Ah, the idea of gazing at a sunset or basking in the glow of twilight on the Moon; so romantic, yet totally unrealistic on a dead world with no atmosphere…or is it? Surprisingly, there’s “lunar horizon glow”, which pictures from space probes’ cameras make look subtle, but sketches and tales from astronauts describe as quite striking to the human eye. Gene Cernan even observed sunbeams peeking through that were analogous to Earth’s crepuscular rays!
“Lunar horizon glow” imaged by NASA Surveyor 7 probe, 1968. It looks subtle on camera, but it proves it is no optical illusion or hallucination…
Gene Cernan’s sketch of “lunar horizon glow” as he observed it with the naked eye, Apollo 17 (1972). Much more vivid and elaborate…
How can this be? After all, on the Moon atmospheric pressure is utterly negligible compared to Earth or any of the other air worlds. To wit, 3×10−15 bars, or 0.3 nanopascals. The entire mass of atmosphere, combined across the entire planet, comes to 10 tonnes on a good day. For comparison, even Mars’s rarefied envelope weighs in at 25 trillion tonnes. The vicinity of the Moon is noticeably denser than the interplanetary medium, but from a human perspective, the gases held, ever so tenuously, by the Moon’s gravity produce no effects at all.
But that’s not the end of the story: the Moon’s very lack of atmosphere fully exposes the surface to the sun’s radiation during the daytime and the charged particles of the solar wind during the nighttime. The light of the day kicks electrons away from the dust, leading to dayside dust being positively charged, repelled from the surface miles high; on the night side, electrons from the solar wind negatively charge dust particles. This phenomenon, known sometimes as the “Moon fountain”, ensures there’s an omnipresent, albeit wispy, haze of moondust near the horizon. Near the terminator, the boundary between the day and night sides, electrical activity, and hence dust transport, is particularly vigorous, and the sun angle is at its lowest. As on Earth or any other planet, at this time the sun is shining through a longer distance of “atmosphere”, so scattering is maximized.
The result? The normally invisible veil of dust becomes very much visible, interacting with the shadows of terrain to create striking visual effects. Crepuscular rays, analogous to those seen in Earth sunsets, though with a distinctly different visual texture.
My understanding is that, at least on a particularly vigorous day, you would see lunar horizon glow present itself as a distinct dusty glow near the setting sun, the entire phenomenon clustering near the horizon by the setting sun, even as the rest of the sky is black, as is usual on the Moon. Already delightfully alien.
But it gets better when you realize that the confluence of conditions that enable visibility at all only appear briefly compared to an Earth sunset; when the sun is well above the horizon, it’s too strong to see the very thin veil of dust, and that same thinness means that it will disappear into invisibility very soon after the sun sets. So the total window of visibility would be short…at least as a fraction of a whole day. But consider the lunar day is 29.5 Earth days long, so the entire sunset process, so to speak, is almost 1/30 the speed we’re accustomed to on Earth. So what looks on a lunar-day-keeping clock like a brief moment might stretch out to an appreciable length of time on a human scale. Near the poles, of course, the sun’s path through the sky becomes less vertical and more horizontal, so certain sites near the “peaks of eternal light” and the “craters of eternal darkness” could see lunar horizon glow that persists for a very extended period of time, with dust dancing to the tune of electrical fields, easily visible to the unaided human eye, as shadows of terrain features sweep through the sky, dictated by the sun’s path.
We’ve never sent astronauts to these terrain features near the poles, nor to anywhere in the higher latitudes of the Moon; every Apollo landing site was at lower latitude, owing to them being easier to reach from the low-inclination lunar orbit that was employed. That’s planned to change with the new “Artemis” missions, which have as their prime objective the establishment of a permanent surface base on the Moon, with the lunar south pole being a favored region.
If you look at that sketch Gene Cernan made, and then you consider that the observation was made from lower latitude, where visually it would be shorter and less impressive than at the poles…well…you start to wonder if the Artemis astronauts on their future surface base might be in for a pleasant surprise.
It wouldn’t even be the first time; it’s widely reported, by Artemis and Apollo astronauts alike, that the Moon looks much less grey in person close-up than it does through a camera lens or from a distance (we can see the disk from Earth, but it’s still small enough that if you hold your thumb outstretched, that little thumb can block out your view of the planet). The human eye, once there, adapts and gradations of color can be discerned. Not vivid, but noticeable. Apollo astronauts on some missions rummaged around and uncovered even more color: distinctly “orange” soil.
A capture of a Shorty Crater landscape by Harrison Schmidt (Apollo 17, 1972), after astronaut footprints unearthed titanium-rich glass beads of volcanic origin, the famous “orange soil”. Notice the similarity to certain Earth deserts; bar the alien combination of black sky in full sunlight, the scene wouldn’t look too out of place in Death Valley.
From a human from behind a spacesuit visor, I’m sure the “lunar horizon glow” at its best would look beautiful, even spectacular. But the real mind-benders, I suspect, will come courtesy of photography. Oh, to a man on the ground, the twilight would hug the horizon, with the dominant sky color being pitch-black…but to a camera, zoomed in on the horizon glow? It would appear to fill the sky as far as you could tell from the image. And situate an astronaut or a piece of equipment just right, so as to appear normal-sized in the camera’s zoom lens as it captures a close-up view of lunar horizon glow, and you would capture some of the most stunning photography any planet has to offer: swirling dust in crepuscular rays, yet with silhouettes looking crisp to an utterly unearthly degree.
Motion is ballistic on the Moon, to boot, due to lack of atmosphere, so if you had a lunar transportation system going based on ballistic, nigh-suborbital short hops? Either for cargo or personnel, as is envisaged in some science fiction? You could see craft and men move in ballistic arcs in the sky, a sky that would be visible. That would look, for a short yet decisive time, like it had substance to it…only with the motion in the video revealing that this atmosphere is nothing like Earth’s.
Normally one assumes that the dusty haze wouldn’t have much color to it, or if it did it wouldn’t be much like Earth’s twilights…but the Moon has one more trick up its sleeve: what we on Earth call a “lunar eclipse”. When the Moon is inside the Earth’s shadow, it famously turns a reddish color, often called a “blood moon” by us. This is because a lunar eclipse as viewed from Earth corresponds to a total solar eclipse as seen from the Moon. The Earth moves in front of the sun, blocking its rays from reaching the lunar surface, turning day into night…only unlike when we get a solar eclipse, the eclipsing body in the Moon’s case has atmosphere. So there is a distinct ring around the limb of the planet that glows red, the combined color of every sunset and sunrise on the Earth. Naturally this is a reddish color, so the Moon is bathed in a dim red glow. Hence why it “turns to blood”.
Spectacular enough even on Earth, but consider that just as on a “normal” sunset, all that dust will be excited by nightfall, and if the eclipsing Earth is near the horizon, the usual crepuscular rays will illuminate the “lunar horizon glow”…only it will be distinctly, vividly red in color. Through the lens of your zoom camera, through a pair of binoculars, even, it would probably look eerily like a true Earth sunset (because, in a way, it is a true Earth sunset), only so alien as to defy terrestrial comprehension…unless, that is, you’re one of a small but hopefully ever-growing number of people who would have the privilege of personally witnessing one of the most obscure yet perhaps romantic natural wonders our solar system has to offer.
And it might come within the realm of human experience as soon as Artemis lands and establishes the first-ever foothold of mankind on another world. We think of the Moon as a feat where we’ve “been there, done that”, but the truth is the journey of discovery has only just begun…