Editing Navigation (section)

===Longitude===
{{Further|Longitude}}
Similar to latitude, the longitude of a place on Earth is the angular distance east or west of the [[prime meridian]] or [[Greenwich meridian]].<ref name="bow4"/> Longitude is usually expressed in [[degree (angle)|degrees]] (marked with °) ranging from [[prime meridian|0°]] at the Greenwich meridian to [[180th meridian|180°]] east and west. [[Sydney]], for example, has a longitude of about [[151st meridian east|151° east]]. [[New York City]] has a longitude of [[74th meridian west|74° west]]. For most of history, mariners struggled to determine longitude. Longitude can be calculated if the precise time of a sighting is known. Lacking that, one can use a [[sextant]] to take a [[Lunar distance (navigation)|lunar distance]] (also called ''the lunar observation'', or "lunar" for short) that, with a [[nautical almanac]], can be used to calculate the time at zero longitude (see [[Greenwich Mean Time]]).<ref name="Norie 1828 pg 222">
 {{cite book
  | last =Norie
  | first =J.W.
  | title =New and Complete Epitome of Practical Navigation
  | year =1828
  | location =London
  | url =http://www.mysticseaport.org/library/initiative/ImPage.cfm?PageNum=3&BibId=13617&ChapterId=30
  | access-date =2007-08-02
  | page =222
  | url-status=dead
  | archive-url =https://web.archive.org/web/20070927203111/http://www.mysticseaport.org/library/initiative/ImPage.cfm?PageNum=3&BibId=13617&ChapterId=30
  | archive-date =2007-09-27
  }}</ref> Reliable [[marine chronometer]]s were unavailable until the late 18th century and not affordable until the 19th century.<ref name="Norie 1828 pg 221">
{{cite book
  | last =Norie
  | first =J.W.
  | title =New and Complete Epitome of Practical Navigation
  | year =1828
  | location =London
  | url =http://www.mysticseaport.org/library/initiative/ImPage.cfm?PageNum=2&BibId=13617&ChapterId=30
  | access-date =2007-08-02
  | page =221
  | url-status=dead
  | archive-url =https://web.archive.org/web/20070927202912/http://www.mysticseaport.org/library/initiative/ImPage.cfm?PageNum=2&BibId=13617&ChapterId=30
  | archive-date =2007-09-27
  }}</ref><ref name="Taylor 1851 pg 195">{{Cite book
| last = Taylor
| first = Janet
| title=An Epitome of Navigation and Nautical Astronomy
| publisher = Taylor
| edition = Ninth
| year = 1851
| url=https://archive.org/details/anepitomenaviga00taylgoog
| quote = Nautical Almanac 1849-1851.
| access-date=2007-08-02
| page = 295f
}}
</ref><ref name="Watchmakers and their Work, pg 230">
{{cite book
| first=Frederick James
| last=Britten
| title = Former Clock & Watchmakers and Their Work
| year = 1894
| publisher=Spon & Chamberlain
| location = New York
|url=https://archive.org/details/formerclockwatc00britgoog
| quote=Chronometers were not regularly supplied to the Royal Navy until about 1825
|access-date=2007-08-08
| page=[https://archive.org/details/formerclockwatc00britgoog/page/n242 230]
}}</ref> For about a hundred years, from about 1767 until about 1850,<ref>Lecky, Squire, ''Wrinkles in Practical Navigation''</ref> mariners lacking a chronometer used the method of lunar distances to determine Greenwich time to find their longitude. A mariner with a chronometer could check its reading using a lunar determination of Greenwich time.<ref name="Norie 1828 pg 221"/><ref name = "Roberts">{{cite book
|last= Roberts |first= Edmund |author-link= Edmund Roberts (diplomat)
|title=1837, Embassy to the Eastern courts of Cochin-China, Siam, and Muscat: in the U.S. sloop-of-war Peacock ... during the years 1832–3–4
|url=  https://books.google.com/books?id=aSgPAAAAYAAJ |publisher=   Harper & brothers  
|page=   373 |no-pp= <!-- 0 hides automatic 'p' or 'pp'.--> 
|chapter=   Chapter XXIV―departure from Mozambique
|isbn=9780608404066 |chapter-url= https://books.google.com/books?id=aSgPAAAAYAAJ&pg=PA365
|access-date=  April 25, 2012
|edition=   Digital
|quote= ...what I have stated, will serve to show the absolute necessity of having first rate chronometers, or the lunar observations carefully attended to; and never omitted to be taken when practicable.
}}</ref>