The Mountains of Eternal Light

Years and years ago, back when I used to know everything, I thought that shadows in a vacuum would be totally, absolutely black, there being no air to "bend" light into the shadowed areas. This misconception was not helped by observing the Moon. The inky blackness of the lunar shadows gave the impression that, were you to be standing in the middle of one of them, you would be unable to see the ground beneath your feet.

I was also led astray by no less than the venerable science fiction author Arthur C. Clarke, whose every word I once took as the Gospel Truth. In his (wonderful) novel, Islands in the Sky, astronauts spacewalking outside a space station would disappear from view whenever they ventured into the shadow of the station, not reappearing until they were once again directly illuminated by the Sun. How Clarke made such a colossal error, I nowadays fail to understand. There would surely be enough reflected light from the surface of the Earth to light up anything in the space station's shadow. And in fact, such is the case.

Astronaut Shane Kimbrough spacewalking in the the shadow of the ISS, plainly visible

In like manner, the shadowed areas on the Moon would be illuminated by Earthlight. This becomes obvious when you realize that you can see the night side of the Moon, even from the Earth, when the Moon is a slender crescent in the evening sky.

Nevertheless, the deep shadows of the taller lunar peaks must be dark indeed - especially near the poles, where the Earth would be just a degree or two above the horizon and shedding very little light indeed onto the floor of craters such as the nearly 3 mile deep Shackleton, which is located directly at 90° South. (There are, in fact, regions of that crater's floor which never receive direct light from either the Sun or the Earth, and which radar observations indicate are sheathed in perpetual ice.)

By contrast, there are portions of that crater's rim which are in perpetual sunlight - they are never in shadow. This was first postulated by the 19th Century lunar mapmakers Beer and Madler. The French astronomer Camille Flammarion gave them the irresistibly poetic name pics de lumière éternelle (The Mountains of Eternal Light). Their existence was positively confirmed by NASA's Clementine spacecraft in 1994. The classic space artist Chesley Bonestell painted a hauntingly beautiful image of them for Willy Ley's 1951 book The Conquest of Space (I have a disintegrating copy in my library - it's on page 76). He pictures them bathed in red light during a total lunar eclipse. Off in the lower right corner, you can just make out the tiny figures of lunar explorers gazing in wonder at the sight. (I unsuccessfully tried to find the painting on the internet, but you'll have to ask me to show it to you in person.)

Well, you too can gaze in wonder at those same mountains - tomorrow evening in fact. Shortly after sunset on 20 January, the Moon will be ideally positioned to spot them, even in a small telescope. Slide your view along the 13 percent illuminated crescent until you get to the southern tip. Just beyond the end of the crescent, you will see a number of disconnected points of light. It's that easy - there they are!

Note the point of light just beyond the edge of the crescent. 

(South is to the top in this image.)

Click on image to get full resolution.

But don't just look at them. Imagine yourself standing at their feet, in an ice-covered landscape so dark that it might as well not be there. And towering miles over your head are the tops of peaks glaringly lit up in direct sunlight. The contrast would be so great that they would likely appear to be floating unsupported in mid-space. Now what a sight that would be!