Different points in the earth are at slightly different distances from the sun and hence experience different forces owing to gravitation. For a rigid body, we know that if various forces act at various points in it, the resultant motion is as if a net force acts on the c.m (centre of mass) causing translation and net torque at the c.m causing rotation around an axis through the c.m. For the Earth-Sun system (approximating the earth as a uniform density sphere)