Methodology

MoonAlibi fact-checks things blamed on the full moon using public statistics. The math is rigorous; the display is one word. This page discloses exactly how that word is decided.

Defining a "full moon day"

• Moon phase is treated as a continuous value. For each event we measure the elapsed time from the nearest full moon instant (the syzygy moment, in UTC)
• A "full moon day" = within ±24 hours of the full moon instant (a 48-hour window). Same for new moons
• Phase instants are computed in-house using the algorithm from Jean Meeus, "Astronomical Algorithms", chapter 49 (no external API)
• All astronomy is computed in UTC; only the display is localized

Several famous "lunar effect" studies have been criticized for vague definitions of the full moon day — move the window arbitrarily and you can manufacture any conclusion. We therefore fix the definition and publish it here.

Verdict criteria

1. We compare the observed event count against the count expected if events were uniform with respect to moon phase 2. We compute a 95% confidence interval for the observed/expected ratio (Poisson approximation) 3. If the interval contains 1.00 (no difference), the verdict is "No difference" — unless the interval is too wide to be informative (half-width above ±10%, meaning even a ±10% effect could not have been detected), in which case the verdict is "Not enough data" rather than a claim of no difference 4. If the interval excludes 1.00, we grade by effect size: under ±3% = "Practically none", ±3–10% = "Slightly higher/lower", over ±10% = "Higher/Lower"

"No difference" reports an observation; it is not proof that no difference exists. Statistical tests cannot prove absence, which is why we avoid wording that asserts it.

Not printing the confidence intervals and effect sizes on every page is a deliberate design choice — the same reason a rain forecast shows "70%" rather than its internal model outputs. The criteria are fixed and published here, which is what keeps the verdicts honest.

Per-topic adjustments

Human-activity data (accidents, births) carries strong weekday, seasonal, and holiday patterns, so those topics will use a "same weekday × same month" baseline as the expected value (details will be published with each topic).

Earthquakes do not follow the human calendar, so no weekday adjustment is needed — instead we remove aftershocks (declustering) and apply a magnitude floor (M6.0). Details are on the earthquakes page.

Reproducibility

• The generator recomputes every verdict daily with the same procedure
• The same input data always produces the same output (idempotent builds)
• If a verdict label changes, the operator is notified and reviews the computation

Sources

• USGS Earthquake Catalog (U.S. Geological Survey, public domain)
• Moon phases: computed in-house from Jean Meeus, "Astronomical Algorithms"