Where'd All the Craters Go?
Today the Moon has just passed it’s full phase, and if you take a look at it after sunset, you may notice something peculiar.
You have probably noticed that at various points over the past two weeks as the Moon has gone from thin crescent to fully illuminated, that it seemed like there were craters and ridges visible. It’s easy to take for granted, like forgetting what a penny looks like when asked to recall on the spot.
Don’t worry, nothing’s wrong with your observations. When the Moon is at partial crescent phases, the line that separates light from dark (called the terminator) is the point on the Moon where sunset (or sunrise, depending on which side of the full moon) is occurring. And just like on Earth, the shadows grow longest at sunrise and sunset when the Sun’s light hits at the steepest angle, allowing even small features to cast long, pronounced shadows.
This is what’s happening on the Moon as the terminator gradually moves across the disc of the Moon. On any given night, the terminator line will have the most striking features visible, as the small elevation changes caused by mountain ridges and craters cast relatively large shadows which makes them stand out quite drastically on the lunar surface.
Conversely, at Full Moon, the Sun’s light is hitting the lunar surface from directly above, which minimizes and all but washes out the shadows from even the most pronounced features in the same way shadows on Earth at high noon appear to shrink to their smallest size.
When we look at the Moon leading up to and just after the full phase, it’s almost impossible to detect the depth in lunar features, we’re only able to see the contrasting seas (or ‘mare’) made of sediments with different composition with slightly different reflectivity, causing some regions to appear darker than others.
Tonight’s image is taken by Andrew McCarthy is a clever composite image. To produce the entire disc of the Moon, he took images of the terminator as it moved gradually each night and blended the images together across the entire face. The resulting image is actually impossible, you’d never be able to see all the craters simultaneously in steep shadow with the Moon’s disc fully illuminated.
Composite image of the terminator spread out across the entire lunar disc, resulting in impossible shadows, but much higher contrast
Compare it to an image of the nearly full Moon taken from the International Space Station last month.
Moon captured from the International Space Station, April 6, 2020
To read more about this unique composite image, check it out here:
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