When we watch Mercury moving relative to the stars, we use the term prograde (Eastward motion) and retrograde (Westward motion). Which of the following statements is true about Mercury's motion?
1) Mercury moves slowly during prograde motion and quickly during the retrograde motion.
2) Mercury moves slowly during retrograde motion and quickly during the prograde motion.
3) Mercury moves the same speed at all times.
4) Mercury alternates speeds as it orbits the sun, because it has a highly elliptical orbit.