Which term describes the faint illumination of the Moon caused by sunlight reflected from Earth?

• A. Earthshine
• B. Moonlight
• C. Sunlight
• D. Starlight

Answer: (A)

Earthshine is the faint illumination of the Moon caused by sunlight reflected from Earth. Sunlight first hits Earth's clouds and surfaces, bounces around, and a portion of that light travels to the Moon and lights its dark side. This subtle glow is easiest to notice when the Moon is a crescent, giving that familiar dim glow on the part of the Moon that would otherwise be dark. Earth reflects more light overall than the Moon because of its oceans and cloud cover, so this reflected light can be seen even though it’s much fainter than direct sunlight.

The other possibilities don’t fit the scenario. Moonlight is light from the Moon itself reflecting sunlight, not light that Earth sends to the Moon. Sunlight is direct light from the Sun, not reflected off Earth. Starlight is light from stars, unrelated to the Earth–Moon interaction in this context.