Earthshine
Written by Fraser Cain
Half of the Moon is always illuminated by the Sun. When we see a new moon, the side of the Moon lit up by the Sun is facing away from us, so we see the side that's in shadow. Since there's no illumination, you would expect that the Moon would be completely dark and invisible from our point of view. And yet, when the Moon is shadow, it's still illuminated by reflected light from the Earth. This light is called Earthshine.
The best time to see Earthshine is just before or after a new moon, during the waxing or waning crescent phase. If you were standing on the Moon, you would see the entire Earth lit up with reflected sunlight. It would be a "full Earth" from your perspective. This reflected light goes off into space, but a lot hits the Moon and lights it up.
Leonardo da Vinci was one of the first people to ever describe this situation. He realized in the 1500s that both the Earth and Moon reflected sunlight, and that the light on the Moon during a new Moon was just reflected from the Earth.
Scientists use Earthshine as a tool to study the Earth's climate. By carefully measuring the amount of Earthshine during each new moon, astronomers have been able to calculate the amount of total cloud cover across the planet. Their studies have shown that cloud cover dipped between 1985 and 1997, and then increased between 1997 and 2003.
We have written a few articles about Earthshine on Universe Today. Here's an article about how decreasing Earth shine might be linked to global warming. And here's a technique that astronomers might use to find planets orbiting other stars.
Here's an article from NASA about Earthshine.
Want more information about the Moon? Here's NASA's World Book on the Moon. And here's NASA's Solar System Exploration Guide.
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?
Filed under: Astronomy
