雙語科技百科(天文) 第12期:仰儀
The Projection Semi-sphere
An ancient astronomic instrument for observation，the projection semi-sphere was a creation by Guo Shoujing，an astronomer of the Yuan Dy-nasty.
仰儀是中國古代的一種天文觀測儀器，由元朝天文學家郭守敬設計製造。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 )de 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.
仰儀的主體是一隻直徑約三米的銅質半球面，好像一口仰放着的大鍋，因而得名。仰儀的內部球面上，縱橫交錯地刻劃出一些規則網格，用來量度天體的位置。在仰儀的鍋口上刻有二圈水槽，用來注水校正鍋口的水平，使其保持水平設置;在水槽邊緣均勻地刻劃出24條線，以示方向。在正南方的刻線上安置着兩根十字交叉的竿子，呈正南北方向，一直延伸到仰儀的中心，把一塊鑿有中心小孔的小方板裝在竿子的北端，並且小方板可以繞着仰儀中心旋轉。
Employing projection，the instrument is simple and convenient for example，when the sun-light goes through the hole，it will project the image of the sun onto the internal spherical 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 efore，the projection semi-sphere was greatly loved by astronomy lovers.
仰儀是採用直接投影方法的觀測儀器，非常直觀、方便。例如，當太陽光透過中心小孔時，在仰儀的內部球面上就會投影出太陽的映像，觀測者便可以從網格中直接讀出太陽的位置了。尤其在日全食時，利用仰儀能清楚地觀看日食的全過程，連同每一個時刻，日面虧損的位置、大小都能比較準確地測量出來。因此，仰儀是一種很受古代天文愛好者喜愛的天文觀測儀器。
