Crescent Moon

A Crescent Moon is simply any phase where less than half the visible lunar disc is lit — the thin, curved sliver most people picture when they think "Moon" in a logo or drawing, even though it only appears this way for about a third of the month. There are two crescent phases, and telling them apart is a matter of which side is lit: a Waxing Crescent (right side lit in the Northern Hemisphere) appears in the days right after New Moon as the Moon builds toward First Quarter, while a Waning Crescent (left side lit) appears in the days right before New Moon as the cycle winds down. The crescent shape itself comes from the terminator — the boundary between the moon's lit and unlit hemispheres — being viewed at a steep angle from Earth; it's the same physical curve as a half-lit sphere photographed from the side, not the Moon changing shape. The famous "old moon in the New Moon's arms," where you can faintly see the rest of the lunar disc during a thin crescent, is earthshine — sunlight bouncing off Earth's clouds and oceans onto the Moon's dark side.