通过天球中心（实际是观测者的眼睛）并与铅垂线(天顶与天底联线)相垂直的平面，称为天球的地平面，地平面与天球相交而成的大圆称为地平圈。
半天球被黄道，天赤道或地平圈分成两半的天球的一半…
Either half of the celestial sphere as divided by the ecliptic, the celestial equator, or the horizon.
这对查看一个精确的天体是否超出当前的地平圈是很有用的，而且可以知道要看到这个天体应朝哪一方向看。
This is very useful for seeing whether or not a particular object is currently above the horizon, and for knowing which direction to look in order to see the object.
当然一个低成本但不是相当直接的方法是，夏至时去北极圈旅行，花一晚上的时间(能在热水缸里最好)看地平线以上的太阳圈。
For a cheaper, although not quite as direct, method, journey north of the Arctic circle around the summer solstice and spend a night (ideally in a hot tub) watching the sun circle above the horizon.
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