The document discusses the phases of the moon. It explains that the moon does not produce its own light, but rather reflects light from the sun. As the moon orbits Earth, different portions are illuminated by the sun, appearing to change shape from our perspective on Earth. This cycle, where the illuminated portion appears to grow and then shrink over the course of around 29.5 days, is what causes the phases of the moon. The document then defines and provides images for each of the 8 main phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent.