What causes the moon to appear bright when it is not full?

 The moon appears bright even when it's not full due to the sunlight reflecting off its surface. The moon itself doesn’t generate any light; it shines because it reflects sunlight. The brightness depends on the phase of the moon and how much of the sunlit portion of the moon is visible from Earth.

The moon has a surface covered in a layer of fine dust and rocks that reflects light. This is why even during phases like the crescent or half-moon, the moon can still appear bright in the night sky. The brightness is determined by two factors: the angle of sunlight hitting the moon’s surface and the amount of the moon visible from Earth.

When the moon is in a crescent or gibbous phase, we still see a significant amount of its surface reflecting sunlight. The moon’s brightness is also affected by its position in the sky and atmospheric conditions, such as how much the Earth’s atmosphere scatters sunlight.

Additionally, the phenomenon called "Earthshine" can contribute to the brightness of the moon in phases like crescent. Earthshine occurs when sunlight reflects off Earth’s surface and illuminates the darker portion of the moon, making it visible, albeit faintly.

In short, the moon appears bright because of sunlight being reflected off its surface, with varying brightness depending on its phase and position in the sky.