No difference

The folklore that "crime rises on full-moon nights"

This is one of the most stubborn of all moon myths. The English word *lunatic* comes from *luna*, Latin for moon: the full moon was thought to unsettle the mind and stir up violence. Among police officers, paramedics, and nurses, the sense that "full-moon nights are busy" is still passed down on the job. That isn't a superstition to laugh off — it's the lived experience of people who work the front line every night. Which is exactly why it deserves a square look at the data.

How the verdict is computed

• The data is daily counts from two U.S. cities' police open data: Chicago (all crime, since 2001) and New York (NYPD complaint records, since 2006) — incident-level public records
• Crime counts have strong weekday patterns (more on weekend nights) and seasonality (more in summer), so the expected value is the average of the "same weekday × same month," and we compare actual ÷ expected as an index (the same adjustment as the accidents and births topics)
• Reporting practices and population shift the level over the years, but normalizing by "weekday × year" keeps that out of the full-moon vs new-moon comparison
• Using the lunar phase at each city's local noon, we classify full-moon days (within ±24h of the instant) and new-moon days, and compare each group's mean index against normal (1.00)

See the methodology for the verdict thresholds.

Reading the data carefully

• These are *reported* incident counts, not true incidence; unreported crime is not included
• This does not dismiss the front line's "full-moon nights are busy" feeling. The page tests only whether recorded counts are visibly higher on calendar full-moon days. Much of the lived impression is also explained by memorability — what happens on a full-moon night is what you remember

Sources

• Chicago Data Portal — Crimes, 2001 to Present
• NYC Open Data — NYPD Complaint Data Historic
• Moon phase instants computed in-house from Jean Meeus, "Astronomical Algorithms" (UTC)