日食、月食都是光的直线传播产生的现象．地球绕着太阳运转一周的时间为一年，月球绕着地球运转一周的时间需一个月．月球本身不发光，我们看见的月光是由月球反射到地球的太阳光．当月亮运行到太阳和地球中间成直线的时侯，遮住射到地球上的太阳光，这种现象叫做日食．如图１ Solar eclipses and lunar eclipses are the phenomenon of linear propagation of light. The time for the earth to orbit the sun is one year. The time it takes for the moon to orbit the earth is one month. The moon itself does not glow. The moonlight we see is the sun reflected by the moon to the earth. When the moon runs into a straight line between the sun and the earth, the sun shines on the earth. This phenomenon is called solar eclipse. Figure 1