Look at a photograph from the Apollo missions and the strangest detail is not the gray dust or the slow-motion leaping; it is the sky. An astronaut stands in blazing sunshine, his white suit and the lunar surface bright enough to hurt your eyes, and directly above him the sky is pure black - not twilight blue, not dusk purple, black, at what was literally high noon for the landing site. The picture is not a defect of the film and not a trick of exposure. It is what a sky looks like when there is almost no air to fill it with light. On Earth, distant ridges turn blue because air scatters sunlight toward your eye; multiply that principle across the whole dome of the atmosphere and you get our daytime sky. Remove the air and you remove the glow. The Moon's surface is lit by the same sun that lights your street, but above the horizon there is nothing to light.
A sky is a lamp, not a curtain
The everyday intuition is that the sky is a blue ceiling with the sun mounted on it, and that nighttime space is what lies behind. Physically it is closer to the reverse. Sunlight arrives from the sun as a parallel beam, and as that beam falls through the atmosphere, air molecules intercept small fractions of it and re-radiate that light in all directions. Short wavelengths are redirected far more strongly than long ones, so blue light bounces around the dome of the sky preferentially, and wherever you stand, some of that redirected blue arrives at your eye from every patch of sky. The blue is not a surface; it is a glow filling the air above you, produced by scattering at every altitude. Space itself has no color in daylight - sunlight passing through vacuum does not scatter, does not spread, and does not glow. It only illuminates what it strikes.
You can watch this fill light at work in every shadow on a summer afternoon. The sunlit side of a wall blazes; the shaded side does not go black but glows a soft blue, because what lights that shade is skylight - sunlight that has already been scattered and re-aimed by the air overhead. Photographers build whole careers on this: open shade on a bright day is gently, evenly lit from the entire dome at once, which is why portraits taken there look soft. On the Moon, that fill light does not exist. A boulder's shadow is lit by little but the dim glow of sunlit ground around it, so the shaded side of a rock can run nearly as dark as the sky above it.
Almost no air, almost no glow
The Moon is not a perfect vacuum, but it is close enough for optical purposes. It wears an exosphere - a scattering of atoms so thin that the particles travel enormous distances before meeting one another, and the whole atmosphere is closer to none than to anything you would call air. Light crossing the lunar sky has essentially nothing to bounce off: no molecules to scatter it, no haze to diffuse it. So the beam of sunlight that misses the ground simply continues into space, and the sky between the stars stays dark while your boots are blindingly lit. The same absence reshapes everything on the surface. Shadows on the Moon are not soft blue-gray pools the way they are on Earth, because there is no scattered skylight to fill them in; they are hard, near-black shapes with knife edges. Photographs from the surface show exactly this - stark, theatrical contrast, with shaded crater floors going black while adjacent slopes glare.
Why the stars still hide
The black sky tempts people to ask why Apollo photographs show no stars, and the answer has nothing to do with the sky's color. Photographing a brilliantly sunlit surface requires fast exposures and small apertures, the same trade-off that turns a full moon into a white blob when you photograph it from your backyard; at those exposure settings, stars are simply too faint to register. Human vision faces its own version of the problem: with the sunlit ground dazzling your eye, your sensitivity is set for daylight, and dim starlight cannot compete. The lesson is that visibility is decided by contrast and adaptation, not only by the presence of light - darkness full of stars and darkness with none look identical to an eye calibrated for brilliance.
Running the experiment in both directions
Earth and Moon make a clean pair because they share a sun and differ almost entirely in atmosphere. Here, a thick scattering blanket gives us a luminous blue dome that washes out the stars all day, softens every shadow, and turns distant mountains faintly blue with distance. There, the absence of that blanket leaves the sun's work unsoftened: fierce highlights, black shadows, black sky. You can watch the same variable change within a single day on Earth, as the twilight after sunset passes through deep blues - tinted by ozone and geometry - before the last scattered light drains away and our own sky, too, goes black. The blue we live under is not the sky's default setting. It is the weather report of the air itself, and on the Moon there is no weather to report.