Schematic of a spectroscopic transit observation of an exoplanetary system. The difference between an out-of-transit spectrum (left) and an in-transit spectrum (middle) gives us two essential quantities: the exoplanetary atmosphere transmission spectrum and the local stellar spectrum of the surface that is hidden behind the exoplanet (right). The temporal evolution of the stellar local spectrum will mostly depend on the projected stellar rotation (represented by the black arrow going through the star from pole to pole), the planetary orbit parameters (represented with the green arrows) and particularly the projected spin-orbit alignment angle. The temporal evolution of the exoplanet atmosphere spectrum will mostly depend on the atmosphere size, the planetary orbital projected velocity, and the local stellar spectrum that is transmitted through. Hence, it is necessary to separate and to measure accurately each component to retrieve the unknown parameters. The present figure is partially inspired by figures in Dravins et al. (2015, 2017); Dumusque et al. (2014); Cegla et al. (2016); Bourrier et al. (2017a).