There is a weak positive feedback mechanism between the astronomy and meteorology of Mars. The mechanism is this: the seasonal waxing and waning polar caps cause small changes in Mars' dynamical flattening. Because the changes in flattening are out of phase with the Sun, there is a net annual solar torque on the planet which increases the angle between the equatorial and orbital planes. On the basis of Viking observations of the present climate and simple atmospheric models of past climates these seasonal shifts of mass between the atmosphere and polar caps are capable of secularly increasing Mars' obliquity by about 1¿ or 2¿ since the origin of the solar system. Thus the climate, driven largely by the axial tilt, reacts back on the planet and slightly enhances the seasons on Mars as time progresses. More sophisticated models will probably not change this result much; therefore, this mechanism probably produced only minor changes in Mars' climate. It causes negligible changes in the axial tilt and climate of the Earth. ¿ American Geophysical Union 1992