Day and night would each be exactly 12 hours long on a spring or fall equinox only if the sun were a single point of light and Earth had no atmosphere.
如果想要昼夜在春秋分日达到精确地12小时等长，除非太阳是个单点光源或者地球上没有大气层。
If the sun were just a tiny point of light and Earth had no atmosphere, then day and night would each be exactly 12 hours long on a spring equinox day.
如果太阳只发出很小一点的光，并且地球周围不存在大气层，那么在春分这天，昼夜等长，都是12个小时。
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