luna

Lunacy comes directly from Latin luna (the Moon), through Late Latin lunaticus (moon-struck, periodically insane), from the belief that mental illness waxed and waned with the lunar cycle. The luna root gives English lunar, lunatic, lunate (crescent-shaped), lunula (the small white crescent at the base of a fingernail), and lune (an archaic word for a fit of lunacy, as well as a geometric figure bounded by two arcs). Luna was also the name of the Roman goddess of the Moon, daughter of the Titans Hyperion and Theia in some traditions and a separate deity in others, worshipped particularly at the full Moon. The word shares a Proto-Indo-European root with Greek selene (the Moon), with English 'light' and 'lucid,' and with the word 'luminous' — all rooted in the PIE *leuk-, to shine. The Moon was the shining one, and lunacy was the madness it caused.

The belief that the full Moon causes or worsens mental illness is among the most widely distributed and persistent folk convictions in recorded history. It appears in ancient Mesopotamian and Egyptian medical texts, in Greek and Roman medical writing, in medieval European medical jurisprudence, and in contemporary surveys showing that medical staff expect more disturbed behavior on full Moon nights. The Roman jurist Ulpian (170–223 CE) defined a lunatic in terms relevant to legal competence: a lunatic was someone whose madness was intermittent rather than constant, and the intermittency was attributed to lunar influence. English common law subsequently distinguished 'lunatics' — those with intermittent madness — from 'idiots,' who were considered permanently incapacitated, a distinction that persisted in legal terminology into the twentieth century.

Paracelsus, the Swiss-German physician and alchemist of the sixteenth century, articulated the theory with unusual precision: the Moon, he argued, draws the fluids of the brain toward itself as it draws the tides, and minds weakened by illness or disposition are overwhelmed by this attraction. The tidal analogy was intellectually serious — if the Moon could move the ocean, why not the water in the human skull? The English word lunatic appears in the King James Bible (Matthew 4:24), where it translates the Greek selēniazomenos (moon-struck, literally 'mooned'), used to describe people healed by Jesus who exhibited convulsive episodes. The identification of epilepsy and other seizure disorders with lunar influence was widespread in ancient medicine.

The scientific investigation of the lunar madness hypothesis has been conducted seriously enough that high-quality studies exist — and consistently find no correlation between the full Moon and psychiatric admissions, emergency room visits, psychiatric incidents in forensic hospitals, or any other measure of disturbed behavior, after controlling for the confounds of increased light on full Moon nights and human expectation bias. The belief persists despite disconfirmation, which is itself instructive: lunacy may be less about actual lunar effects and more about humans' tendency to notice and remember apparent confirmations while forgetting disconfirmations. The Moon did not cause madness, but the belief that it did shaped law, medicine, and language for two thousand years — and the word lunacy preserves that history in four syllables, long after the theory was abandoned.