Fooled by shadow play into thinking lunar mountains were pointy pinnacles? Learn why we often see them that way.
A few winters back, I noticed that a snowbank illuminated by a nearby yard light cast very long and pointy shadows. That seemed odd, as the bank consisted of rounded humps and knobs. But because the light struck it at a low angle, it greatly exaggerated the relief of each and every chunk, stretching bits of topography into long, sharp "teeth".
The effect was dramatic and immediately recalled nights at the telescope observing the spire-like shadows cast by lunar peaks and craters near the lunar terminator, the moving line separating daylight from darkness on the Moon.
Could the snowbank scenario be playing out on the Moon? At first blush, you wouldn't think so. With no atmosphere or running water to soften its hard edges, it's easy to imagine a jagged lunar landscape of pointy peaks just like what the shadows appear to show. Early depictions of the Moon's surface took their cues from telescopic observation, showing a rugged, forbidding landscape. As it turns out, this is an illusion.
The very fact that the moon is airless allows every bit of meteoric dust to zap the surface at tens of thousands of miles per hour. Over the 4.5 billion year lifetime of the moon, myriad micrometeorite impacts have acted like cosmic sandpaper, grinding down the once craggy peaks into the smooth hills and mountaintops so vividly seen in photographs returned from the Apollo missions.
Other erosional forces also come into play. The 500-degree difference between daytime and nighttime lunar temperatures undoubtedly acts to weaken and fracture rock. If I can hear the trees popping on a subzero night here at home, I’ll bet rocks shudder during the long lunar night when the temperature bottoms out around –240° F (–151° C) and shudder again when the Sun rises two weeks later and the temperature soars to 250 °F (123 °C).
And what of boulders perched atop crater walls and dotting the valleys and slopes? Loosened by temperature change and occasional moonquakes, they tumble downhill, further subduing lunar contours over time. No, our satellite turns out to be a softer-looking place than most had imagined, with mountains that more closely resemble the Appalachians than the Himalayas.
One of the great pioneers of space art, Chesley Bonestell, who painted breathtaking scenes of rockets landing on a rough and rocky Moon, grumbled when he saw the photos returned by the orbiting lunar probes and landers of the mid-1960s.
Someone asked Bonestell what he thought about the images. "I thought how wrong I was!" he said. "My mountains were sharp, and they aren't on the Moon. They're round, battered by millions of years of meteorites."
You can watch this shady business play out in the coming week as the Moon waxes from a thick crescent to a three-quarter gibbous. All you need is a small telescope. Use 50x or higher and examine the strip of craters and hills along the lunar terminator lit by the rising Sun.
For evening viewing, the 6-9 day old Moon is best because we face the terminator squarely and shadows display the least amount of foreshortening. But other times work too — just avoid the Full Moon. Keep your wits about you as you enter a land of shadow and illusion!
The Sky & Telescope Field Map of the Moon: the only moon reference you'll need when you're at the telescope!