion of the celestial sphere, a celestial object passes through the meridian from east to west once a day (or more precisely, one sidereal day). This is called the meridian of the celestial object, and its altitude above the horizon is greatest. Telescopes such as meridian instruments, zenith instruments, meridian circles and photographic zenith tubes observe celestial objects at this meridian to determine the positions of the stars, the movements of the planets, the attitude of the Earth relative to the celestial sphere, the time, and the positions of observation points on Earth. This research is called "meridian astronomy."

A meridian on Earth is a great circle, or a longitude line, that passes through the North Pole and the South Pole, and a longitude line that passes through a certain point is called the meridian of that point. The meridian at 0 degrees longitude that passes through the Greenwich Observatory in London is called the prime meridian, and the meridian at 135 degrees east that passes through Akashi (Hyogo Prefecture) in Japan is called the Japanese central meridian.

[Kouichi Nakajima and Masami Ichikawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

天球上で、天頂と天の北極を通る大円をいう。これは、地平線と真北・真南で交わる。昔の呼称で、北は子(ね)の方向、南は午(うま)の方向といっていたので子午線とよばれる。同様に、真東・真西と天頂を通る卯酉線(ぼうゆうせん)という大円もある。天球の日周運動によって、天体は1日（正確には1恒星日）に1回子午線を東から西へ通過するが、これを天体の南中とよび、地平からの高度がもっとも大きくなる。子午儀・天頂儀・子午環・写真天頂筒などの望遠鏡は、この南中時に天体を観測して、恒星の位置や惑星の動き、天球に対する地球の姿勢、時刻、地球上の観測点位置などを決定する。これらの研究を「子午線天文学」とよぶ。

　また地球上での子午線は、北極点と南極点を通る大円すなわち経線のことであり、ある地点を通る経線をその地点の子午線という。ロンドンのグリニジ天文台を通る経度0度の経線を本初子午線、日本の明石(あかし)（兵庫県）を通る東経135度の経線を日本中央子午線と称する。

［中嶋浩一・市川正巳］

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