The Moon is just past first-quarter today. That means that sunlight illuminates a bit more than half of the lunar hemisphere that faces our way. But as the sky darkens this evening, you’ll also be able to see the dark portion of the lunar disk, because it’s illuminated by earthshine — sunlight that’s reflected from the surface of our own Earth.
In recent years, scientists have been watching this dark part of the Moon several nights a month as part of Project Earthshine. They’re interested because earthshine is a novel way of measuring Earth’s albedo — how bright it is. The albedo changes depending on the cloud cover — more clouds means a brighter Earth. The albedo is linked to the planet’s climate, so collecting data on earthshine could lead to a better understanding of the overall health of Earth’s environment.
The project is carried out by astronomers at the Big Bear Observatory in southern California, and at observatories in the Canary Islands and the Crimea. They aim small telescopes at the Moon and carefully record the light from the dark side. They need only small telescopes because the Moon is so bright — even the dark side.
In addition to providing a way to monitor Earth’s cloud cover, earthshine may also prove useful in studies of other phenomena that affect Earth’s albedo, such as volcanic ash in the upper atmosphere.
We’ll talk about a mission that’ll look below the lunar surface tomorrow.
Script by Robert Tindol, Copyright 2011