albedo

earthshine
Originally uploaded by rod lewis

"My borrowed face
And my third-hand grace
Only reflect your glory"
-neil peart

Of course we know that the moon has no light of it's own. Same is true for us, of course. We are both reflectors. In fact, the earth is a much better reflector than the moon, but we're losing our ability it would seem. We are increasingly absorbing and trapping more than we are reflecting.
Though the results of this are easily felt and observed, the cause and effect is much less clear. No one seems to agree as to what are symptoms and what are causes.
The average reflective ability, or albedo of the moon is about 7% of the sunlight that strikes it. Right now, the earth's is somewhere around 30%. If we were to observe a "full earth" from the moon, it would be about 100 times brighter than a full moon seen from earth.
How much sunlight (and thus heat) we continue to reflect can be monitored by observing how much of our light is reflected back from the moon. This is called “earthshine.” There have been several plans in the past decade to send machines way out into space to constantly monitor how much of the sun’s energy we reflect, but this has yet to happen. Meanwhile, as should probably be the most obvious way of knowing what we’re reflecting, we watch to see how much of us is reflected back again.
Earthshine, as viewed from the moon, is an indicator of our own albedo, just as my friends and family give me an indication of how much I reflect the light sent my way, by reflecting it back to me.
And I should never mistake that bright swath of reflected sunlight along the bottom as my own, lest all data be skewed.