Earth Shine

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Earth Shine
You see it, but you might not know...
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Earth Shine Rising before the Sun that day was an exquisite crescent Moon with the bright planet Venus nearby. The Moon was a whopper, swollen by the well-known illusion that makes moons near the horizon seem big. But that wasn't what grabbed my attention. The wonderful thing was the way the "dark" part of the Moon was faintly glowing. Leonardo Da Vinci made this sketch of the crescent moon between 1506 and 1510 Leonardo Da Vinci explained the phenomenon nearly 500 years ago. He realized that both Earth and the Moon reflect sunlight. But when the Sun sets anywhere on the Moon (this happens every 29.5 Earth-days) the landscape remains lit -- illuminated by sunlight reflected from our own planet. Astronomers call it Earthshine. It's also known as the Moon's "ashen glow" or "the old Moon in the New Moon's arms." This month and the next may be the best time to see it. Scientists in recent years have studied the Moon's ashen glow to discover how much sunlight our planet reflects -- a key datum for global warming studies. They've found that Earthshine is most intense -- about 10% brighter than average -- during April and May. Phil Goode of Big Bear Solar Observatory is principal investigator of Project Earthshine, a NASA-supported effort to monitor Earth's albedo (the astronomer's term for reflectivity). He says, "it's not surprising that Earth's albedo changes with the seasons." After all, most of the sunlight reflected from our planet is reflected from clouds -- and cloud cover changes from one season to the next. Clouds dominate the "shininess" of our planet, adds Goode. "They reflect about 50% of the sunlight that hits them -- more than oceans (10%) or land (10% to 25%). Only snow and ice reflect more (40% to 90%) than clouds do, but snowy areas tend to be cloud covered anyway." Although the spring peak wasn't surprising to Goode, its size was. The extra Earthshine in April and May was about twice as much as computer models predicted based on actual satellite observations of ice and cloud cover. It's a lingering mystery. On Monday evening, April 15, the three-day old Moon will lie farther from the Sun and so the sky behind it will be darker than the night before. The Moon should be easy to find about 30 degrees above the western horizon at 8 p.m. local time. Says astronomer Clay Sherrod: "I tell my students that such a crescent Moon is best for spotting Earthshine -- provided that the sky is clear and transparent." That bright star beside the Moon on Monday night will be the planet Mars. By Tuesday, April 16, the Earthshine will begin to fade: it will be hard to see against the glare of the growing sunlit portion of the Moon. Plus, Earth -- which has phases on the Moon just as the Moon has phases on Earth -- will no longer be so "Full" in lunar skies. Sunday and Monday are the evenings to watch. Just don't gawk and drive. You can visit the Earthshine Project's official site at: http://www.bbso.njit.edu/Research/EarthShine/
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