Editing Apollo 8 (section)

==Mission==
===Parameter summary===

[[File:Apollo 8 TLI.jpg|thumb|upright=3.0|Mission profile]]

As the first crewed spacecraft to orbit more than one celestial body, Apollo{{nbsp}}8's profile had two different sets of orbital parameters, separated by a translunar injection maneuver. Apollo lunar missions would begin with a nominal {{convert|100|nmi|km|sigfig=4|adj=on}} circular Earth parking orbit. Apollo{{nbsp}}8 was launched into an initial orbit with an [[apogee]] of {{convert|99.99|nmi|km}} and a [[perigee]] of {{convert|99.57|nmi|km}}, with an [[inclination]] of 32.51° to the [[Equator]], and an [[orbital period]] of 88.19 minutes. Propellant venting increased the apogee by {{convert|6.4|nmi|km}} over the 2{{nbsp}}hours, 44 minutes, and 30 seconds spent in the parking orbit.{{sfn|Orloff|2000|p=45}}

This was followed by a [[trans-lunar injection]] (TLI) burn of the [[S-IVB]] third stage for 318 seconds, accelerating the {{convert|63650|lb|kg|abbr=on}} command and service module and {{convert|19900|lb|kg|abbr=on}} LM test article from an orbital velocity of {{convert|25567|ft/s|m/s}} to the injection velocity of {{convert|35505|ft/s|m/s|abbr=on}}{{sfn|Orloff|2000|p=46}}<ref name="PressKit"/> which set a record for the highest speed, relative to Earth, that humans had ever traveled.{{sfn|Phillips|1975|p=178}} This speed was slightly less than the Earth's [[escape velocity]] of {{convert|36747|ft/s|m/s}}, but put Apollo{{nbsp}}8 into an elongated elliptical Earth orbit, close enough to the Moon to be captured by the Moon's gravity.{{sfn|Woods|2008|pp=108–109}}

The standard lunar orbit for Apollo missions was planned as a nominal {{convert|60|nmi|km|adj=on}} circular orbit above the Moon's surface. Initial lunar orbit insertion was an ellipse with a [[perilune]] of {{convert|60.0|nmi|km}} and an [[apolune]] of {{convert|168.5|nmi|km}}, at an inclination of 12° from the lunar equator. This was then circularized at {{convert|60.7|by|59.7|nmi|km}}, with an orbital period of 128.7 minutes.{{sfn|Orloff|2000|p=46}} The effect of lunar [[mass concentration (astronomy)|mass concentrations]] ("mascons") on the orbit was found to be greater than initially predicted; over the course of the ten lunar orbits lasting twenty hours, the orbital distance was perturbated to {{convert|63.6|by|58.6|nmi|km}}.{{sfn|Orloff|2000|p=39}}

Apollo 8 achieved a maximum distance from Earth of {{convert|203752|nmi|smi km|abbr=off|sp=us}}.{{sfn|Orloff|2000|p=39}}

===Launch and trans-lunar injection===

[[File:Apollo 8 liftoff.jpg|thumb|Apollo 8 launch]]

Apollo 8 was launched at 12:51:00 [[UTC]] (07:51:00 [[Eastern Time Zone (North America)|Eastern Standard Time]]) on December 21, 1968, using the Saturn V's [[Multistage rocket|three stages]] to achieve Earth orbit.{{sfn|Orloff|2000|p=39}} The [[S-IC]] first stage landed in the [[Atlantic Ocean]] at {{Coord|30|12|N|74|7|W|name=Apollo 8 S-IC impact}}, and the [[S-II]] second stage landed at {{Coord|31|50|N|37|17|W|name=Apollo 8 S-II impact}}.{{sfn|Orloff|2000|p=34}} The [[S-IVB]] third stage injected the craft into Earth orbit and remained attached to perform the TLI burn that would put the spacecraft on a trajectory to the Moon.{{sfn|Orloff|2000|p=35}}

Once the [[space vehicle|vehicle]] reached Earth orbit, both the crew and [[Christopher C. Kraft Jr. Mission Control Center|Houston flight controllers]] spent the next 2{{nbsp}}hours and 38 minutes checking that the spacecraft was in proper working order and ready for TLI.{{sfn|Brooks|Grimwood|Swenson|1979|p=276}} The proper operation of the S-IVB third stage of the rocket was crucial, and in the last uncrewed test, it had failed to reignite for this burn.{{sfn|Brooks|Grimwood|Swenson|1979|pp=250–252}} Collins was the first CAPCOM on duty, and at 2{{nbsp}}hours, 27 minutes and 22 seconds after launch he radioed, "Apollo{{nbsp}}8. You are Go for TLI."{{sfn|Brooks|Grimwood|Swenson|1979|p=276}} This communication meant that Mission Control had given official permission for Apollo{{nbsp}}8 to go to the Moon. The S-IVB engine ignited on time and performed the TLI burn perfectly.{{sfn|Brooks|Grimwood|Swenson|1979|p=276}} Over the next five minutes, the spacecraft's speed increased from {{convert|7600|to|10800|m/s|sp=us}}.{{sfn|Brooks|Grimwood|Swenson|1979|p=276}}

After the S-IVB had placed the mission on course for the Moon, the command and service modules (CSM), the remaining Apollo{{nbsp}}8 spacecraft, separated from it. The crew then rotated the spacecraft to take photographs of the spent stage and then practiced flying in formation with it. As the crew rotated the spacecraft, they had their first views of the Earth as they moved away from it—this marked the first time humans had viewed the whole Earth at once. Borman became worried that the S-IVB was staying too close to the CSM and suggested to Mission Control that the crew perform a separation maneuver. Mission Control first suggested pointing the spacecraft towards Earth and using the small [[Apollo CSM#Reaction control system|reaction control system]] (RCS) thrusters on the [[Apollo command and service module#Service module (SM)|service module]] (SM) to add {{convert|1.1|ft/s|m/s|abbr=on|sp=us}} to their velocity away from the Earth, but Borman did not want to lose sight of the S-IVB. After discussion, the crew and Mission Control decided to burn in the Earth direction to increase speed, but at {{convert|7.7|ft/s|m/s|abbr=on}} instead. The time needed to prepare and perform the additional burn put the crew an hour behind their onboard tasks.{{sfn|Orloff|2000|p=35}}{{sfn|Brooks|Grimwood|Swenson|1979|pp=276–278}}
[[File:As8-16-2583.jpg|thumb|Apollo 8 [[S-IVB]] rocket stage shortly after separation. The LM test article, a circular boilerplate model of the LM, is visible with four triangular legs connecting it to the stage.]]

Five hours after launch, Mission Control sent a command to the S-IVB to vent its remaining fuel, changing its trajectory. The S-IVB, with the test article attached, posed no further hazard to Apollo{{nbsp}}8, passing the orbit of the Moon and going into a {{convert|0.99|by|0.92|AU|Gm|lk=on|adj=on}} solar orbit with an [[inclination]] of 23.47° from the Earth's equatorial plane, and an orbital period of 340.80 days.{{sfn|Orloff|2000|p=35}} It became a [[:Category:Derelict satellites in heliocentric orbit|derelict object]], and will continue to [[heliocentric orbit|orbit the Sun]] for many years, if not retrieved.<ref name=ha20130923>{{cite web|title=Saturn S-IVB-503N—Satellite Information |url=http://www.heavens-above.com/SatInfo.aspx?satid=3627&lat=0&lng=0&loc=Unspecified&alt=0&tz=UCT |work=Satellite database |publisher=Heavens-Above |access-date=September 23, 2013}}</ref>

The Apollo 8 crew were the first humans to pass through the [[Van Allen radiation belt]]s, which extend up to {{convert|15000|mi|km}} from Earth. Scientists predicted that passing through the belts quickly at the spacecraft's high speed would cause a radiation dosage of no more than a chest [[X-ray]], or 1{{nbsp}}[[Gray (unit)|milligray]] (mGy; during a year, the average human receives a dose of 2{{nbsp}}to 3{{nbsp}}mGy from [[background radiation]]). To record the actual radiation dosages, each crew member wore a Personal Radiation [[Dosimeter]] that transmitted data to Earth, as well as three passive film dosimeters that showed the cumulative radiation experienced by the crew. By the end of the mission, the crew members experienced an average radiation dose of 1.6 mGy.<ref name="Biomedical">{{cite book|last=Bailey |first=J. Vernon |title=Biomedical Results of Apollo |url=http://lsda.jsc.nasa.gov/books/apollo/eboard2.htm |archive-url=https://web.archive.org/web/20080117135912/http://lsda.jsc.nasa.gov/books/apollo/S2ch3.htm |archive-date=January 17, 2008 |access-date=January 28, 2008 |year=1975 |publisher=[[Lyndon B. Johnson Space Center]] |id=[https://history.nasa.gov/SP-368/sp368.htm NASA SP-368] |chapter=Radiation Protection and Instrumentation |chapter-url=http://lsda.jsc.nasa.gov/books/apollo/S2ch3.htm |url-status=dead}} Sec. 2, Ch. 3.</ref>

===Lunar trajectory===

Lovell's main job as Command Module Pilot was as [[flight officer|navigator]]. Although Mission Control normally performed all the navigation calculations, it was necessary to have a crew member adept at navigation so that the crew could return to Earth in case communication with Mission Control was lost. Lovell navigated by star sightings using a [[sextant]] built into the spacecraft, measuring the angle between a star and the Earth's (or the Moon's) [[horizon]]. This task was made difficult by a large cloud of debris around the spacecraft, which made it hard to distinguish the stars.<ref name="journal day 1 green">{{cite web |url=http://history.nasa.gov/ap08fj/03day1_green_sep.htm |title=Day 1: Green Team |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |date=April 22, 2006 |work=Apollo 8 Flight Journal |publisher=NASA |access-date=January 30, 2008 |archive-url=https://web.archive.org/web/20080114013205/http://history.nasa.gov/ap08fj/03day1_green_sep.htm |archive-date=January 14, 2008 |url-status=dead  }}</ref>

By seven hours into the mission, the crew was about 1{{nbsp}}hour and 40 minutes behind flight plan because of the problems in moving away from the S-IVB and Lovell's obscured star sightings. The crew placed the spacecraft into Passive Thermal Control (PTC), also called "barbecue roll", in which the spacecraft rotated about once per hour around its long axis to ensure even heat distribution across the surface of the spacecraft. In direct sunlight, parts of the spacecraft's outer surface could be heated to over {{convert|200|C|F}}, while the parts in shadow would be {{convert|-100|C|F}}. These temperatures could cause the [[atmospheric reentry#Thermal protection systems|heat shield]] to crack and propellant lines to burst. Because it was impossible to get a perfect roll, the spacecraft swept out a [[Conical surface|cone]] as it rotated. The crew had to make minor adjustments every half hour as the cone pattern got larger and larger.<ref name="journal day 1 Maroon">{{cite web|url=https://history.nasa.gov/ap08fj/04day1_maroon.htm |title=Day 1: Maroon Team |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |work=Apollo 8 Flight Journal |publisher=NASA |date=April 22, 2006 |access-date=February 4, 2008 |archive-url=https://web.archive.org/web/20080107002315/https://history.nasa.gov/ap08fj/04day1_maroon.htm |archive-date=January 7, 2008 |url-status=dead}}</ref>
[[File:AS08-16-2593 remastered.jpg|thumb|left|The [[first images of Earth from space|first image taken by humans of the whole Earth]], probably photographed by [[William Anders]].<ref name="journal day 1 green"/> (time tag:  003:42:55) South America is visible in the lower half.]]

The first mid-course correction came eleven hours into the flight. The crew had been awake for more than 16 hours. Before launch, NASA had decided at least one crew member should be awake at all times to deal with problems that might arise. Borman started the first sleep shift but found sleeping difficult because of the constant radio chatter and mechanical noises. Testing on the ground had shown that the [[Apollo command and service module#Service propulsion system|service propulsion system]] (SPS) engine had a small chance of exploding when burned for long periods unless its [[combustion chamber]] was "coated" first by burning the engine for a short period. This first correction burn was only 2.4 seconds and added about {{convert|20.4|ft/s|m/s|abbr=on|sp=us}} velocity [[Retrograde and prograde motion|prograde]] (in the direction of travel).{{sfn|Orloff|2000|p=35}} This change was less than the planned {{convert|24.8|ft/s|m/s|abbr=on|sp=us}}, because of a bubble of [[helium]] in the [[nitrogen tetroxide|oxidizer]] lines, which caused unexpectedly low propellant pressure. The crew had to use the small RCS thrusters to make up the shortfall. Two later planned mid-course corrections were canceled because the Apollo{{nbsp}}8 trajectory was found to be perfect.<ref name="journal day 1 Maroon"/>

About an hour after starting his sleep shift, Borman obtained permission from [[flight controller|ground control]] to take a [[Secobarbital|Seconal]] [[Barbiturate|sleeping pill]]. The pill had little effect. Borman eventually fell asleep, and then awoke feeling ill. He vomited twice and had a bout of diarrhea; this left the spacecraft full of small globules of vomit and feces, which the crew cleaned up as well as they could. Borman initially did not want everyone to know about his medical problems, but Lovell and Anders wanted to inform Mission Control. The crew decided to use the Data Storage Equipment (DSE), which could tape voice recordings and telemetry and dump them to Mission Control at high speed. After recording a description of Borman's illness they asked Mission Control to check the recording, stating that they "would like an evaluation of the voice comments".<ref name="journal day 2 green">{{cite web |url=https://history.nasa.gov/ap08fj/06day2_green.htm |title=Day 2: Green Team |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |date=April 22, 2006 |work=Apollo 8 Flight Journal |publisher=NASA |access-date=January 30, 2008 |archive-url=https://web.archive.org/web/20080311114001/https://history.nasa.gov/ap08fj/06day2_green.htm |archive-date=March 11, 2008 |url-status=dead }}</ref>

The Apollo 8 crew and Mission Control medical personnel held a conference using an unoccupied second-floor control room (there were two identical control rooms in Houston, on the second and third floors, only one of which was used during a mission). The conference participants concluded that there was little to worry about and that Borman's illness was either a [[Gastroenteritis|24-hour flu]], as Borman thought, or a reaction to the sleeping pill.{{sfn|Collins|2001|p=306}} Researchers now believe that he was suffering from [[space adaptation syndrome]], which affects about a third of astronauts during their first day in space as their [[Labyrinth (inner ear)|vestibular system]] adapts to [[weightlessness]].<ref>{{cite journal |last=Quine |first=Tony |date=April 2007|title=Addicted to space: An appreciation of Anousheh Ansari, Part II |journal= Spaceflight|volume=49 |issue=4 |page=144|issn=0038-6340 |publisher=[[British Interplanetary Society]]}}</ref> Space adaptation syndrome had not occurred on previous spacecraft ([[Project Mercury|Mercury]] and [[Project Gemini|Gemini]]), because those astronauts could not move freely in the small cabins of those spacecraft. The increased cabin space in the Apollo command module afforded astronauts greater freedom of movement, contributing to symptoms of space sickness for Borman and, later, astronaut [[Rusty Schweickart]] during Apollo{{nbsp}}9.<ref>{{cite web |url=http://lsda.jsc.nasa.gov/scripts/experiment/exper.aspx?exp_index=747 |title=The Effects of Long-Duration Space Flight on Eye, Head, and Trunk Coordination During Locomotion |last1=Kozlovskaya |first1=Inessa B |last2=Bloomberg |first2=Jacob J. |last3=Layne |first3=Charles S. |year=2004 |work=Life Sciences Data Archive |publisher=Lyndon B. Johnson Space Center |id=LSDA Exp ID: 9307191 |access-date=June 28, 2013 |display-authors=2}}</ref>
[[File:Ap8-S68-56531.jpg|thumb|right|Still from film of the crew taken while they were in orbit around the Moon. Frank Borman is in the center.]]

The cruise phase was a relatively uneventful part of the flight, except for the crew's checking that the spacecraft was in working order and that they were on course. During this time, NASA scheduled a television broadcast at 31 hours after launch. The Apollo{{nbsp}}8 crew used a {{convert|2|kg|lb|adj=on}} camera that broadcast in [[black-and-white]] only, using a [[Video camera tube|Vidicon]] tube. The camera had two [[lens (optics)|lenses]], a very [[wide-angle lens|wide-angle (160°) lens]], and a [[telephoto lens|telephoto (9°) lens]].{{sfn|Orloff|2000|p=36}}<ref name="journal day 2 Maroon">{{cite web|url=http://history.nasa.gov:80/ap08fj/07day2_maroon.htm |title=Day 2: Maroon Team |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |work=Apollo 8 Flight Journal |publisher=NASA |date=April 22, 2006 |access-date=February 4, 2008 |archive-url=https://web.archive.org/web/20080404232523/http://history.nasa.gov/ap08fj/07day2_maroon.htm |archive-date=April 4, 2008 |url-status=dead}}</ref>

During this first broadcast, the crew gave a tour of the spacecraft and attempted to show how the Earth appeared from space. However, difficulties aiming the narrow-angle lens without the aid of a monitor to show what it was looking at made showing the Earth impossible. Additionally, without proper [[Optical filter|filters]], the Earth image became saturated by any bright source. In the end, all the crew could show the people watching back on Earth was a bright blob.{{sfn|Orloff|2000|p=36}} After broadcasting for 17 minutes, the rotation of the spacecraft took the [[high-gain antenna]] out of view of the receiving stations on Earth and they ended the transmission with Lovell wishing his mother a happy birthday.<ref name="journal day 2 Maroon" />

By this time, the crew had completely abandoned the planned sleep shifts. Lovell went to sleep {{frac|32|1|2}} hours into the flight&nbsp;– three-and-a-half hours before he had planned to. A short while later, Anders also went to sleep after taking a sleeping pill.<ref name="journal day 2 Maroon" /> The crew was unable to see the Moon for much of the outward cruise. Two factors made the Moon almost impossible to see from inside the spacecraft: three of the five windows fogging up due to out-gassed oils from the [[silicone]] [[sealant]], and the [[Orientation (geometry)|attitude]] required for passive thermal control. It was not until the crew had gone behind the Moon that they would be able to see it for the first time.{{sfn|Brooks|Grimwood|Swenson|1979|p=277}}

Apollo 8 made a second television broadcast at 55 hours into the flight. This time, the crew rigged up filters meant for the still cameras so they could acquire images of the Earth through the telephoto lens. Although difficult to aim, as they had to maneuver the entire spacecraft, the crew was able to broadcast back to Earth the first television pictures of the Earth. The crew spent the transmission describing the Earth, what was visible, and the colors they could see. The transmission lasted 23 minutes.{{sfn|Orloff|2000|p=36}}

===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}}

===Lunar orbit===

The SPS was ignited at 69 hours, 8{{nbsp}}minutes, and 16 seconds after launch and burned for 4{{nbsp}}minutes and 7{{nbsp}}seconds, placing the Apollo{{nbsp}}8 spacecraft in orbit around the Moon. The crew described the burn as being the longest four minutes of their lives. If the burn had not lasted exactly the correct amount of time, the spacecraft could have ended up in a highly [[ellipse|elliptical]] lunar orbit or even been flung off into space. If it had lasted too long, they could have struck the Moon. After making sure the spacecraft was working, they finally had a chance to look at the Moon, which they would orbit for the next 20 hours.<ref name="nssdc orbit">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1968-118A |title=Apollo 8 |work=[[National Space Science Data Center]] |publisher=NASA |access-date=June 28, 2013 }}</ref>

On Earth, Mission Control continued to wait. If the crew had not burned the engine, or the burn had not lasted the planned length of time, the crew would have appeared early from behind the Moon. Exactly at the calculated moment the signal was received from the spacecraft, indicating it was in a {{convert|193.3|by|69.5|mi|km|adj=on}} orbit around the Moon.<ref name="nssdc orbit" />

After reporting on the status of the spacecraft, Lovell gave the first description of what the lunar surface looked like:
{{Blockquote|The Moon is essentially grey, no color; looks like [[plaster of Paris]] or sort of a grayish beach sand. We can see quite a bit of detail. The [[Mare Fecunditatis|Sea of Fertility]] doesn't stand out as well here as it does back on Earth. There's not as much contrast between that and the surrounding craters. The craters are all rounded off. There's quite a few of them, some of them are newer. Many of them look like—especially the round ones—look like hit by [[meteorite]]s or projectiles of some sort. [[Langrenus (crater)|Langrenus]] is quite a huge crater; it's got a central cone to it. The walls of the crater are terraced, about six or seven different [[wiktionary:terrace|terraces]] on the way down.<ref name="journal day 4-123">{{cite web |url=https://history.nasa.gov/ap08fj/13day4_orbits123.htm |title=Day 4: Lunar Orbits 1, 2 and 3 |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |date=April 22, 2006 |work=Apollo 8 Flight Journal |publisher=NASA |access-date=September 20, 2007 |archive-url=https://web.archive.org/web/20070922045003/https://history.nasa.gov/ap08fj/13day4_orbits123.htm |archive-date=September 22, 2007 |url-status=dead |df=mdy-all }}</ref>}}
[[File:The Lunar Farside - GPN-2000-001127.jpg|thumb|A portion of the [[Far side of the Moon|lunar far side]] as seen from Apollo{{nbsp}}8]]

Lovell continued to describe the terrain they were passing over. One of the crew's major tasks was [[reconnaissance]] of planned future landing sites on the Moon, especially one in [[Mare Tranquillitatis]] that was planned as the Apollo{{nbsp}}11 landing site. The launch time of Apollo{{nbsp}}8 had been chosen to give the best lighting conditions for examining the site. A [[film camera]] had been set up in one of the spacecraft windows to record one frame per second of the Moon below. Bill Anders spent much of the next 20 hours taking as many photographs as possible of targets of interest. By the end of the mission, the crew had taken over eight hundred 70&nbsp;mm still photographs and {{convert|700|ft}} of 16&nbsp;mm movie film.{{sfn|Orloff|2000|p=37}}

Throughout the hour that the spacecraft was in contact with Earth, Borman kept asking how the data for the SPS looked. He wanted to make sure that the engine was working and could be used to return early to the Earth if necessary. He also asked that they receive a "go/no go" decision before they passed behind the Moon on each orbit.<ref name="journal day 4-123" />

As they reappeared for their second pass in front of the Moon, the crew set up equipment to broadcast a view of the lunar surface. Anders described the craters that they were passing over. At the end of this second orbit, they performed an 11-second LOI-2 burn of the SPS to circularize the orbit to {{convert|70.0|by|71.3|mi|km}}.<ref name="nssdc orbit" /><ref name="journal day 4-123" />

Throughout the next two orbits, the crew continued to check the spacecraft and to observe and photograph the Moon. During the third pass, Borman read a small prayer for his church. He had been scheduled to participate in a service at St. Christopher's [[Episcopal Church (United States)|Episcopal Church]] near [[Seabrook, Texas]], but due to the Apollo{{nbsp}}8 flight, he was unable to attend. A fellow parishioner and engineer at Mission Control, Rod Rose, suggested that Borman read the prayer, which could be recorded and then replayed during the service.<ref name="journal day 4-123" />

===''Earthrise'' and Genesis broadcast===
{{Main|Earthrise|Apollo 8 Genesis reading}}
[[File:Earthrise.jpg|thumb|The ''[[Earthrise]]'' image]]
[[File:As8 genesis1a.ogv|thumb|Apollo 8's 1968 Christmas Eve [[Apollo 8 Genesis reading|broadcast and reading]] from the [[Book of Genesis]]]]
When the spacecraft came out from behind the Moon for its fourth pass across the front, the crew witnessed an "Earthrise" in person for the first time in human history.<ref name="journal day 4-456" /> NASA's [[Lunar Orbiter 1]] had taken the first picture of an Earthrise from the vicinity of the Moon, on August 23, 1966.<ref>{{cite web |url=https://sservi.nasa.gov/articles/the-other-lunar-orbiter-1-earthrise-image/ |title=The 'Other' Lunar Orbiter 1 Earthrise Image |publisher=Solar System Exploration Research Virtual Institute |access-date=November 28, 2018 }}</ref> Anders saw the Earth emerging from behind the lunar horizon and called in excitement to the others, taking a black-and-white photograph as he did so. Anders asked Lovell for color film and then took ''[[Earthrise]]'', a now famous color photo, later picked by ''[[Life (magazine)|Life]]'' magazine as one of its hundred photos of the century.<ref name="journal day 4-456">{{cite web |url=https://history.nasa.gov/ap08fj/14day4_orbits456.htm |title=Day 4: Lunar Orbits 4, 5 and 6 |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |date=April 22, 2006 |work=Apollo 8 Flight Journal |publisher=NASA |access-date=September 20, 2007 |archive-url=https://web.archive.org/web/20071002204030/https://history.nasa.gov/ap08fj/14day4_orbits456.htm |archive-date=October 2, 2007 |url-status=dead }}</ref><ref>{{cite magazine |url=https://www.smithsonianmag.com/science-nature/who-took-legendary-earthrise-photo-apollo-8-180967505/ |title=Who Took the Legendary Earthrise Photo From Apollo 8? |magazine=Smithsonian |first=Andrew |last=Chaikin |access-date=November 28, 2018 }}</ref>

Due to the [[tidal locking|synchronous rotation]] of the Moon about the Earth, Earthrise is not generally visible from the lunar surface. This is because, as seen from any one place on the Moon's surface, Earth remains in approximately the same position in the lunar sky, either above or below the horizon. Earthrise is generally visible only while orbiting the Moon, and at selected surface locations near the Moon's [[lunar limb|limb]], where [[libration]] carries the Earth slightly above and below the lunar horizon.<ref>{{cite web |url=https://www.nasa.gov/image-feature/goddard/lro-earthrise-2015 |title=NASA Releases New High-Res Earthrise Image |date=December 16, 2015 |publisher=NASA |access-date=November 28, 2018 }}</ref>

Anders continued to take photographs while Lovell assumed control of the spacecraft so that Borman could rest. Despite the difficulty resting in the cramped and noisy spacecraft, Borman was able to sleep for two orbits, awakening periodically to ask questions about their status. Borman awoke fully when he started to hear his fellow crew members make mistakes. They were beginning to not understand questions and had to ask for the answers to be repeated. Borman realized that everyone was extremely tired from not having a good night's sleep in over three days. He ordered Anders and Lovell to get some sleep and that the rest of the flight plan regarding observing the Moon be scrubbed. Anders initially protested, saying that he was fine, but Borman would not be swayed. Anders finally agreed under the condition that Borman would set up the camera to continue to take automatic pictures of the Moon. Borman also remembered that there was a second television broadcast planned, and with so many people expected to be watching, he wanted the crew to be alert. For the next two orbits, Anders and Lovell slept while Borman sat at the helm.<ref name="journal day 4-456"/>{{sfn|Kluger|2017|pp=240–241}}

[[File:Apollo 8 genesis reading.ogg|left|thumb|[[Apollo 8 Genesis reading]]]]

As they rounded the Moon for the ninth time, the astronauts began the second television transmission. Borman introduced the crew, followed by each man giving his impression of the lunar surface and what it was like to be orbiting the Moon. Borman described it as being "a vast, lonely, forbidding expanse of nothing".{{sfn|De Groot|2006|p=229}} Then, after talking about what they were flying over, Anders said that the crew had a message for all those on Earth. Each man on board [[Apollo 8 Genesis reading|read a section from the Biblical creation story]] from the [[Book of Genesis]]. Borman finished the broadcast by wishing a Merry Christmas to everyone on Earth. His message appeared to sum up the feelings that all three crewmen had from their vantage point in lunar orbit. Borman said, "And from the crew of Apollo{{nbsp}}8, we close with good night, good luck, a Merry Christmas and God bless all of you—all of you on the good Earth."<ref name="moonport">{{cite book |last1=Benson |first1=Charles D. |last2=Faherty |first2=William Barnaby |title=Moonport: A History of Apollo Launch Facilities and Operations |url=http://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html |archive-url=https://web.archive.org/web/20080123133438/http://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html |archive-date=January 23, 2008 |access-date=February 7, 2008 |year=1978 |publisher=NASA |id=NASA SP-4204 |chapter=Apollo 8—A Christmas Gift |chapter-url=http://www.hq.nasa.gov/office/pao/History/SP-4204/ch20-9.html |url-status=live}} Ch.20-9.</ref>

The only task left for the crew at this point was to perform the [[trans-Earth injection]] (TEI), which was scheduled for {{frac|2|1|2}} hours after the end of the television transmission. The TEI was the most critical burn of the flight, as any failure of the SPS to ignite would strand the crew in lunar orbit, with little hope of escape. As with the previous burn, the crew had to perform the maneuver above the far side of the Moon, out of contact with Earth.{{sfn|Kluger|2017|pp=250–252}} The burn occurred exactly on time. The spacecraft telemetry was reacquired as it re-emerged from behind the Moon at 89 hours, 28 minutes, and 39 seconds, the exact time calculated. When voice contact was regained, Lovell announced, "Please be informed, there is a [[Santa Claus]]", to which Ken Mattingly, the current CAPCOM, replied, "That's affirmative, you are the best ones to know."<ref name="journal day 4 TEI">{{cite web |url=https://history.nasa.gov/ap08fj/16day4_final_orbit_tei.htm |title=Day 4: Final Orbit and Trans-Earth Injection |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |date=April 22, 2006 |work=Apollo 8 Flight Journal |publisher=NASA |access-date=February 7, 2008 |archive-url=https://web.archive.org/web/20080116172357/https://history.nasa.gov/ap08fj/16day4_final_orbit_tei.htm |archive-date=January 16, 2008 |url-status=dead }}</ref> The spacecraft began its journey back to Earth on December 25, [[Christmas Day]].{{sfn|Orloff|2000|p=39}}

===Unplanned manual realignment===

Later, Lovell used some otherwise idle time to do some navigational sightings, maneuvering the module to view various stars by using the [[Apollo guidance computer|computer]] keyboard. He accidentally erased some of the computer's memory, which caused the [[Apollo PGNCS#Inertial measurement unit|inertial measurement unit]] (IMU) to contain data indicating that the module was in the same relative orientation it had been in before lift-off; the IMU then fired the thrusters to "correct" the module's attitude.<ref name="Benke">{{cite news |title=Astronauts look back 30 years after historic lunar launch |first=Richard |last=Benke |url=http://cgi.canoe.ca/SpaceArchive/981221_30.html |archive-url=https://archive.today/20121206004702/http://cgi.canoe.ca/SpaceArchive/981221_30.html |url-status=usurped |archive-date=December 6, 2012 |agency=[[Associated Press]] |newspaper=[[The Augusta Chronicle]] |date=December 21, 1998 |access-date=June 28, 2013}}</ref>

Once the crew realized why the computer had changed the module's attitude, they realized that they would have to reenter data to tell the computer the module's actual orientation. It took Lovell ten minutes to figure out the right numbers, using the thrusters to get the stars [[Rigel]] and [[Sirius]] aligned,{{sfn|Kluger|2017|pp=260–261}} and another 15 minutes to enter the corrected data into the computer.{{sfn|Orloff|2000|p=39}} Sixteen months later, during the [[Apollo 13|Apollo{{nbsp}}13]] mission, Lovell would have to perform a similar manual realignment under more critical conditions after the module's IMU had to be turned off to conserve energy.{{sfn|Lovell|Kluger|1994|pp=282–283}}

===Cruise back to Earth and reentry===

[[File:Apollo 8 reentry, December 27, 1968.jpg|thumb|left|Reentry, December 27, 1968, photographed from a [[KC-135 Stratotanker]] at 40,000 feet|alt=White streaks of light, with bright spots on the right side of them, fill the bottom of the frame. A larger yellow-tinted sphere with a streak is in the center of the frame. The background is black space.]]

The cruise back to Earth was mostly a time for the crew to relax and monitor the spacecraft. As long as the trajectory specialists had calculated everything correctly, the spacecraft would reenter Earth's atmosphere two-and-a-half days after TEI and [[splashdown (spacecraft landing)|splash down]] in the Pacific.{{sfn|Orloff|2000|p=39}}

On Christmas afternoon, the crew made their fifth television broadcast.<ref>
{{Cite AV media
| date      = 2003
| title     = Apollo 8: Leaving the Cradle
| url       = http://www.collectspace.com/resources/reviews/dvd/apollo8_leaving_the_cradle.html
| time      = Chapter 5, Disk 2
| medium    = DVD
| publisher = Spacecraft Films/[[20th Century Fox Home Entertainment]]
}}
</ref> This time, they gave a tour of the spacecraft, showing how an astronaut lived in space. When they finished broadcasting, they found a small present from Slayton in the food locker: a real turkey dinner with stuffing, in the same kind of pack given to the troops in Vietnam.{{sfn|Wilford|1973|p=68}}

Another Slayton surprise was a gift of three [[Miniature (alcohol)|miniature bottles]] of [[brandy]], which Borman ordered the crew to leave alone until after they landed. They remained unopened, even years after the flight.{{sfn|Schefter|1999|p=275}} There were also small presents to the crew from their wives. The next day, at about 124 hours into the mission, the sixth and final TV transmission showed the mission's best video images of the Earth, during a four-minute broadcast.<ref>
{{Cite AV media
| date      = 2003
| title     = Apollo 8: Leaving the Cradle
| url       = http://www.collectspace.com/resources/reviews/dvd/apollo8_leaving_the_cradle.html
| time      = Chapter 6, Disk 2
| medium    = DVD
| publisher = Spacecraft Films/20th Century Fox Home Entertainment
}}
</ref> After two uneventful days, the crew prepared for reentry. The computer would control the reentry, and all the crew had to do was put the spacecraft in the correct attitude, with the blunt end forward. In the event of computer failure, Borman was ready to take over.{{sfn|Chaikin|1998|pp=127–128}}

[[File:The crew of Apollo 8 addresses the crew of the USS Yorktown after a successful splashdown and recovery.jpg|thumb|Crew of Apollo 8 addressing the crew of USS ''Yorktown'' after successful splashdown and recovery]]
Separation from the service module prepared the command module for reentry by exposing the heat shield and shedding unneeded mass. The service module would burn up in the atmosphere as planned.{{sfn|Chaikin|1998|pp=127–128}} Six minutes before they hit the top of the atmosphere, the crew saw the Moon rising above the Earth's horizon, just as had been calculated by the trajectory specialists.<ref name="splashdown1">{{cite web |url=https://history.nasa.gov/ap08fj/22day6_maroon_splash.htm |title=Day 6: The Maroon Team—Splashdown |last1=Woods |first1=W. David |last2=O'Brien |first2=Frank |date=April 22, 2006 |work=Apollo 8 Flight Journal |publisher=NASA |access-date=February 4, 2008 |archive-url=https://web.archive.org/web/20080206092307/https://history.nasa.gov/ap08fj/22day6_maroon_splash.htm |archive-date=February 6, 2008 |url-status=dead }}</ref> As the module hit the thin outer atmosphere, the crew noticed that it was becoming hazy outside as glowing [[Plasma (physics)|plasma]] formed around the spacecraft.{{sfn|Orloff|2000|p=40}} The spacecraft started slowing down, and the deceleration peaked at {{convert|6|g0}}. With the computer controlling the descent by changing the [[Flight dynamics (fixed-wing aircraft)|attitude]] of the spacecraft, Apollo{{nbsp}}8 rose briefly like a skipping stone before descending to the ocean. At {{convert|30000|ft|km}}, the drogue parachute deployed, stabilizing the spacecraft, followed at {{convert|10000|ft|km}} by the three main parachutes. The spacecraft splashdown position was officially reported as {{Coord|8|8|N|165|1|W|name=Apollo 8 estimated splashdown}} in the North Pacific Ocean, southwest of Hawaii at 15:51:42 UTC on December 27, 1968.<ref name="MissionReport" />

[[File:Ap8-S68-56310.jpg|thumb|Command module on the deck of {{USS|Yorktown|CV-10|6}}]]

When the spacecraft hit the water, the parachutes dragged it over and left it upside down, in what was termed Stable{{nbsp}}2 position. As they were buffeted by a {{convert|10|ft|m|adj=on}} swell, Borman vomited, waiting for the three flotation balloons to right the spacecraft.{{sfn|Kluger|2017|p=277}} About six minutes after splashdown, the command module was righted into a normal apex-up (Stable 1) orientation by its inflatable bag uprighting system.{{sfn|Orloff|2000|p=40}} The first [[frogman]] from [[aircraft carrier]] {{USS|Yorktown|CV-10|6}} arrived 43 minutes after splashdown. Forty-five minutes later, the crew was safe on the flight deck of the ''Yorktown''.<ref name="splashdown1" />{{sfn|Orloff|2000|p=40}}