The heliocentric path of the Moon

In his Astronomie populaire, Camille Flammarion points out that the heliocentric path of the Moon, which, according to him, has generally been represented as a sinuous curve, is actually concave everywhere towards the Sun. Flammarion’s observation is the starting point of this study which goes backwards in time, via often misinformed authors, to the mathematician who first established this counterintuitive property by means of a purely geometrical proof. The story also includes a heated debate between readers of a British periodical. Beginning in France at the end of the 19th century, the journey finishes in Scotland in the first half of the previous century.