Lunatic is an informal term referring to a person who is considered as mentally ill, dangerous, foolish or unpredictable, conditions once attributed to lunacy. The term may be considered insulting in serious contexts in modern times, but is now more likely to be used in friendly jest. The word derives from lunaticus meaning "of the moon" or "moonstruck". The term was once commonly used in law.
The term "lunatic" derives from the Latin word lunaticus, which originally referred mainly to epilepsy and madness, as diseases thought to be caused by the moon. KJV records "lunatick" in Mt 17:15 and Mt 4:24. By the fourth and fifth centuries[clarification needed] astrologers were commonly using the term to refer to neurological and psychiatric diseases. Philosophers such as Aristotle and Pliny the Elder argued that the full moon induced insane individuals with bipolar disorder by providing light during nights which would otherwise have been dark, and affecting susceptible individuals through the well-known route of sleep deprivation.[clarification needed] Until at least 1700 it was also a common belief that the moon influenced fevers, rheumatism, episodes of epilepsy and other diseases.
In the British jurisdiction of England and Wales the Lunacy Acts 1890-1922 referred to "lunatics", but the Mental Treatment Act 1930 changed the legal term to "person of unsound mind", an expression which was replaced under the Mental Health Act 1959 by "mental illness". "Person of unsound mind" was the term used in 1950 in the English version of the European Convention on Human Rights as one of the types of person who could be deprived of liberty by a judicial process. The 1930 Act also replaced the term "asylum" with "mental hospital". Criminal lunatics became Broadmoor patients in 1948 under the National Health Service (Scotland) Act 1947.
On December 5, 2012 the US House of Representatives passed legislation approved earlier by the US Senate removing the word "lunatic" from all federal laws in the United States. President Barack Obama signed this legislation into law on December 28, 2012.
The term lunatic was sometimes used to describe those who sought to discover a reliable method of determining longitude (before John Harrison developed the marine chronometer method of determining longitude, the main theory was the Method of Lunar Distances, advanced by Astronomer Royal Nevil Maskelyne). The artist William Hogarth portrayed a "longitude lunatic" in the eight scene of his 1733 work A Rake's Progress. Twenty years later, though, Hogarth described John Harrison's H-1 chronometer as "one of the most exquisite movements ever made."