When the Moon is just a crescent, how come we can faintly see the darkened side?
When the Moon is just a crescent, how come we can faintly see the darkened side? asks a reader.
It all starts with the Sun. The Sun makes its own glow, by fusing hydrogen atoms into helium atoms, releasing photons of light in the process. Presto: Sunshine. The Moon is lit up by sunlight, reflected back to our eyes. Voila: Moonlight-or Moonshine. But what’s the night side of the Moon lit by, since it’s turned away from the blazing Sun?
The surprising answer: Earthshine.
Walk outdoors on a sunny day into the bright light. Just as sunlight lights up the moon, it lights up the Earth. Likewise, sunlight reflects off the Earth, and some of it is reflected to the Moon. The shadowed side of the Moon is softly illuminated by the light of planet Earth. The Earth is the Moon’s own night light.
So when we look at a crescent moon, our eyes receive both reflected sunlight (from a sliver of the bright side) and Earthlight (from the darkened side). Of course, the Earthlight streaming into our eyes from the Moon is just doubly-reflected sunlight, accounting for its dim glow. One way or another, stars are ultimately responsible for lighting up their solar systems.
Earthshine is also known as “ashen light” and the “da Vinci glow.” Artist and scientist Leonardo da Vinci actually solved the moon mystery in the early 1500s, suggesting that light reflected from the Earth created that “ghostly glow.”
The intensity of Earthshine varies, depending on the Earth’s cloud cover, weather, and the shifting position of land masses as the planet rotates. For example, as the Earth turns and the Sun rises over Asia, its big land mass reflects more light than the Pacific Ocean. This makes the night side of the Moon appear slightly brighter to observers on the night side of Earth.
And measuring the light reflected from the darkened part of the Moon may help scientists studying climate change. About 30 percent of the Sun’s radiation striking Earth ends up reflected back into space. The intensity of the reflected radiation depends on the reflectivity of our planet, otherwise known as albedo. If the average albedo decreases — if more sunlight is absorbed, rather than reflected – the Earth’s temperature rises. So as the planet gets hotter, the night side of the Moon may get darker.
Just as moonlight on Earth seems romantic, so does Earthshine seem to give the Moon a dimly romantic glow. In fact, another name for the crescent moon is “the old Moon in the new Moon’s arms.” Just as we walk around on a full-moon night without a flashlight, observing moon shadows and moonlit gardens of white flowers, so might someone on the night side of the moon cross craters bathed dimly in Earthshine, looking up at the source: a shining blue and white planet in the night sky.
Likewise, someone visiting the moons of other worlds would experience planetshine, too. To see the night side of Saturn’s moon Rhea lit by Saturnshine, visit http://www.saturntoday.com/news/viewsr.html?pid=25230.
