The Lunar Society of Birmingham was a dinner club and informal learned society of prominent figures in the Midlands Enlightenment, including industrialists, natural philosophers and intellectuals, who met regularly between 1765 and 1813 in Birmingham, England. At first called the Lunar Circle, "Lunar Society" became the formal name by 1775. The name arose because the society would meet during the full moon, as the extra light made the journey home easier and safer in the absence of street lighting. The members cheerfully referred to themselves as "lunarticks", a pun on lunatics. Venues included Erasmus Darwin's home in Lichfield, Matthew Boulton's home, Soho House, Bowbridge House in Derbyshire, and Great Barr Hall.
The Lunar Society evolved through various degrees of organisation over a period of up to fifty years, but was only ever an informal group. No constitution, minutes, publications or membership lists survive from any period, and evidence of its existence and activities is found only in the correspondence and notes of those associated with it. Historians therefore disagree on what qualifies as membership of the Lunar Society, who can be considered to have been members, and even when the society can be said to have existed.Josiah Wedgwood, for example, is described by some commentators as being one of five "principal members" of the society, while others consider that he "cannot be recognized as [a] full member" at all. Dates given for the establishment of the society range from "sometime before 1760" to 1775. Some historians argue that it had ceased to exist by 1791; others that it was still operating as late as 1813.
Despite this uncertainty, fourteen individuals have been identified as having verifiably attended Lunar Society meetings regularly over a long period during its most productive eras: these are Matthew Boulton, Erasmus Darwin, Thomas Day, Richard Lovell Edgeworth, Samuel Galton, Jr., James Keir, Joseph Priestley, William Small, Jonathan Stokes, James Watt, Josiah Wedgwood, John Whitehurst and William Withering.