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What on Earth is that moon glow? Actually, it is Earth

What on Earth is that moon glow? Actually, it is Earth
Photo courtesy Flick user jurvetson (Creative Commons 2.0 License)

Ever been out when the moon is full and bright and feel like maybe you can even read a book?

Believe it or not, the Earth returns the favor.

I received an e-mail during the last crescent moon that wondered why the "dark" part of the moon actually had a faint glow to it. Turns out, it's "Earthshine"

When the moon and Earth align just right, the sunlight reflecting off the Earth can act like a dim flashlight shining toward the moon -- in effect, providing a faint glow on what would usually be the dark part of the crescent moon. says the light shone by Earth is 50 times brighter than the moon's light on Earth.

When the moon is nearly new here, to those standing on the moon, the Earth is nearly "full."

Scientists discovered long ago that the "full Earth" reflects enough light from the sun to dimly illuminate what is usually the dark part of the moon. Leonardo da Vinci was one of the first credited with observing this effect

The interesting part is that the effects are most visible in April and May, as that tends to be when the Earth as a whole is the cloudiest. Clouds can reflect as much as 50 percent of the incoming sunlight back into space -- in essence, being a brighter flashlight. But apparently the planet must have been somewhat cloudy this autumn. (Ironically, if it's clear enough to see this effect, your location is not doing its best job in contributing to the light :) )

For more information on "Earthshine" check out