Large-scale variations in the obliquity of the planet Mars are produced by a coupling between the motion of its orbit plane due to the gravitational perturbations of the other planets and the precession of its spin axis which results from the solar torque exerted on the equatorial bulge of the planet. The obliquity oscillates on a time scale of approximately 1.2 × 105 years. The amplitude of this oscillation itself varies periodically on a time scale of 1.2 × 106 years. The present-day obliquity is approximately 25.1 degrees. The maximum possible variation is from about 14.9 to 35.5 degrees. Significant climatic effects must be associated with the phenomenon.