Here's why:

* The Earth isn't a light source: The Earth itself doesn't produce its own light. It's illuminated by the Sun.

* Reflection: When sunlight hits the Earth, a portion of it is reflected back into space. Some of this reflected light reaches the moon.

* Earthshine: This reflected light from the Earth is what causes the faint glow sometimes visible on the dark side of the moon, known as "earthshine."

So, while the moon primarily reflects sunlight, it also receives a weaker reflection of sunlight from the Earth.