Riemann was the first to talk about elliptic geometry: Bolyai and Lobačevskij (even Gauss too) studied only hyperbolic geometry. But of course some theorems of (planar) elliptic geometry were known since more than fifteen centuries, since the surface of a sphere is a space of constant positive curvature even if it's not a real model of elliptic geometry.
Did Riemann actually notice this? If not, who was the first to point out the equivalence?
[EDIT: I tried to explain what I meant. The author of the approved answer read anyway into my mind!]