During which phase of the Moon do lunar eclipses occur?

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

Answer: (B)

A lunar eclipse happens when Earth lies directly between the Sun and the Moon, casting its shadow onto the Moon. That alignment only occurs when the Moon is at the Full Moon phase—when the Moon is opposite the Sun in the sky. At full Moon the Sun, Earth, and Moon line up enough for Earth’s shadow to reach the Moon, so the Moon can appear dark or take on a reddish hue during the eclipse.

Other phases don’t produce this alignment. At the First Quarter or Third Quarter, the Moon is 90 degrees away from the Sun, so its shadow doesn’t fall on Earth’s shadow line and you don’t see the Earth-shadow event on the Moon. At New Moon, the Moon is between the Sun and Earth, so its shadow would be on Earth or invisible to us, and Earth’s shadow isn’t cast onto the Moon from our viewpoint. Also, even at full Moon, eclipses don’t happen every time because the Moon’s orbit is tilted about 5 degrees to the Sun–Earth line, so only when it crosses a node near full Moon will an eclipse occur.