During a lunar eclipse, why does the Moon sometimes appear red?

During a lunar eclipse the Earth blocks the direct sunlight that normally lights the Moon, but the Sun’s rays still reach the Moon after passing through the Earth’s atmosphere. The atmosphere bends the light and filters out most of the blue and green wavelengths, letting the longer red wavelengths travel through. As a result, the light that finally reaches the Moon is mostly red, which makes the Moon look reddish or orange. For example, when the Earth’s atmosphere is dusty from a recent volcanic eruption, the eclipse can appear a deeper burgundy because even more of the shorter wavelengths are scattered away. This same scattering effect is why sunsets and sunrises look red.