Which phase is the halfway point of the illumination?

Which phase is the halfway point of the illumination?

• A. Full Moon
• B. First Quarter
• C. New Moon
• D. Third Quarter

Answer: (B)

The moment the Moon shows half of its disk illuminated is when the illuminated fraction reaches 50% as seen from Earth. This happens during the First Quarter phase, about a week after New Moon, as the Moon has waxed to half of its surface being lit and the illumination is increasing toward a Full Moon. At this point the line between light and dark (the terminator) cuts the Moon in half. There’s another half-illuminated moment later during the Third Quarter, but then the illumination is waning. So, the halfway point of the waxing illumination is First Quarter.

The moment the Moon shows half of its disk illuminated is when the illuminated fraction reaches 50% as seen from Earth. This happens during the First Quarter phase, about a week after New Moon, as the Moon has waxed to half of its surface being lit and the illumination is increasing toward a Full Moon. At this point the line between light and dark (the terminator) cuts the Moon in half. There’s another half-illuminated moment later during the Third Quarter, but then the illumination is waning. So, the halfway point of the waxing illumination is First Quarter.