Bouyugi (English spelling) prime vertical instrument

Japanese: 卯酉儀 - ぼうゆうぎ（英語表記）prime vertical instrument

A telescope whose axis of rotation is set horizontally in the north-south direction, allowing observations to be made only in the direction of the Uyou Line. There is no special device; an altazimuth or portable meridian instrument is used with its horizontal axis set north-south. When observing with this, the time when a star with a known declination passes through the Uyou Line is measured, and the latitude of the observation site is determined from that hour angle. When the same star is observed on the eastern and western Uyou Lines, half the difference in the time of passage, expressed in sidereal time, is the hour angle, and because observations are made on both sides of the meridian, any errors in the placement of the telescope can be eliminated.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

望遠鏡の回転軸を南北方向で水平において，卯酉線の方向だけを観測できるようにしたもの。特別なものがあるわけではなく，経緯儀や携帯用の子午儀の水平軸を南北にして用いる。これによる観測では，赤緯のわかっている星が卯酉線を通過する時刻を測定し，その時角から観測地の緯度を決定する。同じ星を東と西の卯酉線で観測すると，それぞれの通過時刻の差の半分を恒星時間で表したものが時角となり，子午線の両側で観測するため望遠鏡の設置誤差が消去できる。

出典　株式会社平凡社世界大百科事典第２版について　情報

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