Which phase is the fully illuminated Moon?

• A. New Moon
• B. First Quarter
• C. Full Moon
• D. Waning Gibbous

Answer: (C)

The amount of the Moon that looks lit from Earth depends on where the Moon is in its orbit around us. When the Moon is opposite the Sun—so the Sun, Earth, and Moon are aligned in a straight line—the Sun illuminating the Moon shines directly toward Earth, and we see the entire sunlit face. That is the Full Moon: the Moon appears fully illuminated, rises at sunset, is highest at midnight, and sets at sunrise.

The other phases describe different alignments. A New Moon happens when the Moon is between the Sun and Earth, so the side facing us is dark. The First Quarter occurs when a half of the near side is lit, with the Moon about a quarter of the way through its orbit. A Waning Gibbous happens after the Full Moon as the illuminated portion shrinks, though still more than half is visible.