在农历十五、十六，月亮运行到和太阳相对的方向。这时如果地球和月亮的中心大致在同一条直线上，月亮就会进入地球的本影，而产生月全食。如果只有部分月亮进入地球的本影，就产生月偏食。当月球进入地球的半影时，应该是半影食，但由于它的亮度减弱得很少，不易察觉，故不称为月食，所以月食只有月全食和月偏食两种。
月食都发生在望（满月），但不是每逢望都有月食，这和每逢朔不都出现日食是同样的道理。在一般情况下，月亮不是从地球本影的上方通过，就是在下方离去，很少穿过或部分通过地球本影，因此，一般情况下就不会发生月食。每年月食最多发生3次，有时一次也不发生。
The Moon is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own but shines by sunlight reflected from its surface. The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases:New, New Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Last Quarter, Old Crescent and back to New again.
The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month. Did you realize that the word month is derived from the Moon's 29.5 day period?
To many of us, Full Moon is the phase of love and romance. When the Moon is Full, it rises at sunset and is visible all night long. At the end of the night, the Full Moon sets just as the Sun rises. None of the Moon's other phases have this unique characteristic. It happens because the Moon is directly opposite the Sun in the sky when the Moon is Full. Full Moon also has special significance with regard to eclipses.
An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of the Earth's shadow. The shadow is actually composed of two cone-shaped components, one nested inside the other. The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.
月食的过程分为初亏、食既、食甚、生光、复圆五个阶段。 ＊初亏：月球刚接触地球本影，标志月食开始。 ＊食既：月球西边缘与地球本影的西边缘内切，月球刚好全部进入地球本影内。 ＊食甚：月球的中心与地球本影的中心最近。 ＊生光：月球东边缘与地球本影东边缘相内切，这时全食阶段结束。 ＊复圆：月球的西边缘与地球本影东边缘相外切，这时月食全过程结束。
月球被食的程度叫“食分”，它等于食甚时月轮边缘深入地球本影最远距离与月球视经之比。
