Lunar Terminator Paradox
The author investigates the lunar terminator paradox, an optical illusion where the moon appears to point upward even when the sun is below the horizon. By developing a simulation program, the author demonstrates that this effect occurs because observers view the moon from below, revealing the dark side at the bottom edge. The article also notes that current AI models failed to correctly explain the phenomenon, highlighting limitations in their reasoning capabilities.
- ▪The lunar terminator paradox describes the counterintuitive appearance of the moon pointing up after sunset.
- ▪The author created a program to simulate the geometry of the sun, moon, and observer to clarify the visual effect.
- ▪The illusion is most pronounced with a full or nearly full moon located high in the sky.
- ▪The author argues that AI tools like Claude and Gemini lacked the reasoning ability to accurately explain the paradox.
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Record
| Original publisher | Secretsauce |
| Canonical URL | https://notes.secretsauce.net/notes/2026/09/27_lunar-terminator-paradox.html |
| Publication time | Sun, 27 Sep 2026 21:06:17 +0000 |
| Retrieval time | 2026-09-27T21:21:06.378Z |
| Last seen | 2026-09-27T21:21:06.378Z |
| Headline source | Publisher (no WeSearch rewrite) |
| Excerpt source | publisher body |
| Excerpt method | First ~120 words (~800 chars) of extracted publisher body, fair-use limited. |
| Summary | WeSearch · cerebras-chat (WeSearch summarizer) |
| Summary source text | contentText |
| Citation coverage | Summary is a WeSearch-generated derivative; primary citation is the original publisher URL. |
| Cluster | CuVQrWX_AfVY · 1 stories |
| Cluster logic | Grouped by semantic title/content similarity across sources within a rolling window. Same-publisher template collisions are excluded from coverage comparison. |
| Ranking reason | Story pages are not engagement-ranked. Hub feeds use recency, with optional source-diversified chronological ordering (cap consecutive stories per source). No personalized ranking. |
| Publisher visit | Yes — open original |
| Substitutes article? | No — link-out required for full text |
Rights status (four layers)
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| Indexing | May the item be indexed (stored, ranked, made findable)? | Allowed |
| Snippet | May a short excerpt of the publisher's text be shown? | Allowed |
| AI summary | May WeSearch generate its own short summary of the article? | Limited |
| Retrieval / RAG | May the content be exposed for third-party retrieval-augmented generation? | Not asserted |
| Model training | May the content be used to train AI models? | Not asserted |
| Commercial reuse | May the content be reused commercially? | Not permitted |
Basis: Derived from the published RSS/Atom feed. Contact: [email protected]. Reviewed: 2026-07-24.
Opening excerpt (first ~120 words) tap to expand
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: The moon is illuminated by the sun 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 isn't what always happens! 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.
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Excerpt limited to ~120 words for fair-use compliance. The full article is at Secretsauce.
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