Why does the moon wax and wane?
These are just terms that we use to describe the moon’s appearance as its month-long lunar day unfolds for us. The phases of the moon are simply the way the moon appears to us as we watch one lunar day pass. As we look at the moon, we can describe its phase changes this way. When the moon is new and before it reaches half-lit, it is ‘waxing crescent’; it’s getting bigger, and it’s still shaped like a crescent. After the half-illumination point, it is waxing gibbous; it’s getting bigger, and it’s beyond the crescent stage. After the full moon and before it reaches half-lit, it is waning gibbous, and after that it is waning crescent.
