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天球座标系统中,位于观测者正下方,与天顶相对的点称为地底(Nadir)。在地平座标系统中,此点的高度为-90度。 Nadir或有称为“天底”,但以“地底”一词更能表达在天顶的对面位置,在地平面下方的底部的意味。

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