INTO THE MOON’S SPHERE OF INFLUENCE

At 061:40, Apollo 11 entered the Moon’s sphere of influence. The strength of a gravitational field is directly proportional to the mass of the gravitating body and inversely proportional to the square of the range from the body. With Earth fully 81 times more massive than the Moon, the ‘neutral point’ was roughly 90 per cent of the way to the Moon; more specifically, 186,437 nautical miles from Earth and 33,822 nautical miles from the Moon. By this milestone, the velocity of the spacecraft had slowed to a relative crawl of 2,990 feet per second with respect to Earth and 3,272 feet per second with respect to the Moon. When Apollo 8 first blazed this trail, the less technically minded members of the press expressed surprise that the crew did not feel a jolt. The origin of this misunderstanding was a comment at a press conference by flight dynamics officer Philip C. Shaffer. After pointing out that Apollo 8 had slowed as it climbed from Earth, and beyond the neutral point accelerated towards the Moon, he added that as the spacecraft crossed the neutral point, the computers in the Real-Time Computer Complex, which constantly calculated its position and velocity, switched from a terrestrial to a lunar frame of reference and the way the numbers were crunched made it appear that the position of the ship had jumped several miles. There was, in fact, no physical manifestation of passing from one gravitational field to the other. Although Apollo 11 entered the Moon’s sphere of influence, it was not committed to it because, being on a free-return trajectory, if it were to do nothing it would pass around the back of the Moon and be deflected back to Earth.

Meanwhile, at home

During the overnight shift, those members of Glynn Lunney’s Black Team who had been asleep during the day and had missed the telecast of the LM inspection, watched a replay.