The ratio of the mass of the earth to the mass of the moon in M_e/m_m = 81.3. The radius of the earth is about 6370 km, and the distance from the earth to the moon is about 384,000 km. 

(a) Locate the center of mass of the earth--moon system relative to the surface of the earth. 

(b) What external forces act on the earth- moon system? 

(c) In what direction is the acceleration of the center of mass of this system? 

(d) Assume that the center of mass of this system moves in a circular orbit around the sun. How far must the center of the earth move in the radial direction (toward or away from the sun) during the 14 days between the time the moon is farthest from the sun (full moon) and the time it is closest to the sun (new moon)?