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The Water一driven Armillary Sphere
漏水转浑天仪
In the middle of the Eastern Han Dynasty, Zhang Heng，an outstanding astronomer, made bold innovations to the armillary sphere，and de-signed and made the first water-driven armillary sphere in the world to measure the position of celestial bodies，which was carved with known important astronomical phenomena. People could observe the movement of the sun，the moon and the stars.
汉中期，杰出的天文学家张衡对浑仪作了大胆创新，设计并制作了漏水转浑天仪。张衡还制造出了世界上第一架测量天体位置的水运浑天仪，凡是已知的重要天文现象，都刻在这架仪器上。人们可以通过浑天仪观测到日月星辰运行的情况。
To enable the armillary sphere to rotate，Zhang employed the gear system，linking the celestial globe with the kettle clepsydra. Powered by the water leaked out of the kettle，the gear drove the celestial globe to rotate around the axis，one day per circuit, in synchronization with the celestial body(sun).In this way，the celestial changes were presented accurately.By observing the instrument，one could know when a star rose in the east and set in the west.
漏水转浑天仪的主体是一个代表天球的球体模型。球里面有一根铁轴贯穿球心，轴的方向就是天球的方向，也是地球自转轴的方向。轴和球有两个交点，一个是北极(北天极)，一个是南极(南天极)。为了让浑天仪能自己转动，张衡采用齿轮系统把浑象和记时用的漏壶联系起来，用漏壶滴出来的水的力量带动齿轮，齿轮带动浑象绕轴旋转，一天一周，与天球同步转动，这样，就可以准确地把天象的变化表示出来。人在屋子里看着仪器，就可以知道某星正从东方升起，某星就要从西方落下。
The world’s first water-driven astronomic instrument with explicit historical re-cords，the new armillary sphere employed an extremely complex gear system. It was a proud achievement for ancient Chinese people to work out such a complex instrument. Unfortunately，the driving system was not handed down，as made in the remote past.
漏水转浑天仪是有明确历史记载的世界上第一架用水力驱动的天文仪器。浑天仪应用到的齿轮机构和凸轮机构十分复杂。远在1800多年前，中国古人就可以造出这样复杂的仪器是很值得自豪的。可惜的是，这套复杂的传动系统因为年代久远没有能够流传下来。
