In December 1877, Emmanuel Liais appointed Cruls to the Commission on Longitude, a position at the Imperial Observatory. (After Brazil became a republic in 1889, the Observatory's name was changed to Observatory of Rio de Janeiro, and in 1909, to the National Observatory of Brazil, the name by which it is known today.) Over the next two years, under the tutelage of Liais, Cruls produced several works that helped to build his international reputation as an astronomer: