The moon appears half lit, half dark from the surface of Earth on two days each month. What causes this?
The moon appears half lit and half dark from the surface of Earth during its first quarter and third quarter phases. This occurs because of the positioning of the moon, Earth, and the sun during these phases.
During the first quarter phase, the moon is exactly halfway between the new moon phase and the full moon phase. From Earth, we can see half of the moon illuminated by the sun’s light on the side facing Earth, while the other half of the moon remains dark.
Similarly, during the third quarter phase, the moon is again halfway between the full moon phase and the new moon phase. This time, the side of the moon facing Earth appears to be half dark and half lit by the sun’s light.
These phases occur as the moon orbits around Earth, and the relative positions of the sun, Earth, and moon change, causing different portions of the moon to be illuminated by the sun as seen from Earth.