Phases happen because only the side of Venus (or of the Moon) facing the Sun is lighted. The phases of Venus supported the theory of Copernicus that the planets go around the Sun.
相变的发生是因为金星（或月球）只有面向太阳的一边被照到。金星的相变支持了哥白尼的理论，即行星围绕太阳旋转。