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有些老师来信问:为什么日食常发生在初一,月食常发生在十五?为什么并不是每一个初一、十五都发生日食、月食? 要理解这个问题,我们必须了解日,地、月三者的运动情况及相互位置关系。 我们都知道,地球围着太阳转,每年转一圈,月亮绕着地球转,每月(阴历)转一圈,运动的轨迹都是椭圆。太阳在地球椭圆轨道的一个焦点上,地球在月亮楠圆轨道的一个焦点上。显然,有可能出现日、地、月在同一直线上的情况。而这种情况又有两种位置排列情形。
Some teachers wrote a letter asking why solar eclipses usually occur on the first day and eclipse often occurs on the fifteen? Why is not every first and fifteen solar eclipses and eclipses occur? To understand this issue, we must understand that day, Ground and the three movements and mutual positional relationship. We all know that the earth rotates around the sun and turns around once a year. The moon turns around the earth and every month (lunar calendar) turns around, and its trajectory is an ellipse. The sun is at a focus of the Earth elliptical orbit, the Earth is at a focal point of the moon's orbicular orbit. Obviously, there may be cases where the days, places and months are on the same line. And this situation has two kinds of position arrangement situation again.