His mother, Ann Caird, was not married to his father, John Duncan, a weaver of Drumlithie, eight miles from Stonehaven, and she supported herself and the boy by harvesting and by weaving stockings.

His master, Charles Pirie, an ill-tempered man who had almost conquered the celebrated Captain Robert Barclay Allardice, carried on an illicit still and smuggled gin.

In 1824, Duncan became a travelling or household weaver, varying his work with harvesting, and taking a half-yearly spell of training as a militiaman at Aberdeen for nearly twenty years.

Duncan never possessed a watch after he left Aberdeen, but became an expert dialler, and made himself a pocket sun-dial on James Ferguson's model.

From his outdoor habits of astronomical observation he was nicknamed Johnnie Meen, or Moon, and also 'the Nogman', from his queer pronunciation of the word gnomon, which he often used.

In 1878, Mr. W. Jolly of Inverness, who had visited him the preceding year, gave an account of Duncan in Good Words, which brought him some assistance; in 1880 a public appeal was made on his behalf.

He died on 9 August 1881 in his eighty-seventh year, having left the balance of the fund raised for him to furnish prizes for the encouragement of natural science, especially botany, among the school children of the Vale of Alford.