A thin crescent Moon high in a deep blue twilight sky is a lovely sight that anyone can appreciate. There’s something about seeing that slim sliver of silvery light that delights nature lovers (and fans of alliteration). And, if you make a point of enjoying this lunar phase, you’ve probably noticed that quite often, you can even see the ghostly visage of the “unlit” side of the Moon too. This is no illusion — it’s the result of a phenomenon called Earthshine. Earthshine is simply sunlight that has taken an indirect journey.
How’s that? Let’s imagine that you’re a ray of sunshine. Sure, you’ve been called that many times before, but this time, let’s take it literally. You begin your journey at the surface of the Sun, and travelling at 300,000 kilometres per second, you arrive at Earth about eight minutes later.
Next, you bounce off Earth (after all, you’re light!) and travel a shade more than a second before reaching the Moon. From there, you retrace your journey back to Earth, where you reach the eyes of an attentive observer. You are, in the words of noted lunar scientist Charles A. Wood, “twice reflected sunlight.” And how lucky you must feel!
Earthshine is faint because neither Earth nor the Moon is particularly reflective. Indeed, Earth (despite its bright oceans and clouds) manages to reflect back only about 37 percent of the sunlight it receives. This property — a planet’s ability to reflect sunlight — is known as its albedo. Some planets are very shiny. Venus, for example, is the albedo champ: Its hazy atmosphere returns an impressive 65 percent of the sunlight it receives, which is the main reason it appears so bright in our sky. Mercury, by comparison, is dull and dark, with an albedo of only 11 percent. The Moon isn’t much better. Its albedo is 12 percent, which makes it about as reflective as pavement. So what does all this add up to? Simply this: By the time our ray of sunshine has completed its double-bounce journey from Earth to the Moon and back again, it’s only a ghost of its former self. That’s why Earthshine is so faint.
But why don’t we see Earthshine most of the time? Two things are happening at once. First, as the Moon waxes, the amount of “dark” Moon shrinks, making the Earthlit side tougher to see because there’s less of it. Second, and more important, as the Moon’s phase waxes, the Earth’s phase (as seen from the Moon) wanes. Indeed, whatever phase the Moon displays in our sky, Earth is exhibiting the inverse phase to an observer on the lunar surface. So when we see a thin sliver of a Moon, Earth appears nearly full to an astronaut looking back home — and a nearly full Earth is much, much brighter than a full Moon. But by the time the Moon reaches first-quarter phase, only half the Earth’s disc is sending light to the lunar surface, which makes for tremendously diminished Earthshine.
Binoculars and telescopes will allow you to widen your window of opportunity for viewing Earthshine. I especially enjoy scanning the Earthlit side of the Moon in a telescope equipped with enough magnification to limit the view to just the unlit portion of the lunar disc. Bright features, such as the crater Aristarchus or Tycho’s rays, stand out surprisingly well. The Earthlit lunar surface also has a strangely cold appearance that’s really quite striking. The view makes it easy to imagine being an astronaut looking back at the home planet, surrounded by a lunar terrain bathed in a faintly blue luminance.