What color does the moon appear during a lunar eclipse and why?

During a lunar eclipse, the moon appears copper-red due to the phenomenon of Rayleigh scattering, which affects the way sunlight interacts with Earth's atmosphere. When the Earth is positioned directly between the sun and the moon, it blocks direct sunlight from reaching the moon. However, some sunlight does filter through the Earth’s atmosphere. This light is bent, or refracted, around the Earth's edges and hits the moon.

The atmosphere is more effective at scattering shorter wavelengths of light, which are the blues and greens, and allows the longer wavelengths, such as reds and oranges, to pass through. As a result, when this refracted light reaches the moon, it takes on a reddish hue, giving it the characteristic copper-red appearance during a lunar eclipse. This is often referred to as a "blood moon."

The other choices present misunderstandings about the lunar environment during an eclipse. While blue light is scattered, it does not dominate the light that reaches the moon, nor does a lack of sunlight merely render it gray or white. The scattering process uniquely alters the light to create the striking copper-red color instead.