将观察点的铅垂直线无限延伸后与天球交于两点，向上于天球的交点称为天顶，而正相对向下延伸与天球的交点，成为天底。
莴苣姑娘长成了天底下最美丽的孩子。
想想吧，在白天，在蓝天底下，走过荒原！
To think of going over the moor in the daylight, and when the sky was blue!
如果你询问美国的女大学生，就会知道她们对另一半的期望包含了天底下的一切：除了忠诚之外，伴侣还必须风趣，还要是个朋友。
If you ask American college women, they expect everything under the sun: in addition to being committed, partners have to be amusing, funny and a friend.
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