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The Projection Semi-sphere
仰仪
An ancient astronomic instrument for observation，the projection semi-sphere was a creation by Guo Shoujing，an astronomer of the Yuan Dy-nasty.
仰仪是中国古代的一种天文观测仪器，由元朝天文学家郭守敬设计制造。
The main body of the projection sphere is a bronze semi-sphere with a diame-ter of three meters，like an upturned pot，hence its name“Yang Yi”(Upturned Sphere ).Inside the spherical surface，there are drawings of many orderly grids that are used for measuring the position of celestial bodies. Along the pot’s mouth is carved a groove，which is used to contain water to see whether the sphere is in a horizontal position. At the edge of the groove are orderly carved 24 lines，indica-ting directions. Above the reticle(network of lines)in the south direction is in-stalled a cross，which extends along the south-north direction until it reaches they center of the instrument，where a small wooden plate with a hole in the center is placed at the end of the cross，evolving around the center of the projection semi-sphere.
仰仪的主体是一只直径约三米的铜质半球面，好像一口仰放着的大锅，因而得名。仰仪的内部球面上，纵横交错地刻划出一些规则网格，用来量度天体的位置。在仰仪的锅口上刻有二圈水槽，用来注水校正锅口的水平，使其保持水平设置;在水槽边缘均匀地刻划出24条线，以示方向。在正南方的刻线上安置着两根十字交叉的竿子，呈正南北方向，一直延伸到仰仪的中心，把一块凿有中心小孔的小方板装在竿子的北端，并且小方板可以绕着仰仪中心旋转。
Employing projection，the instrument is simple and convenient for observation.For example，when the sun-light goes through the hole，it will project the image of the sun onto the internal spherical surface.Then the observer can read the sun’s position from the grids. Par-ticularly，during a total solar eclipse，the whole process of the solar eclipse can be observed with the instrument. Moreover, both the exact position and size of the e-clipse at different times can also be measured.Therefore，the projection semi-sphere was greatly loved by astronomy lovers.
仰仪是采用直接投影方法的观测仪器，非常直观、方便。例如，当太阳光透过中心小孔时，在仰仪的内部球面上就会投影出太阳的映像，观测者便可以从网格中直接读出太阳的位置了。尤其在日全食时，利用仰仪能清楚地观看日食的全过程，连同每一个时刻，日面亏损的位置、大小都能比较准确地测量出来。因此，仰仪是一种很受古代天文爱好者喜爱的天文观测仪器。
