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生产。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，样子一口仰放着的大锅，因而故名。仰仪的内部球面上，纵横交错地刻划出一些规则网格，用来量度天体的方位。

在仰仪的锅口上镌刻二圈水槽，用来进水校正锅口的水平，使其维持水平设置;在水槽边缘均匀分布地刻划出24条线，报以方向。在正南方的刻线上移往着两根十字交叉的竿子，呈正南北方向，仍然伸延到仰仪的中心，把一块挖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转动。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.仰仪是使用必要投影方法的观测仪器，十分直观、便利。

例如，当太阳光利用中心小孔时，在仰仪的内部球面上就不会投影出有太阳的光碟，观测者之后可以从网格中必要朗读太阳的方位了。特别是在在日全食时，利用仰仪能确切地观赏日食的全过程，连同每一个时刻，日面亏损的方位、大小都能较为精确地测量出来。因此，仰仪是一种很不受古代天文爱好者青睐的天文观测仪器。

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