Fooled by shadow play into thinking lunar mountains were pointy pinnacles? Learn why we often see them that way.

Frank McCabe
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.

Bob King
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).

NASA
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.

NASA
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.

Damian Peach
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.

Virtual Moon Atlas - Patrick Chevalley, Christian Legrande
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!
Comments
Nzubenel
November 26, 2014 at 4:30 pm
Excellent post, Bob - Thanks!
Doug Z
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Bob KingPost Author
November 28, 2014 at 9:31 pm
Thank you Doug!
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Roger-Keith
November 29, 2014 at 2:44 am
Well, it would be good for the article to explain Why the shadows look so pointy. Is it the lunar roundness, or some other factor? It seems like the question is asked, but not answered.
Best regards and clear skies to all -
Roger
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Nzubenel
November 29, 2014 at 12:42 pm
Roger-Keith,
For me, it is an issue of angular perspective, and can be illustrated thus: If I stand in the sun with the shadow of my head (DEFINITELEY not a sharp pinnacle - though some may argue otherwise!) projected upon a flat surface ~ 4 feet away, the shadow subtends an angle of ~ 7 degrees. The same shadow ~ 20 feet farther away subtends ~ 1 degree. The farther away the shadow goes (due to a lower sun angle), the smaller my head's shadow gets. I we took an extremely high magnification look at the "pinnacle" shadows on the moon, they may not be as pointy as they appear at lower magnigications.
Doug Z
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Nzubenel
November 29, 2014 at 12:46 pm
Sorry about the bad spelling!
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Nzubenel
November 29, 2014 at 2:29 pm
Actually, a better way to illustrate this, because the observer is not where my possibly pinnacle-shaped head is, but rather what the observer would see in a right-angle view of the Earth's (or Moon's) shadow cast during an eclipse. The umbral - or full - shadow is a gradually narrowing cone, because the object casting the shadow (Earth or Moon, in this case), is so much smaller than the light source of the sun. The cone would eventually comet to a point - even though the Earth and Moon are both round and have no pinnacles at such a scale. Anyone care to do the math and tell us how far from the sun the shadow "cones" cast by the Earth and Moon would be, respectively?
DZ
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November 29, 2014 at 7:35 pm
Good article. its the perspective of the viewer seeing those shadows
so it makes sense that the best lunar viewing is at the quarters, or ninety
degrees. Its also when the apparent speed of terminator is greatest.
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