Which statement correctly describes the Moon during the first and third quarters?

• A. The Moon is fully illuminated.
• B. The Moon is half illuminated.
• C. The Moon is one-quarter illuminated.
• D. The Moon is not illuminated.

Answer: (B)

During the first and third quarters, about half of the Moon’s disk that we can see from Earth is illuminated. This happens because the Sun lights up half of the Moon, and at these points in the orbit the angle between the Sun, Moon, and Earth makes the terminator—the line between lit and dark areas—run right down the middle of the Moon. In the Northern Hemisphere, the lit side looks on the right during the first quarter and on the left during the third quarter. The other descriptions correspond to different phases: a full Moon would be fully lit, a new Moon would be largely not lit, and one-quarter illuminated doesn’t describe these quarters since the illuminated fraction is 50%.