Moon phases

The Moon is not a lamp; it shines by reflecting the Sun's light. Half of the Moon is always lit and half is always dark, just like a ball in sunlight. As the Moon orbits Earth, we see that sunlit half from a different angle each day. Sometimes we face most of the lit side and sometimes mostly the dark side. That shifting view makes the cycle of shapes we call phases.

New moon comes when the Moon sits between the Sun and Earth, with its lit half turned away from us. As it moves along its orbit, a growing sliver appears: waxing crescent, then first quarter, then waxing gibbous, until full moon, when the Moon stands opposite the Sun and its whole lit face points at us. Then the shapes repeat in reverse, shrinking through waning gibbous and last quarter to a waning crescent.

The whole cycle takes about 29.5 days, which is why a lunar month is a month. The Moon rises about 50 minutes later each night because it moves along its orbit while Earth spins. In the Northern Hemisphere, a waxing crescent is lit on its right side and a waning crescent on its left. Phase names describe what we see from Earth, and they never require anything to cover the Moon.

Earth's shadow does land on the Moon sometimes, during a lunar eclipse, but that is rare and lasts under two hours, and the Moon turns a dull coppery red. The monthly phases are not an eclipse. You can recreate every phase in a dark room with a flashlight, a ball and your head: walk the ball around your head and the same geometry, with no shadow at all, reproduces the whole month.