Geometry of the Gaia NSL on the celestial sphere, with ecliptic north at the top. The scanning motion of Gaia is represented by the red dashed line. The precession of the spin axisdescribes the two cones, aligned on the solar–anti-solar direction, with an aperture of 90°. As a consequence, the scan plane, here represented at a generic epoch, is at any time tangent to the cones. When the spin axis is on the ecliptic plane, Gaia scans the ecliptic perpendicularly, at extreme solar elongations. The volume inside the cones is never explored by the scan motion.