What is the waning moon?

The Moon doesn't emit its own light: the Sun always illuminates exactly half of its surface, and what changes over the course of the month is how much of that lit half faces Earth. As the orbit brings the daylit face closer to our point of view, the bright area grows and we call it a waxing moon. When the Moon begins turning back toward the Sun's direction, that bright area seems to shrink: that's the waning phase. Nothing actually darkens on the satellite; only our perspective changes.

NASA divides the cycle into eight phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent. The last three are the waning ones. In waning gibbous, more than half the disk is lit, but already less than at full moon; at last quarter exactly half is visible; in waning crescent only a sliver of light remains, growing thinner until it disappears at new moon. The full cycle, from new moon to new moon, lasts 29.5 days, while the Moon takes 27.3 days to orbit Earth once.

Two practical clues. The first is timing: the waning Moon rises later and later each night and is mostly visible in the second half of the night and early morning, while the waxing Moon dominates the evening sky. The second is the orientation of the lit arc, which in the northern hemisphere points one way during waxing and the other during waning, and which reverses if observed from the southern hemisphere. That reversal is why mnemonic rules learned in Spain don't work in Chile.