The pursuit on subdivision precision Length: Diagonal scale (Gunter rule) Widely used on cartography and maps Vernier scale Angles: Cross staff Astrolabe Tycho Brahe's armillary sphere and quadrant Idea from Levi ben Gerson 10” accuracy Nonius scale Mariner's quadrant original nonius scale I guess in practice, the observer just read off the angle as 900/87 degrees, and later on convert it to DMS. 元史四海測驗 由晷影轉換成角度.北宋 sub degree 用 半 = 1/2 小 = 1/3 強 = 2/3 半小= 1/6？ 半強=5/6？ 太強=？ 太= 3/4=6/8？ 太弱=5/8？ 渾天儀: 0.1古度, linear 沒有用“magnification of scale" Cf: 郭守敬把數據精確到1/20度（how?)，晷影轉換到角， 時的角度單位？averaging? 永樂大典(失傳？），分度矩（估計是舶來品）： WeChat Reference: Curiosite geometrique internet archive 元史 四海 元史天文志疏漏 163 Astronomiae Instauratae mechanica Tycho Brahe Pansci Chapman , YouTube Dividing circle by Henry Hindley Wiki James watt micrometer