Position of the surface element on the asteroid body. The big circle represents the asteroid; the north pole is at the top, the equator in the middle. The axes x₁ and z₁ are linked to the surface element from Fig. 2. The latitude ψ is defined as the angle between the equatorial plane and the global normal to the surface −y₁. The axes x₂, y₂, z₂ are defined in the asteroid’s body-fixed frame. The axis z₂ is aligned with the rotational axis of the asteroid, whereas x₂ and y₂ are aligned with its equatorial plane and co-rotate with the asteroid.