An ancient astronomic instrument for observation, the projection semi-sphere was a creation by Guo Shoujing, an astronomer of the Yuan Dynasty (1271-1368).

The main body of the projection sphere is a bronze semi-sphere with a diameter 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.

Above the reticle (network of lines) in the south direction is installed a cross, which extends along the south-north direction until it reaches the center of the instrument, where a small wooden plate with a hole in the center is placed at the end of the cross..

Employing projection, the instrument is simple and convenient for observation. For example, when the sunlight 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. Particularly, 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 eclipse at different times can also be measured. Therefore, the projection semi-sphere was greatly loved by astronomy lovers.

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Note added at 10分钟 (2008-06-28 11:32:03 GMT)
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中文介绍请看这里：

http://www.hongen.com/art/twdg/gtwyq/tg0011.htm