Editing Tranquility Base (section)

==Site selection==
For more than two years, [[NASA]] planners considered a collection of 30 potential sites for the first crewed landing. Based on high-resolution photographs taken by the [[Lunar Orbiter]] spacecraft, and photos and data taken by the uncrewed [[Surveyor Program|Surveyor]] landers, this list was narrowed down to five sites located near the lunar equator. They ranged between 45 degrees east and west, and 5 degrees north and south of the center of the Moon's facing side. They were numbered 1 to 5, going from east to west. Site number 2, centered at {{coord|0|42|50|N|23|42|28|E|globe:moon}}, was the Sea of Tranquility site ultimately chosen.<ref>{{cite press release
 | title = Apollo 11 Lunar Landing Mission Press Kit
 | publisher = [[NASA]]
 | date = July 6, 1969
 | pages = 82–85
 | url = https://history.nasa.gov/alsj/a11/A11_PressKit.pdf
 }}
</ref> Since a precision landing was not expected on the first mission, the target area was an ellipse measuring {{convert|11.5|mi|km}} east and west by {{convert|3.0|mi|km}} north and south.<ref name="chaikin_88">{{Cite book |author=Chaikin, Andrew |title=A Man on the Moon: The Triumphant Story Of The Apollo Space Program |date=2007|publisher=[[Penguin Group]] |location=New York |page=88|isbn=978-0-14-311235-8}}</ref>

On the landing, a combination of thrust from residual pressure in the docking tunnel that connected the Lunar Module with the [[Apollo command and service module|command module]] ''[[Command module Columbia|Columbia]]'' in orbit, and an imperfect understanding of the Moon's uneven gravitational field, resulted in navigation errors which pushed the powered descent initiation point about {{convert|3|mi|km}}, and thus the computer-targeted landing spot about {{convert|4|mi|km}}, downrange (west) of the planned target.<ref>{{cite web| url=http://www.history.nasa.gov/alsj/a11/a11.landing.html| last=Jones| first=Eric M.| title=The First Lunar Landing, time 102:36:21| publisher=Apollo Lunar Surface Journal| via=NASA| access-date=2013-08-09| archive-date=2017-12-25| archive-url=https://web.archive.org/web/20171225232200/https://history.nasa.gov/alsj/a11/a11.landing.html| url-status=dead}}</ref> The automated targeting system was taking ''Eagle'' toward what Armstrong described as a "[[American football field|football-field]] sized crater, with a large number of big boulders and rocks for about one or two crater diameters around it", which he avoided by assuming manual control and flying a bit farther downrange.<ref>{{cite web| url=http://www.history.nasa.gov/alsj/a11/a11.postland.html| last=Jones| first=Eric M.| title=Post-landing Activities, time 102:55:16| publisher=Apollo Lunar Surface Journal| via=NASA| access-date=2013-08-23| archive-date=2021-04-18| archive-url=https://web.archive.org/web/20210418132829/https://history.nasa.gov/alsj/a11/a11.postland.html| url-status=dead}}</ref> The landing was still within the target ellipse.