極夜,也叫永夜是勒地球個兩極地區,一日天裏向太陽儕勒地平線下頭個現象。
太陽直射點勒囖個半球,另外箇個半球個極地附近就會出現極夜現象。
極夜個範圍搭太陽直射點緯度搭界,極點搭俚個邊界交差個緯度就是太陽直射點個緯度。
春分過後,南極附近就會出現極夜,後頭極夜範圍越來越大;到夏至達到頂大,邊界碰著南極圈;過脫夏至,南極附近極夜範圍逐漸縮小,秋分辰光縮到0;秋分過後,北極附近出現極夜,後頭北極附近個極夜範圍越來越大;到冬至達到頂大,邊界碰著北極圈;過脫冬至,北極附近極夜範圍逐漸縮小,春分辰光縮到0。必過因為太陽折射,冬天極夜個範圍弗會完全達到北極圈或者南極圈。
該浪弗斷重複,周期是一個回歸年。
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