Editing Apollo 8 (section)

===Lunar sphere of influence===

[[File:Apollo 8 Image of the Moon (AS08-14-2506).jpg|thumb|left|This photograph of the Moon was taken from Apollo{{nbsp}}8 at a point above 70 degrees east longitude.]]

At about 55 hours and 40 minutes into the flight, and 13 hours before entering lunar orbit, the crew of Apollo{{nbsp}}8 became the first humans to enter the gravitational [[Sphere of influence (astrodynamics)|sphere of influence]] of another celestial body. In other words, the effect of the Moon's [[Newton's law of universal gravitation|gravitational force]] on Apollo{{nbsp}}8 became stronger than that of the Earth. At the time it happened, Apollo{{nbsp}}8 was {{convert|38759|mi|km}} from the Moon and had a speed of {{convert|3990|ft/s|m/s|abbr=on}} 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 [[Frame of reference|reference frame]] based on ideal orientation for the second engine burn they would make in lunar orbit.{{sfn|Orloff|2000|p=36}}

The last major event before Lunar Orbit Insertion (LOI) was a second mid-course correction. It was in [[Retrograde and direct motion|retrograde]] (against the direction of travel) and slowed the spacecraft down by {{convert|2.0|ft/s|m/s|abbr=on}}, effectively reducing the closest distance at which the spacecraft would pass the Moon. At exactly 61 hours after launch, about {{convert|24200|mi|km}} from the Moon, the crew burned the RCS for 11 seconds. They would now pass {{convert|71.7|mi|km}} from the [[Geology of the Moon#Lunar landscape|lunar surface]].{{sfn|Orloff|2000|p=46}}

At 64 hours into the flight, the crew began to prepare for Lunar Orbit Insertion{{nbsp}}1 (LOI-1). This maneuver had to be performed perfectly, and due to [[orbital mechanics]] had to be on the far side of the Moon, out of contact with the Earth. After Mission Control was polled for a "[[Launch status check|go/no go]]" decision, the crew was told at 68 hours that they were Go and "riding the best bird we can find".<ref name="journal day 3 LOI">{{cite web |url=https://history.nasa.gov/ap08fj/11day3_black_approach.htm |title=Day 3: The Black Team—Approaching the Moon |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |date=April 22, 2006 |work=Apollo{{nbsp}}8 Flight Journal |publisher=NASA |access-date=February 7, 2008 |archive-url=https://web.archive.org/web/20080204015340/https://history.nasa.gov/ap08fj/11day3_black_approach.htm |archive-date=February 4, 2008 |url-status=dead }}</ref> Lovell replied, "We'll see you on the other side", and for the first time in history, humans travelled behind the Moon and out of radio contact with the Earth.<ref name="journal day 3 LOI"/> [[Frances Northcutt|Frances "Poppy" Northcutt]], who was the first woman in NASA's mission control and helped calculate the return to Earth trajectory for this mission, recounts what it was like when Apollo 8 went behind the Moon for the first time in an interview: "That was a very nerve-racking period on the team I was on, and I think it was a very nerve-racking period in general because of this thing with losing signal. You've got this big mystery going on there on the backside of the Moon. You do not know what's happening and there's not a darn thing anybody here can do about it until we hear from them."<ref>{{Cite web |date=November 14, 2018 |title=NASA Johnson Space Center Oral History Project – Edited Oral History Transcript |url=https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/NorthcuttFM/NorthcuttFM_11-14-18.pdf}}</ref>

With ten minutes remaining before LOI-1, the crew began one last check of the spacecraft systems and made sure that every switch was in its correct position. At that time, they finally got their first glimpses of the Moon. They had been flying over the unlit side, and it was Lovell who saw the first shafts of sunlight [[wiktionary:oblique|oblique]]ly illuminating the lunar surface. The LOI burn was only two minutes away, so the crew had little time to appreciate the view.{{sfn|Lovell|Kluger|1994|pp=48–49}}