Which phase is the Moon when it is a fully illuminated disk?

Which phase is the Moon when it is a fully illuminated disk?

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

Answer: (A)

The Moon looks like a fully lit disk when it is opposite the Sun as viewed from Earth. In this position, the Sun’s light shines directly on the side of the Moon that faces us, so the entire near side is illuminated. That arrangement is what we call the Full Moon. If the Moon were between the Sun and Earth, the side facing us would be in darkness, which is the New Moon. When the Moon is about 90 degrees away from the Sun in the sky, we see a half-illuminated disk: that’s the First Quarter (the illuminated half is the right side for observers in the Northern Hemisphere). When it’s 90 degrees on the other side, we also see half illumination, but the opposite half of the disk, which is the Third Quarter.

The Moon looks like a fully lit disk when it is opposite the Sun as viewed from Earth. In this position, the Sun’s light shines directly on the side of the Moon that faces us, so the entire near side is illuminated. That arrangement is what we call the Full Moon.

If the Moon were between the Sun and Earth, the side facing us would be in darkness, which is the New Moon. When the Moon is about 90 degrees away from the Sun in the sky, we see a half-illuminated disk: that’s the First Quarter (the illuminated half is the right side for observers in the Northern Hemisphere). When it’s 90 degrees on the other side, we also see half illumination, but the opposite half of the disk, which is the Third Quarter.