 At about 55 hours and 40 minutes into the flight , the crew of Apollo 8 became the first humans to enter the gravitational sphere of influence of another celestial body . In other words , the effect of the Moon 's gravitational force on Apollo 8 became stronger than that of the Earth . At the time it happened , Apollo 8 was 38 @,@ 759 miles ( 62 @,@ 377 km ) from the Moon and had a speed of 3 @,@ 990 ft / s ( 1 @,@ 220 m / s ) relative to the Moon . This historic moment was of little interest to the crew since they were still calculating their trajectory with respect to the launch pad at Kennedy Space Center . They would continue to do so until they performed their last mid @-@ course correction , switching to a reference frame based on ideal orientation for the second engine burn they would make in lunar orbit . It was only 13 hours until they would be in lunar orbit . 
