Lalande was fascinated by astronomy and, although he continued to study law, he also attended Delisle's astronomy lectures at the Collège Royale as well as Pierre Lemonnier's lectures on mathematical physics.
Lalande ay nabighani sa pamamagitan ng astronomy, at, kahit na siya ay patuloy na pag-aaral ng batas, siya rin, nag-aral Delisle's astronomy lektura sa kolehiyo Royale pati na rin Pierre Lemonnier ng lektura sa matematika physics.