Lunar Terminator Paradox
An author explores the optical illusion known as the lunar terminator paradox, where the moon appears to point upwards at sunset even when illuminated from the side. Through programming simulations, they discovered this visual anomaly occurs because observers look up at the moon from below rather than straight across.
Why it matters
It explains a common everyday optical illusion using amateur programming and geometry, satisfying scientific curiosity about celestial mechanics.
There's an optical illusion known by names like the "lunar terminator paradox". This has much discussion and descriptions online, but none made sense in my head, so I wrote this program to figure out what was going on. The paradox:
I was camping in the mountains with a friend recently. We were looking up at the sky, just after the sun has set. The moon was clearly pointing up.
After playing with this program , I have clarity: we're looking up at the moon from below. The most pathological case is a full moon. Let's say the moon is on the horizon: elevation=0. If we're looking at this low moon, and the moon is full, the sun is directly behind us. The moon is fully illuminated, and we see 100% of the moon disc lit up.
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