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.
[6] 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., Robert Augustus Johnson, James Keir, Joseph Priestley, William Small, Jonathan Stokes, James Watt, Josiah Wedgwood, John Whitehurst and William Withering.
[21] This lack of a defined membership has led some historians to criticise a Lunar Society "legend", leading people to "confuse it and its efforts with the general growth of intellectual and economic activities in the provinces of eighteenth century Britain".
[24] Darwin was a physician and poet who had studied at Cambridge and Edinburgh; Boulton had left school at fourteen and started work in his father's business making metal goods in Birmingham at the age of 21.
His arrival with a letter of introduction to Matthew Boulton from Benjamin Franklin was to have a galvanising effect on the existing circle, which began to explicitly identify itself as a group and actively started to attract new members.
[25] Another new recruit, Richard Lovell Edgeworth, met Darwin, Small and Boulton in 1766 through a shared interest in carriage design, and he in turn introduced his friend and fellow Rousseau-admirer Thomas Day, with whom he had studied at Corpus Christi College, Oxford.
Joseph Priestley, then living in Leeds and a close friend of John Michell, became associated with the Society in 1767 when Darwin and Wedgwood became involved with his work on electricity.
[5] In the same year James Watt visited Birmingham on the recommendation of his business patron John Roebuck, being shown around the Soho Manufactory by Small and Darwin in Boulton's absence.
[34] If William Small's arrival in 1765 had been the catalyst to the development of the Lunar Circle as a cohesive group, his death – probably from malaria[7] – in 1775 was to mark another change in its structure.
[35] John Whitehurst's move to London in 1775 had a less dramatic effect: he kept in regular contact with other members of the society and remained an occasional attender of meetings.
"[36] This reliance on Boulton was also to prove a weakness, however, as the period coincided with the peak of his work building up his steam engine business and he was frequently absent.
[39] The botanist and physician Jonathan Stokes, who had known William Withering as a child, moved to Stourbridge and started attending Lunar Society meetings from 1783.
His contribution to the society was significant but short-lived: after collaborating with Withering on his Botanical Arrangement of British Plants the two quarrelled bitterly and Stokes severed his relations with the main Lunar members by 1788.
[41] Most significantly, Erasmus Darwin moved to Derby in 1781, but although he complained of being "cut off from the milk of science", he continued to attend Lunar Society meetings at least until 1788.
[44] Joseph Priestley himself was driven from the town, leaving England entirely for the United States in 1794, William Withering's house was invaded by rioters and Matthew Boulton and James Watt had to arm their employees to protect the Soho Manufactory.
[45] Regular meetings are recorded into the nineteenth century – eight in 1800, five or six before August 1801 and at least one in 1802,[6] while as late as 1809 Leonard Horner was describing "the remnant of the Lunar Society" as being "very interesting".
[51] In the latter part of the 20th century, the University of Birmingham Lunar Society met every Thursday to debate and discuss all manner of topics in the Guild bar.