In his doctoral thesis, written in 1868, Tisserand extended Delaunay's work on the three-body problem on the orbit of the Moon. This work was a great contribution to mathematical astronomy coming after the publication of the second volume of Delaunay's work on lunar theory "La Théorie du mouvement de la lune" which had been published in 1867. Tisserand was then appointed as an assistant-astronomer at the Paris Observatory. This was an unfortunate time at the Paris Observatory since its Director Le Verrier had become very unpopular following his drive for efficiency and attempts were being made to have him removed. In 1870 Delaunay was appointed as Director of the Paris Observatory to replace Le Verrier. Tisserand showed remarkable abilities in his work at the Observatory and it was clear that he would soon achieve an elevated position in the world of astronomy. …
