clockwise motion. The general day formula for non-extreme axial tilt is P_rev x P_rot P_day = ---------------------- . P_rev - P_rot In the case of Mercury , P_rot = (2/3) P_rev and both are positive. Thus (2/3) P_rev**2 P_day_Mercury = ------------------------- = 2 x P_rev . P_rev - (2/3) P_rev The