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Shine On, Earthshine

1-20-12 Earthshine_ChumackIn snowy states, it can be hard to sleep in winter when there’s a full moon, because the glow coming off the snow lights up the outside world. The brightness behind my curtains on a snow-covered, full-moon night is comparable to my neighbors leaving their garage light on during a summer night during new moon.

Fortunately for my beauty rest, a new moon is on the way. January’s new moon occurs on Monday the 23rd, assuring me of darkness out the window, if cloud cover hasn’t already taken care of that for me.

Many people find the moon most interesting when it is rising in the east in full phase, often showing a yellow or orange hue and looking gargantuan when compared with earthly objects. While this is undeniably a pretty sight, I find the moon most intriguing on the nights following new moon, as the slender crescent creeps into sunset skies and slowly reveals more of its lit face.

The fourth week of January will be a great time to watch the moon emerge as a crescent and wax toward its first quarter phase, on January 31. The sunshine hitting the edge of the moon is a lovely sight, but what also draws the eye is the faint light coming from the night portion of the moon. This is called Earthshine, because light from the sun is hitting Earth and reflecting into space, hitting the dark side of the moon and then reflecting back toward us. The degree of brightness of Earthshine is influenced by what is happening on Earth; for example, if the side of Earth that the sunlight is reflecting off is especially cloudy, it will cause more light to be reflected and light up the dark side of the moon a bit more.

Another factor that figures into the scenario of the light on both snow and the moon is their albedo. Albedo refers to the amount of reflectivity an object has. Fresh snow has one of the highest albedos, with 80 to 95 percent of the light that hits the snow reflected back. The moon, while it looks white and bright (at least outside of the mare regions), is actually not bright at all. It’s just that the only thing we really can compare it to is the blackness of space. But in actuality, the surface of the moon is quite dark and only reflects back 12 percent of the light that hits it.

Look this coming week for the dark side of the moon lit by Earthshine. You may even be able to make out a number of the dark mare under the lunar night. Earthshine is sometimes poetically referred to as seeing the old moon in the new moon’s arms.

1-20-12 Astronaut Duke Apollo 16

This image from the Apollo 16 mission shows Astronaut Duke's spacesuit covered with dark moon dust. Credit: NASA. Above photo credit: Credit: John Chumack.

-- Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She writes the SkyGuide for AstronomyToday.com. You can follow her on Twitter at twitter.com/Astronomommy.

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