Thursday, September 2, 2010

Why Is the Moon So Bright?

The brightest object in the nighttime sky is, by far, the Moon. In fact, it can seem blindingly bright, particularly when compared to other celestial objects. With its face only half illuminated, a First Quarter or Last Quarter Moon (sometimes called a “Half Moon”) is still around 250 times brighter than the next brightest celestial object, the planet Venus.

The Moon does not emit its own light, but rather shines by reflecting light from the Sun. And as reflective surfaces go, it’s just not that impressive. The reflectivity of a body or surface is known as its albedo (al-BEE-doh), a term derived from the Latin word for white. The Moon’s albedo is around 0.12, meaning that it reflects an average of only 12 percent of the sunlight reaching its surface.

The rest of the sunlight is absorbed by the Moon’s regolith (REGG-uh-lith), the loose surface layer of broken rock and dust covering the lunar bedrock.

So, if it’s reflecting only a small fraction of the sunlight that’s striking it, why does the Moon—even a slender crescent Moon—look so darn bright? The fact is our Sun is so bright that even a fraction of its light reflected is still a lot of light. Add to that the relatively close proximity of our Moon (about a quarter of a million miles away), which gives it a large apparent size, that is, the amount of sky it covers as seen from Earth.

Now consider this: planet Earth’s albedo, or reflectivity, is 0.37. Can you imagine how bright a “Full Earth” would look if you were standing on the Moon?