Schematic representation of the orientation of one of Gaia's detectors (red arrow) compared to a star’s direction (green arrow). The solid circle represents the celestial sphere as seen from the Gaia center of mass, and the dashed-line circle represents the celestial equator. The two quadrilaterals represent Gaia's preceding (light red) and following (light yellow) FoV detectors. On the bottom right, we show the possible location of the star on the detector and the η angle that is measured projected along the AL axis (u_AL). Arbitrary north and east directions are shown with the definition of the PA of the AL direction. They are not intended to exactly correspond to the top-left drawing but allowed us to define θ_AL, the eastward-oriented angle between u_AL and the north.