a person or thing that retracts. Anatomy. a muscle that retracts an organ or protruded part. Surgery. an instrument or appliance for drawing back an impeding part, as the edge of an incision. a mechanism, device, or the like that regulates retraction: to adjust the retractor on a seat belt.
The Moon orbits Earth in the prograde direction and completes one revolution relative to the stars in approximately 27.323 days (a sidereal month). Earth and the Moon orbit about their barycentre (common center of mass), which lies about 4,600 km (2,900 mi) from Earth's center (about 34 of the radius of Earth).
Apollo 8 took 69 hours, 8 minutes to enter orbit around the moon. Apollo 11, which placed the first humans on the moon, took 75 hours and 56 minutes to enter orbit around the moon. Long before they entered orbit, however, both spacecraft entered the moon's sphere of influence, a region 33,823 nautical miles (62,630 km) from the moon. For Apollo 11, this occurred after 61 hours and 56 minutes, while for Apollo 8 it took only 55 hours 40 minutes.