Editing Longitude (section)

== Determination ==
{{further|Celestial navigation#Longitude|History_of_longitude#Methods_of_determining_longitude}}
{{see also|Latitude determination}}

The main conventional methods for determining longitude are listed below. With one exception (magnetic declination), they all depend on a common principle, which is to determine the time for an event or measurement and to compare it with the time at a different location. Longitude, being up to 180° east or west of a [[prime meridian]], is mathematically related to time differences up to 12 hours by a factor of 15. Thus, a time differential (in hours) between two points is multiplied by 15 to obtain a longitudinal difference (in degrees).

Historically, times used for calculating longitude have included [[apparent solar time]], [[local mean time]], and [[ephemeris time]], with mean time being the one most used for navigation of the sea. See also the [[equation of time]] for details on the differences.

* [[Lunar distance (navigation)|Lunar distances]] and moon [[culmination]]s. In its orbit around the Earth, the Moon moves relative to the stars at a rate of just over 0.5°/hour. The angle between the limb of the Moon and a suitable star, planet, or sun is measured with a [[sextant]], and, after consulting [[ephemeris]] tables, a value for the [[local mean time|mean time]] at a reference meridian, usually Greenwich, can be calculated. For a culmination, the observer simply records the time and compares it with the reference time in the ephemerides after correcting for [[refraction]] and other errors. This method was established by [[Nathaniel Pigott]] around 1786.<ref name=time>{{cite EB9 |wstitle = Measurement of Time |volume= XXIII |last= Dreyer |first= John Louis Emil |author-link= John Louis Emil Dreyer |pages= 392-396 |short= 1}}</ref> [[The Nautical Almanac]] was published in the UK beginning in 1767 and the [[American Ephemeris and Nautical Almanac]] starting in 1852.
* Satellites of Jupiter. [[Galileo Galilei|Galileo]] proposed that with sufficiently accurate knowledge of the orbits of the satellites, their positions could provide a measure of absolute time. The method requires a telescope, as the moons are not visible to the naked eye. Ephemeris tables are employed for comparison to a reference meridian.
* Appulses, occultations, transits, and eclipses. An [[appulse]] is the least apparent distance between two objects (the Moon, a star or a planet); an [[occultation]] occurs when a star or planet passes behind the Moon — essentially a type of eclipse. Lunar eclipses continued to be used. The times of these events are compared to those of a reference meridian. Major observatories used the [[Meridian circle|transit circle or meridian circle]] to establish very accurate longitude values for their country, often establishing their own [[prime meridian]] at the longitude of the instrument.<ref name=transit>{{cite EB1911|wstitle=Transit Circle|last=|first=|page=}}</ref>
* Transport of [[Marine chronometer|chronometers]]. A clock is set to the local time of a starting point whose longitude is known, and the longitude of any other place can be determined by comparing its [[local mean time]] with the clock time. While marine chronometers are relatively stable, they are also relatively large and expensive. Prior to the quartz crystal, chronometers were susceptible to time drift from temperature fluctuations and vibration.
* Signals. Rockets and lights were occasionally used in the 18th and 19th century, although the method is impractical except for short distances and demonstrations.<ref name=time /> It was a rudimentary form of [[Time and frequency transfer|synchronizing time]] and establishing longitude. However, [[time ball|signaling by "ball drop"]] was extensively used in the US Navy and Royal Navy in the 19th century. In each case, there were observatories near bodies of water that would drop a ball from a tower, alerting the ships of the correct time, and hence enabling them to maintain stable longitudinal position fixes while at sea.<ref name=ball>[https://prancer.physics.louisville.edu/modules/time/articles/how_time_balls_worked.pdf How Time Balls Work]</ref>
* [[Telegraph]]ic determination of longitude. First suggested by the American astronomer [[Sears Cook Walker]], the [[United States Coast Survey]] began deploying it in 1849.<ref name=time /> Europe quickly followed. As the American West was settled, mapping and surveying was greatly improved by the use of the telegraph to determine time and longitude differences between stations. The laying of [[transatlantic telegraph cable]]s also helped establish coordinated global mapping and navigation.
* [[Magnetic declination]]. A compass needle does not in general point [[true north]]. The variation from true north varies with location, and it was suggested that this could provide a basis for determination of longitude.
With the exception of magnetic declination, all proved practicable methods. Developments on land and sea, however, were very different.

Several newer methods for navigation, location finding, and the determination of longitude exist. [[Radio navigation]], [[satellite navigation]], and [[Inertial navigation system]]s, along with [[celestial navigation]], are a few of the more prevalent ones.