During the first quarter, approximately what fraction of the Moon's disk is illuminated?

• A. Three quarters
• B. Full
• C. Half
• D. One quarter

Answer: (C)

Understanding how much of the Moon’s disk is illuminated as seen from Earth during a given phase. At the first quarter, the Sun, Moon, and Earth form about a right angle, so sunlight washes over exactly half of the Moon’s visible surface. The line separating day and night on the Moon—the terminator—runs through the middle of the disk, leaving about one-half illuminated. This is why the Moon appears half-lit at this phase. The other options describe more or less illumination (full is all lit, a smaller crescent is less than half, and three quarters is more than half), so the half-lit appearance at first quarter is the correct sense of the phase.