The Moon doesn't change — your seat does. Phases are the one astronomy lesson you can verify with your own eyes every single week.
Half the Moon is always lit
At any moment, the Sun lights exactly half of the Moon, just as it lights half of the Earth. The phases exist because as the Moon circles us each month, we see that lit half from a changing angle. New moon: the lit side faces away from us. Full moon: it faces straight at us. Everything between — crescent, quarter, gibbous — is geometry, nothing more.
A month of moonrise
The cycle takes 29.5 days from new moon to new moon, and it comes with a rhythm you can feel: the full moon always rises around sunset, the new moon travels with the Sun and hides in daylight, and a first-quarter moon hangs highest at dusk. Once you know the pattern you can tell the time of month from the sky — sailors and farmers did exactly that for millennia.
The part you can prove tonight
Look at tonight's moon and note which side is lit — that's the direction the Sun sits. The dark portion sometimes glows faintly too: that's earthshine, sunlight bouncing off the Earth, hitting the Moon, and coming back to your eye — light that has made a million-kilometer round trip. Our tracker computes the current phase from the same geometry described here, labeled 'computed' because it comes from a model, not a measurement.