Lunar Terminator Paradox
A developer writes a simulation to explain the lunar terminator paradox, prompting commenters to point out fundamental misunderstandings of 3D celestial geometry.
summary is enough Skim the short post to see the author's premise, but rely on the comment section for clear explanations of spherical astronomy.
summary
The author attempts to decode the optical illusion where the moon appears to point away from the setting sun by building a custom visualization program. Commenters quickly push back, arguing that the author conflates lunar phase with elevation and struggles with basic 3D perspective. Rather than a genuine paradox, the discussion clarifies that the phenomenon is simply how spherical illumination looks from a rotating Earth. The thread ultimately provides much clearer mental models than the original post.
key points
- The author built a simulation program to resolve confusion over why the moon's illuminated edge sometimes points upward at sunset.
- Commenters pointed out that the author conflated lunar phase and elevation, creating a nonexistent paradox out of standard 3D perspective.
- The comment thread corrects the premise using spherical geometry explanations and external resources like Vsauce videos.
who should read it
Programmers and amateur astronomers interested in spatial visualization and celestial mechanics.
discussion skeptical
consensus
Commenters largely agree there is no paradox, attributing the author's confusion to ambiguous terminology and a flawed mental model of 3D space.
This isn’t really a paradox if you just visualize the Moon as a sphere and visualize where the Sun is shining from, and I think that’s what the plots in OP demonstrate.
The “paradox” might be in the initial assertion which does not use an accurate mental model: “After sunset, the sun is below, so we would expect the illuminated portion of the moon to point down, towards where the sun is.” This assertion might be true if at sunset the Sun ducks just behind the horizon, around the same distance or closer than the Moon is to the Earth. In reality, the Sun is of course much farther from Earth …
It cleanly explains that the false paradox stems from imagining the sun's distance as comparable to the moon's instead of factoring in the scale of the solar system.
Generated by an LLM from the article and thread. Check the sources before relying on it.