The moon's position around the Earth and the sun determines the phases
As you may have noticed, unless the sky is really clouded, there was a full moon last night and it's going to be one tonight as well.It's the 13th full moon of the year, something that happens rather infrequently, when the first full moon is very early in the year and the last full moon is very late in the year.
But, for something universally observed, plenty of people still don't understand why the phases of the moon happen. One common misconception is that the phases of the moon are caused by the shadow of the Earth.
In fact, it has to do with the position of the moon, the Earth and the sun. As the moon revolves around Earth, different parts are illuminated by the sun.
While we always see the same side of the moon, not all of what we see is lit up by the sun, depending on where the moon is around the Earth.