“There is no dark side of the moon, really. Matter of fact, it’s all dark.” —Pink Floyd

With my apologies to Roger Waters, who wrote those lyrics, there is a dark side of the moon. And sometimes it’s not all that dark.

You can see that for yourself over the next few nights. Go outside just after sunset, and turn your gaze to the west. You’ll see brilliant Venus setting not far above the horizon and, depending on the exact day you look, the thin crescent moon nearby. If you go out tonight (Friday, July 21, 2023), the moon will be an extremely thin crescent just above Venus and next to the much fainter red planet Mars. But over the next few nights, the moon will pull away to the east (to the upper left, that is, for Northern Hemisphere observers) as its lunar crescent grows thicker.

Once you spot the moon, take a close look at it. The part of it lit by the sun forms a tight crescent with the tips pointing away from the sun. And as for the unlit remainder, that should be invisible, right?

Well, look again. If the twilight is deep enough, you might see that the dark part of the moon isn’t, well, dark. It appears to be faintly glowing, softly illuminated and forming an eerie outline of the rest of the moon’s disk. In a pale and gentle shade of its full glory, it looks like a ghost.

Normally the dark side of the moon is truly dark. Like Earth, the moon is spherical, and sitting in space, one half of it is always lit by the sun. As the moon orbits our planet, we see different parts of its sunbathed Earth-facing hemisphere, which creates the moon’s familiar phases. Where the sun shines on the lunar surface, it’s daytime, and where it’s dark, it’s nighttime.

Astronomers call the line between day and night the terminator, and when the moon is new and a thin crescent, that line marks the location of sunrise across the moon’s face. As seen from the moon’s surface, anywhere along the terminator, the sun is rising and still low in the lunar sky. The rest of the moon, which we see as dark, is still in night, with the sun yet to rise to start the long lunar day. That, in turn, means the part of the lunar surface that isn’t lit should be as dark as space.

Yet, looking up at the moon’s thin crescent, you can see it’s not, can’t you? Instead it glows dimly. So if the sun isn’t up yet, how can that part of the moon be lit? What’s illuminating it?

The answer’s in the name of this phenomenon: earthshine.

An explanation of some basic viewing geometry can help explain what exactly we’re talking about. When the moon is new, it’s more or less between Earth and the sun, with its unlit side facing us. As it moves along in its orbit, we see more and more of the lit part forming a crescent that grows wider—called a waxing crescent—over time.

But from the moon’s point of view, Earth and sun are on opposite sides of the sky. It doesn’t see a dark Earth—it sees the half of our planet that is illuminated by the sun, a “full Earth,” in its sky. From Earth, the full moon is pretty bright. You can read by it, and it casts noticeable shadows. But Earth is four times the diameter of the moon and, therefore, more than 15 times its area in the sky, so it appears much brighter. Not only that, on average Earth is also about two to three times more reflective than the moon, so overall it can be as much as 50 times brighter in the moon’s sky than the full moon is in ours. From the moon’s point of view, Earth is bright.

That’s what’s illuminating the moon’s dark side! The sun is far brighter than Earth, of course, so the lunar crescent we see is brilliant, but Earth is still enough of a light source that it softly lights up the rest of the moonscape for our viewing, too. If you look carefully—binoculars or a small telescope help here—there’s enough earthshine to actually see features on the moon’s surface, such as craters and the large dark areas called maria (Latin for “seas”).

Earthshine is also called “the old moon in the new moon’s arms”—a lovely turn of phrase that not nearly enough astronomers use.

And think about what you’re seeing: Light from the sun travels 150 million kilometers to Earth and lights it up. Earth reflects some of that light, some of which travels the 380,000 km to the moon, which again reflects a fraction of that light back to us, where it finally hits your eyes and allows you to see the velvety glow of its surface. From the sun to the Earth to the moon and back again, this light has a complicated journey. Appreciating it adds to the wonder of this apparition.

But this phenomenon offers more than just a notional pretty picture. Some of that sunlight travels through Earth’s atmosphere before it travels to the moon, and this can affect earthshine’s brightness, especially if the daylit part of our planet is strewn with highly reflective clouds. Scientists have used this fact to measure how clouds, the oceans and even forests or grasslands can change that light as a proxy for studies of far more distant exoplanets—alien worlds orbiting other stars—to see if we can puzzle out their surface features as well. Earthshine, it seems, could even help reveal the presence of life on other worlds!

Also, there are craters located near the lunar poles where the sun never rises very high above the horizon. The bottoms of these craters can be deep enough that sunlight never shines there, making them extremely cold. There’s a lot of evidence that many of them contain water ice locked up in that permanent darkness—ice that could be a huge boon for future human exploration. While the sun doesn’t shine there, Earth can, and it’s possible to use earthshine to look for water ice in the bottoms of these craters. Scientists found that nearly half these permanently shadowed polar craters are occasionally bathed in earthshine and have pinpointed the times when it would be easier to directly explore them in search of promising deposits.

Earthshine is a wonderful and beautiful thing to see, but we can use our equally wonderful brains to use it, like so many other natural phenomena, to learn more about the universe around us. And that, in turn, allows me to not only talk about its science but also its beauty—if I may wax poetic.