Schematic representation of a meridional cut of a rotating star. The plane shown contains the line connecting the centre of the star to the observer and the rotational axis. In the text, the angle i is called the inclination angle (i = 0 when the star is seen pole-on). A given point on the surface is defined by the angle θ that we call the colatitude in the text. Note that in a deformed rotating star, the unit vector perpendicular to the surface is not aligned with the direction connecting the considered position to the centre of the star. The same is true for the effective gravity which is the classical gravity modified by the effect of the centrifugal force.