Different points in the earth are at slightly different distance from the sun and hence experience different force due 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 `CM` (centre of mass) causing translation and a net torque at the `CM` causing rotation around an axis through the `CM`. for the earth-sun system (approximating the earth as a uniform density sphere).