Editing Celestial navigation (section)

====Use of time====
{{Main|Longitude by chronometer|Marine chronometer}}

The considerably more popular method was (and still is) to use an accurate timepiece to directly measure the time of a sextant sight. The need for accurate navigation led to the development of progressively more accurate chronometers in the 18th&nbsp;century (see [[John Harrison]]). Today, time is measured with a chronometer, a [[Quartz clock|quartz watch]], a [[Time signal#Radio time signals|shortwave radio time signal]] broadcast from an [[atomic clock]], or the time displayed on a [[Atomic clock#Global navigation satellite systems|satellite time signal]] receiver.<ref>{{cite web |last=Mehaffey |first=Joe |title=How accurate is the TIME DISPLAY on my GPS? |url=http://gpsinformation.net/main/gpstime.htm |url-status=live |archive-url=https://web.archive.org/web/20170804120341/http://gpsinformation.net/main/gpstime.htm |archive-date=4 August 2017 |access-date=9 May 2018 |website=gpsinformation.net}}</ref> A [[Watch#Electronic|quartz wristwatch]] normally keeps time within a half-second per day. If it is worn constantly, keeping it near body heat, its rate of drift can be measured with the radio, and by compensating for this drift, a navigator can keep time to better than a second per month. When time at the [[prime meridian]] (or another starting point) is accurately known, celestial navigation can determine longitude, and the more accurately latitude and time are known, the more accurate the longitude determination. [[Earth's rotation#Angular speed|The angular speed of the Earth]] is latitude-dependent. At the poles, or latitude 90°, the rotation velocity of the Earth reaches zero. At 45° latitude, one second of time is equivalent in longitude to {{convert|1077.8|ft|2|lk=on|abbr=on}}, or one-tenth of a second means {{convert|107.8|ft|2|abbr=on}}<ref>[http://adsabs.harvard.edu/pdf/1914JRASC...8...85M Errors in Longitude, Latitude and Azimuth Determinations — I] by F. A. McDiarmid, The Royal Astronomical Society of Canada, 1914.</ref> At the slightly bulged-out equator, or latitude 0°, the rotation velocity of Earth or its equivalent in longitude reaches its maximum at {{convert|465.10|m/s|1|lk=on|abbr=on}}.<ref name="Cox2000">{{cite book |editor=Arthur N. Cox |title=Allen's Astrophysical Quantities |url=https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA244 |edition=4th |date=2000 |publisher=AIP Press |location=New York |isbn=978-0-387-98746-0 |page=244 |access-date=17 August 2010}}</ref>

Traditionally, a navigator checked their chronometer(s) with their sextant at a geographic marker surveyed by a professional astronomer. This is now a rare skill, and most [[harbormaster]]s cannot locate their harbor's marker. Ships often carried more than one chronometer. Chronometers were kept on [[gimbals]] in a dry room near the center of the ship. They were used to set a [[hack watch]] for the actual sight, so that no chronometers were ever exposed to the wind and salt water on deck. Winding and comparing the chronometers was a crucial duty of the navigator. Even today, it is still logged daily in the ship's deck log and reported to the captain before [[Ship's bell|eight bells]] on the forenoon watch (shipboard noon). Navigators also set the ship's clocks and calendar. Two chronometers provided [[dual modular redundancy]], allowing a backup if one ceases to work but not allowing any [[error correction]] if the two displayed a different time, since in case of contradiction between the two chronometers, it would be impossible to know which one was wrong (the [[error detection]] obtained would be the same as having only one chronometer and checking it periodically: every day at noon against [[dead reckoning]]). Three chronometers provided [[triple modular redundancy]], allowing [[error correction]] if one of the three was wrong, so the pilot would take the average of the two with closer readings (average precision vote). There is an old adage to this effect, stating: "Never go to sea with two chronometers; take one or three."<ref>{{Cite book |title=[[The Mythical Man-Month]] |last=Brooks |first=Frederick J. |publisher=Addison-Wesley |year=1995 |isbn=0-201-83595-9 |page=[https://archive.org/details/mythicalmonth00broo/page/64 64] |author-link=Fred Brooks |orig-year=1975}}</ref> Vessels engaged in survey work generally carried many more than three chronometers{{snd}} for example, [[HMS Beagle|HMS ''Beagle'']] carried [[List of chronometers on HMS Beagle|22 chronometers]].<ref>{{cite web |url=http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F10.2&pageseq=41 |title=Volume II: Proceedings of the Second Expedition |page=18 |author=R. Fitzroy}}</ref>