He modestly entitled his work a Gnomon or index, his object being rather to guide the reader to ascertain the meaning for himself, than to save him from the trouble of personal investigation.
The building of the Gnomon inside the Church of Saint-Sulpice occurred at a time when Rome was relaxing its stance against the theories of Galileo Galilei, as his works were being printed in Rome with the agreement of the Holy See, and in 1757 the Pope removed the works of Galileo from the Index Librorum Prohibitorum.
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Le Monnier further used the gnomon from 1744 to establish the variations of the ecliptic, or the variations in the obliquity of the Earth's axis.