De revolutionibus orbium coelestium
The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of "De revolutionibus orbium coelestium" in 1543. Until the discovery of the Galilean satellites in 1610 there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as "Planetæ" ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.