Reference frame employed.  { u_i }  is the basis of the equator J2000 reference frame (explained in the text). O_b represents the position of the solar center of mass, which we chose as origin. For convenience, we introduced a new coordinate frame (O_b,  { e_i } ). This system uses as z-axis the solar rotation axis Ω. Here δ and α are the declination and right ascension of the solar rotation axis at epoch J2000.