This image shows the
CNSA's Chang'e-6 probe on the surface of the Moon, where it drilled and collected samples, using the (deployed) robotic arm, for humanity's first sample return mission from the lunar far side. Although the far side itself can only be thought of as "dark" in a metaphorical sense, as it receives just as much sunlight as the more familiar near side, many of its properties still have yet to be investigated in thorough detail.
Credit: Xinhua/Shutterstock
Key Takeaways
- When we think about the Moon, we recognize that there’s a “near side” that always faces us, and a “far side” that always faces away from us. But sometimes, we also talk about the “dark side” of the Moon.
- Initially, “dark side” was simply an analogy for the side we couldn’t see, as it wasn’t until the rise of rocketry and spaceflight that we got our first glimpses of the far side, revealing its terrain to us.
- But it turns out that there really are places on the Moon that live in eternal darkness, and are never illuminated by sunlight at all. There really is a dark side of the Moon, but it’s not where you might expect!
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For nearly all of human history, there was a mystery that showed up, recurrently, on a nearly nightly basis. The Moon, visible during at least some portion of the night except during the once-per-month “new moon” phase, always shows its same face to us: the face of its near side. The opposite side of the Moon — the far side — surely must have existed, but because we’re stuck here on Earth and the same side of the Moon always faces the Earth, we’ve never been able to view it. Many, in poetic fashion, have called it the dark side of the Moon: a phrase that still occasionally shows up in popular culture, including in songs from Pink Floyd and Disney’s Mulan, for example.
But is there really a “dark side” of the Moon, or is that just a flowery expression that doesn’t actually apply in reality? That’s what Graeme Cree wants to know, posing the following inquiry to me:
“Of course, there is no literal “Dark Side of the Moon”. There’s a side that faces permanently away from the Earth, but no side that faces permanently away from the Sun. “Dark Side” is a metaphorical way of saying Unknown Side… Astrophysicist Ethan Siegel might get a kick out of this. He’s probably dealt with the same misconception himself, and can probably answer the question of whether or not anybody still calls it the dark side.”
Although “dark side” did originally mean “far side,” and the “far side” is only slightly darker than the “near side,” there really is a “dark side” to the Moon. Here’s what we’ve discovered so far.
Although one half of the Moon, only, is ever illuminated by the Sun, both the portion of the Moon that’s illuminated by the Sun and the illuminated portion that’s visible from Earth change over the course of a lunar month. A complete cycle, from new phase to new phase, defines the length of a lunar month. Note that, from someone’s perspective on the Earth-facing (near) side of the Moon, it’s never truly dark, as either the Earth or Sun (or both) is always shining light on the Moon.
Credit: Horst Frank & Nethac DIU/Wikimedia Commons
To be clear, there is a real and important difference between something that’s “obscure to us,” like the far side of the Moon (as viewed from Earth), and something that is an actual “dark side,” where the Sun doesn’t shine on it. When it comes to the near side of the Moon — the side that always faces us — we know that there is no dark side there because, even with the naked eye, we can observe that there isn’t. As the Moon goes through its phases every month, and as its phases shift with respect to apogee (when the Moon is farthest from Earth) and perigee (when the Moon is closest to Earth), the entirety of the near side gets illuminated by direct sunlight for about two continuous weeks in a row, followed by two weeks of darkness.
Even during those dark periods, however, the Moon is never truly dark, as it’s illuminated by the reflected light from the Earth: Earthshine. Just as the Moon goes through the full gamut of phases — from new to full and back to new again — as seen from the Earth, the Earth does the same thing as seen from the Moon. A “new Earth” corresponds to a “full Moon,” however, so that the only time there isn’t a bright Earth shining in the near side of the Moon’s sky is when the Sun is shining on entirety of the Moon’s near side. In a sense, it’s never dark on the near side of the Moon, because the Moon is always illuminated by either the Earth or the Sun or both; there’s never a moment where it’s darker than that.
Although the Moon is tidally locked to the Earth so that the same side always faces our planet, the fact that the Moon’s orbit is elliptical and follows Kepler’s laws of motion ensures that it appears to rock back-and-forth while growing and shrinking in apparent size over the course of a month: a phenomenon known as lunar libration. Overall, 59% of the total lunar surface, not 50%, is visible from Earth over time, and eclipses can occur during the new or full phase if the Moon happens to be passing through the Earth-Sun plane at that time.
Credit: Tomruen/Wikimedia Commons
However, even if you take into account the effect of lunar libration (as illustrated above), which is the apparent “rocking” of the Moon as it both:
- rotates on its axis (which it does at a constant speed),
- and revolves around the Earth (which it does at a maximum speed near perigee and a minimum speed near apogee),
we find that we can only see approximately 59% of the Moon’s area from our vantage point on Earth. The remaining 41% of the lunar surface can never be seen from Earth; it’s always on the “away-facing” side, which is known as the far side.
For centuries and millennia, that opposite side of the Moon remained obscure. Whatever was located there was completely “dark” to us, as it couldn’t be glimpsed from anywhere on Earth’s surface: where humanity was trapped by the force of gravity for so long. However, beginning in the mid-20th century, the development of rocketry led to our earliest spaceflights. Sputnik 1 was launched by the Soviet Union in 1957, becoming the first spacecraft to successfully orbit the Earth, and launching what’s today known as the space race. Just two years later, in 1959, the USSR launched Luna 3, which took a total of 29 photographs of the lunar far side, covering 70% of the area that had previously been invisible to humans. For the first time, we had knowledge of the Moon’s other side.
This photograph is the first view of the lunar far side ever acquired by humanity, as the USSR’s Luna 3 spacecraft flew past the Moon’s far side and snapped a series of more than 20 pictures, covering over 70% of the far side’s surface. Subsequent views have been far more informative, illuminating the lunar far side over the past 66 years in a way that would have amazed and delighted all the humans who came before the advances that made this possible.
Credit: Union of Soviet Socialist Republics (USSR)
To be certain, there were several surprises in store for us when we began to look, over time, with greater resolution and fuller coverage of both hemispheres of the Moon. Whereas the Moon’s near size has a large portion of it covered by deep, dark “seas” (or maria) where very few craters reside, the far side only has one prominent, small deep one and one large shallow one. The far side’s average elevation is a few kilometers greater than the near side’s average elevation. And there’s a very interesting and compelling theory that the chemical composition of the far side ought to be different from that of the near side: a theory we’re on the brink of testing now that we’ve returned samples from the Moon’s far side as well.
However, we were able to confirm something else with superior coverage of the Moon’s surface along with higher-resolution imaging: everywhere on the far side, just as is the case on the near side, appears to get illuminated by the Sun at some point during each lunar month. Most locations, just like on the near side, alternate between spending two weeks in darkness and two weeks in direct sunlight. The only major difference, for most of the lunar surface on the far side, is that there’s no “Earth” to shine in their skies: either during the day or to illuminate the lunar night. The lunar nights are darker on the far side of the Moon, but the far side is, during the day, just as light as the near side.
This two-faced mosaic from NASA’s Lunar Reconnaissance Orbiter shows the near side (left) and the far side (right) of the Moon with modern technology. By looking at the ratios and sizes of craters found on the Moon with respect to the age of that portion of the Moon, Mars, Mercury, and Earth, we can learn how cratering rates have varied over the Solar System’s history. Now, with our first samples from the lunar far side having been returned to Earth, we might finally learn more about the Moon’s ultimate origins.
Credit: NASA/GSFC/Arizona State University
So when it comes to the “dark side” of the Moon, if you mean the “far side” of the Moon, you should say “far side” rather than “dark side.” The only way that the far side is darker than the near side comes from the lack of having a large, bright, reflective Earth in the sky, especially during the lunar night, when the Sun isn’t shining at all. It’s also true — but not quite the point — that the far side is much darker and quieter in one particular set of wavelengths of light: in the radio band.
In fact, the far side of the Moon is arguably the quietest place in the entire Solar System, for the exact reason that it never receives Earthshine: it’s always facing away from the Earth. Since nearly all of the Solar System’s radio signals are generated by either the Earth or the (radio-loud) Sun, “night on the Moon’s far side” is the most radio-quiet place of all. This is why scientists have long hoped to build an enormous lunar radio telescope in a far-side crater, because it’s the one place that has the lowest noise floor in all the Solar System, and it’s right here in Earth’s backyard.
The greatest danger to the radio-quiet status that the lunar far side has is actually part of humanity’s plans for human exploration of the Moon: to place an array of radio-loud satellites in lunar orbit. If we actually do this, then the one way the Moon’s far side actually is dark — in radio light — will disappear.
Constructing either a very large radio dish, perhaps in a lunar crater, or alternatively an array of radio telescopes, on the far side of the Moon, could enable unparalleled radio observations of the Universe, including in the all-important 21 centimeter range, both nearby and across cosmic time. The ability to map out where neutral hydrogen has newly formed within the past ~20 million years would advance our understanding of cosmic history like nothing else.
Credit: Saptarshi Bandyopadhyay
However, it isn’t true that there’s no such thing as a “dark side” of the Moon. In terms of our common understanding, “dark” means that it’s shielded from the Sun, and never receives direct sunlight. On the Moon, that’s all it takes. Because there is no atmosphere at all:
- no air molecules to scatter and reflect sunlight,
- no lunar dust that gets kicked up and reflects scattered light,
- no clouds, hazes, or other particles that reflect light,
- and no medium to reflect direct sunlight that might fall on other, more faraway portions of the Moon’s surface,
simply being in a region of permanent shadow, that never receives direct sunlight itself, would equate to being on a “dark place” on the Moon.
It might not take up an entire hemisphere — or “side” — of the Moon, but there actually are several locations on the Moon that do meet these criteria: the permanently shadowed craters at and sufficiently close to the lunar north and south geographic poles. Here on Earth, our north and south poles spend six months in continuous light followed by six months in continuous darkness, and that’s due to our large (roughly 23.5°) axial tilt. When one polar hemisphere is tilted towards the Sun, it’s always illuminated; when it’s tipped away, it’s never illuminated.
However, even though the Moon revolves around the Earth, its axial tilt with respect to the Sun is extremely small: just 1.54°, or only about 6-7% as great as Earth’s much more pronounced axial tilt.
Whereas the Earth has a significant axial tilt with respect to the Sun of 23.44 degrees, the Moon’s inclination is much less severe: at just 1.54 degrees. (The difference between the 6.68 and the 5.14 degree figures shown in the diagram here.) This small axial tilt, combined with the depth of craters with high-rimmed crater walls on the Moon, enables the existence of true locations of darkness.
Credit: NASA
This means that, at or near the lunar poles, a sufficiently large and deep depression in the ground — including just a regular old impact crater — will prevent any and all sunlight from ever striking the low points of that depression. When the lunar pole you’re at is tipped towards the Sun, you only need to remain a couple of degrees below the rimmed wall of that depression (again, most craters have high walls) and sunlight will never directly strike you. When the lunar pole you’re at is tipped away from the Sun, it’s even easier. This makes not just the poles themselves, but the sufficiently deep and expansive craters within 5-to-even-10° of the poles good candidates for being dark.
In planetary science, we don’t often refer to these regions as the “dark side of the Moon,” but they are indeed some of the darkest places within the Solar System that once can venture to. We usually refer to them as permanently shadowed craters, and note that they behave as “cold traps,” which warrants further explanation. Whereas the majority of the Moon’s surface is inhospitable to volatile molecules, such as ices, persisting for any meaningful duration of time, because direct sunlight will heat them up and sublimate them into a vaporous state, these “cold traps” represent excellent places for volatile ices, should they ever enter these dark places, to persist indefinitely far into the future.
These Lunar Reconnaissance Orbiter mosaics of the North and South poles of the Moon showcase several candidate craters whose insides likely never receive direct sunlight. The walls of these craters are candidates for being peaks of eternal light, while the insides of the craters never rise above temperature of 100 K, serving as “cold traps” for volatile ices and more.
Credit: NASA/GSFC/Arizona State University
The north pole of the Moon does have plenty of craters that appear to be good candidates for being permanently shadowed, or dark, in precisely this fashion. Observations at radio frequencies that were conducted as far back as the 1990s indicate that there are likely reservoirs of volatiles, including water-ice, present within at least some of those craters. Many have long speculated that if you:
- set up solar panels around the crater rims,
- created a pressurized environment either inside or outside of the crater,
- and gather/mine the ice — which will be more like a dirty slushball, most likely — inside of those permanently shadowed craters,
you’d be able to perform all sorts of tasks, including tasks towards creating habitable environments, without having to bring your own materials, such as water and air, from Earth.
However, the south pole of the Moon is an even better location than the north pole of the Moon in this regard, with very large prominent craters (such as Shackleton crater) remaining in permanent shadow. Although a layer of lunar regolith, or Moon dust/soil/rocks, likely covers whatever exists in the “cold traps” within these craters, direct observations indicate that the temperatures inside the crater never rise above a maximum 100 K (-173 °C/-279 °F), meaning that not only water ice, but dry ice (carbon dioxide) and possibly even methane ice could persist, long-term, within these craters.
This rendering of the South Pole area of the Moon shows 13 candidate landing regions for Artemis III. Each region, shown in blue, is approximately 225 square kilometers, with some of them atop “peaks of eternal light” that always receive sunlight, or that border permanently shadowed craters on the Moon.
Credit: NASA
These permanently shadowed regions are as close as one can actually get to a true “dark side” of the Moon. At both the north and south lunar poles, these coveted pieces of lunar real estate are likely the perfect locations to stake out and claim if one wants the most resource-rich regions of the Moon for themselves. The combination of ancient material that’s likely found within these craters (and nowhere else on the Moon), the fact that they have access to continuous sunlight at the top of the crater walls (rather than having to reckon with two continuous weeks of darkness), and where they’ll be able to either communicate with Earth (at the crater wall tops) or be shielded from Earth (within the crater) makes them unlike anyplace else on the Moon.
Even though there are treaties prohibiting “ownership” of any locations on the Moon for any public, private, or national entity, there is a loophole that several nations are racing to exploit: the ability to declare a keep-out zone, for safety reasons, if one is conducting nuclear activity on the Moon. This is what’s behind the race to build a nuclear reactor on the Moon:
- to claim these peaks of eternal light for ones self,
- while excluding everyone else from the permanently shadowed polar craters that house large stores of water-ice.
These locations are suspected to be highly sought after as a means to begin experimenting with human colonization of other worlds here in the 21st century, with nuclear power plants on the Moon providing justification for activity that would otherwise violate the international law of the Outer Space Treaty.
Humanity has long dreamed of establishing a robust and continuous presence on another world, such as the Moon or Mars. One of the keys to such a successful colony would be the continuous generation of power, with nuclear power plants remaining a prime, and arguably our best, option. However, claiming the real estate of peaks of eternal light and permanently shadowed craters for one’s own interests, whether national or business interests, is likely the true motivation behind the desire for nuclear power plants on the Moon.Credit: NASA; edited by E. Siegel
Sure, if you want to get technical about it, there is no “dark side” of the Moon. What we once called the “dark side” was only the obscure side, or the side permanently facing away from us. But this is the lunar far side, not the lunar “dark side,” as it receives just as much light as the near side, or Earth-facing side, does. It’s more correct to call it the far side of the Moon, and to note that it’s no longer mysterious, as it was for all the years, decades, centuries and millennia prior to 1959. We’ve mapped it out, we know what it looks like, and while it’s quite different in many important ways from the more familiar near side, it isn’t “dark” in the conventional sense of the word.
However, there are regions of the Moon that actually are eternally dark. It’s true that they’re not a “side” of the Moon in any real sense; they’re just a series of locations that lie incredibly close to (or at) the true geographic north and south poles of the Moon itself: within the permanently shadowed craters of sufficient size that are located there. The insides of these craters likely have not been disturbed other than the small amount of lunar dust, dirt, and debris that’s landed in them due to other, subsequent strikes on the Moon in hundreds of millions or even billions of years. Within them, not only darkness, but stores of volatile compounds and ices from our Solar System’s ancient history likely reside. There are indeed dark sides to the Moon; you just need to know where to go to find them!
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