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.
