The Moon reflects sunlight rather than producing its own visible light. Except during an eclipse, roughly half of its surface is illuminated by the Sun. The phase depends on how much of that illuminated half we can see from Earth. As the Moon orbits Earth, the viewing geometry changes. A crescent, quarter, gibbous phase, and full Moon are different views of the sunlit portion. Ordinary phases are not caused by Earth’s shadow; that shadow is involved in a lunar eclipse. A model using a lamp and a ball can help, provided you distinguish the model’s scale and geometry from reality. Keep the lamp stationary and observe the ball from one position while moving it around you. Use a cool, safe light source and never use the Sun as a lamp to stare toward. Practice: Draw the Sun, Earth, and Moon for a quarter phase. Explain why “quarter Moon” describes the orbital phase even though half the visible disc appears illuminated.