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測量： 星過中天 meridian transit . Ulugh beg的地下天文台同功能。


窺幾之制，長六尺，廣二尺，高倍之。下為趺，廣三寸，厚二寸，上跂廣四寸，厚如趺。以板為面，厚及寸，四隅為足，撐以斜木，務取正方。面中開明竅，長四尺，廣二寸。近竅兩旁一寸分畫為尺，內三寸刻為細分，下應圭面。几面上至梁心二十六尺，取以為準。窺限各各長二尺四寸，廣二寸，脊厚五分，兩刃斜閷，取其於幾面相符，着限兩端，厚廣各存二寸，銜入幾跂。俟星月正中，從幾下仰望，視表梁南北以為識，折取分寸中數，用為直景。又於遠方同日窺測取景數，以推星月高下也。

可視為現代重要光學測量optical astrometry 儀器eg carlsberg meridian telescope 的雛形。可惜當年還沒有改良發展下去。




Reference :

Baidu

Carlsberg meridian telescope wiki

Meridian circle 

Ulugh beg observatory 

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