Question: Why is the moon white?

Sara Imari Walker answered on 17 Jun 2011:

Hello crunchijelly! Great question! Some of it has to do with the moon’s albeido. Albeido is what we use to measure the reflectance of a surface. Albedo is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Surfaces with an albedo near one reflect almost all light that is incident on their surface. An example of a substance with high albedo is snow which looks very white! The craters of the moon have a lower albeido than the highlands so they reflect less light and look darker to us. So why does the moon look white if it is reflecting the yellow light of the sun and the surface is gray? Well on average the moon has a fairly low albedo – around 0.12 (compare to fresh snow’s glaring value of about 0.9), so not much light is reflected from its surface. We view only a small amount of this reflected light here on Earth and it has to pass through our atmosphere. So the light from the Sun has been diluted quite a bit once it reaches us from the moon! During the day this light additionally has to compete with sunlight directly from the Sun. Our eyes are not super sensitive so we can’t really pick up on the color in this diluted and reflected light coming from the moon – so it looks white!! Did you ever notice during the day that sometimes you can’t even see the darker regions on the moon? They look blue! That is because their reflectance is so low. Physics a work 😉