In a non-inertial reference frame, of which the motion is given, the differential equations describing the rotation of a planet around its center of mass can be derived either under the form of Euler type equations, or from certain relations between angular velocity vectors or even angles. Unfortunately these equations are not well adapted to an analytical integration. Nevertheless, with some simplifications, Ward (1973), and Christensen (1977), by considering the motion of the spin axis in a non-inertial reference frame attached to the moving orbital plane, have found very large periodic variations in the obliquity of the Mars planet over a period of the order 1.2 10+5 years, which would be of great importance to the climatic history of the planet.