Viewing geometry: a stellar feature represented by the dark blue point moves as the star rotates and is visible only a fraction of the time. It modifies the local emission, and velocity of the plasma, which appears to move outwards because of its convective motion. We parametrize the position of the feature with the angle between the sky plane and rotation axis, $\overline{i},$ the longitude of the feature, ϕ, and its latitude, δ. As a remark, $\overline{i}=\pi /2-i$ where i is the inclination classically used in the projected mass and velocities m sin i and V sin i.