In a previous post, I explained how all the manned moon landings were made with the sun low in the sky behind the Lunar Module, so that long shadows accentuated terrain features, making it easier to locate a safe place to land. But this meant that the LM landed facing into its own shadow, so that the astronauts descended the ladder to the surface in the shade of their own vehicle. It seems as if they should have been fumbling around in the dark, to a large extent, because there is no air on the moon to scatter light into shadowed areas. But, as you can see from the Apollo 11 photograph above, although the shadows on the ground appear very dark, the shadowed face of the LM is quite well illuminated. That light is being reflected from the lunar surface, but it’s being reflected in a peculiar and interesting way, which is what I want to talk about here.
Take a look at this picture of an area of cropland, taken from a balloon.
The area around the balloon’s basket appears strangely bright compared to the rest of the field. In fact, this patch of brightness is centred on the antisolar point of the camera taking the photograph—it’s directly opposite the sun.
What’s happening is called shadow hiding. The field of standing grain is full of shadows cast by the plants, so in most directions we can see a mixture of sunlight and shade. But when we look directly down-sun, we see only the illuminated surfaces of the plants—they hide their own shadows from our view. So the region around the antisolar point appears bright, compared to the rest of the field where shadows are visible.
A high vantage point above a field of vegetation is a good way to see this effect. A person looking down into the field will see the bright patch concentrated around the shadow of their head, and that gives the phenomenon another name—it’s called heiligenschein, German for “holy light”, because the bright patch resembles the depiction of a halo around a saint’s head. In particular, the shadow-hiding version of the effect is called dry heiligenschein—there’s also a “wet” version that occurs when drops of water (for instance, beads of dew) act as retro-reflectors of the kind I discussed in my post about signalling mirrors.
So what’s the relevance to the moon? The moon is largely covered in a layer of compacted rocks and dust called regolith, the product of billions of years of meteor impacts. This surface has never been weathered by the action of air and water, and so is jagged, on the small scale, beyond anything we commonly encounter on Earth. So it produces exactly the same sort of shadow-hiding heiligenschein as a field of wheat on Earth.
We knew about this long before we went to the moon, for two reasons. The first is that the full moon is so very much brighter than the half-phase moon—ten times brighter, rather than just the factor of two you might expect. The second is that the full moon looks like a uniformly illuminated flat circle, rather than a sphere—this is because the edges of the full moon appear just as bright as the centre of its disc.
We’re used to surfaces that are inclined to our line of sight (like those around the edge of the moon) reflecting less light than transverse surfaces (like the middle of the lunar disc), but the moon’s surface doesn’t seem to obey that rule. Both of these effects are explicable in terms of dry heiligenschein—the whole full moon is behaving like that bright patch of shadow-hiding cropland.
The way in which the moon’s surface brightens dramatically when it is opposite the sun in the sky is called the opposition surge. It’s also sometimes called the Seeliger effect, after Hugo von Seeliger, an astronomer who used a similar opposition surge in the brightness of Saturn’s rings to deduce that the rings consisted of multiple self-shadowing particles.
One thing we didn’t know, until the Apollo missions reached the moon, was how bright the exact antisolar point on the moon’s surface would look. Here on Earth, we can never see the brightly illuminated full moon exactly opposite the sun, because in that position it is eclipsed by the Earth’s shadow. But when Apollo 8 went into orbit around the moon, the astronauts were able to look down on the illuminated moon’s surface with the sun precisely behind them. A publication in the Astronomical Journal soon followed*, showing a distinctive sharp peak in reflectance close to the antisolar point.
So when the Apollo 11 astronauts landed on the moon, they were already expecting to see bright dry heiligenschein on the regolith around them—in fact, some time was set aside in their busy schedule for them to record their observations of this “zero phase angle” effect. Here’s a view of the checklist attached to Aldrin’s spacesuit glove, reminding him to check and describe the reflectance of the lunar surface “UP/DOWN/CROSS SUN” during his time on the lunar surface:
And the effect was immediately obvious. Here’s a view of the lunar surface and the shadow of the Lunar Module, taken from the right-hand window of the LM shortly after landing:
That bright reflection coming from around the shadow zone is bouncing straight back, like a spotlight, to illuminate the shadowed face of the LM. And as the astronauts moved around the surface, they continually observed a patch of “holy light” around the shadow of their helmets. We can actually see what this heiligenschein halo looked like, if we zoom in on Armstrong’s famous portrait of Aldrin†:
Examining the reflection in Aldrin’s helmet visor, we can see (among other interesting things) his long shadow stretching off into the distance, and the patch of heiligenschein he was able to observe around his own head. Here’s what he reported at Mission Elapsed Time 110 hours, 28 minutes:
As I look around the area, the contrast, in general, is … comes about completely by virtue of the shadows. Almost [garbled] looking down-Sun at zero-phase very light-colored gray, light gray color [garbled] a halo around my own shadow, around the shadow of my helmet.
Then, as I look off cross-Sun, the contrast becomes strongest in that the surrounding color is still fairly light. As you look down into the Sun [garbled] a larger amount of [garbled] shadowed area is looking toward us. The general color of the [garbled] surrounding [garbled] darker than cross-Sun. The contrast is not as great.
(Aldrin, you’ll notice, had recurrent problems with his comms during lunar surface activities.)
But the moon was really just our first extraterrestrial encounter with this effect. We now know that heiligenschein is common on the airless, meteor-battered worlds of the solar system, most of which are surfaced with regolith like the moon’s.
It’s an absolutely perfect illustration of the sort of self-shadowing surface that produces heiligenschein.
* Pohn, HA, Radin, HW, Wildey, RL. The Moon’s photometric function near zero phase angle from Apollo 8 photography. Astrophysical Journal (1969) 157: L193-L195
† If the image looks a little unfamiliar, that’s because it’s the rather poorly composed picture direct from Armstrong’s Hasselblad camera. The story of how that original image AS11-40-5903 was processed to produce the more familiar (and now thoroughly iconic) Aldrin portrait is told here.