Reflecting on Earthshine

The moon will be full Thursday, which means we'll see it in all its illuminated glory. But when the moon is just a sliver, we sometimes see our own reflection shining back at us from the moon's shrouded side, in a phenomenon called "earthshine."

Now scientists say the difference in light reflection from the Earth's land masses vs. the oceans can be seen on the moon. By tracking changes in earthshine as Earth rotates, scientists measured brightness variations that correspond to the brilliant, mirror-like reflections from oceans compared to the dimmer reflections from land.

Earthshine was first proposed by Leonardo da Vinci, who suggested that sunlight could bounce off our planet and be reflected back to us by the moon. This light is only visible when there is little sunlight reflecting directly off the moon, which would otherwise drown out the much dimmer earthshine. Thus, Earth's reflection is only visible to the naked eye on the darker portion of thin crescent moons, and not full moons.

The phenomenon can sometimes be seen by the naked eye as a ghostly glow, and is easily visible with a telescope. It is best seen once a month when the crescent moon hangs just above the western horizon right after sunset.