Editing Electrical telegraph (section)

==Telegraphy and longitude==
Main article § Section: {{Section link|History of longitude|Land surveying and telegraphy}}.

The telegraph was very important for sending time signals to determine longitude, providing greater accuracy than previously available. Longitude was measured by comparing local time (for example local noon occurs when the sun is at its highest above the horizon) with absolute time (a time that is the same for an observer anywhere on earth). If the local times of two places differ by one hour, the difference in longitude between them is 15° (360°/24h). Before telegraphy, absolute time could be obtained from astronomical events, such as [[eclipses]], [[occultations]] or [[Lunar distance (navigation)|lunar distances]], or by transporting an accurate clock (a [[Marine chronometer|chronometer]]) from one location to the other.

The idea of using the telegraph to transmit a time signal for longitude determination was suggested by [[François Arago]] to [[Samuel Morse]] in 1837,<ref name="Walker 1850">{{cite journal |last1=Walker |first1=Sears C |author-link=Sears Cook Walker |title=Report on the experience of the Coast Survey in regard to telegraph operations, for determination of longitude &c. |journal=American Journal of Science and Arts |date=1850 |volume=10 |issue=28 |pages=151–160 |url=https://archive.org/details/appendix-telegraphic-longitude-the-american-journal-of-science-and-arts}}</ref> and the first test of this idea was made by [[Charles Wilkes|Capt. Wilkes]] of the U.S. Navy in 1844, over Morse's line between Washington and Baltimore.<ref name="Briggs 1858">{{cite book |last1=Briggs |first1=Charles Frederick |last2=Maverick |first2=Augustus |title=The Story of the Telegraph, and a History of the Great Atlantic Cable: A Complete Record of the Inception, Progress, and Final Success of that Undertaking: a General History of Land and Oceanic Telegraphs: Descriptions of Telegraphic Apparatus, and Biographical Sketches of the Principal Persons Connected with the Great Work |date=1858 |publisher=Rudd & Carleton |location=New York |url=https://archive.org/details/storyoftelegraph01brig}}</ref> The method was soon in practical use for longitude determination, in particular by the U.S. Coast Survey, and over longer and longer distances as the telegraph network spread across North America and the world, and as technical developments improved accuracy and productivity<ref name="Loomis 1856">{{cite book |last1=Loomis |first1=Elias |title=The recent progress of astronomy, especially in the United States |edition=3rd |date=1856 |publisher=Harper and Brothers |location=New York |url=https://archive.org/details/recentprogressof00loomrich}}</ref>{{rp|318–330}}<ref name="Stachurski">{{cite book |last1=Stachurski |first1=Richard |title=Longitude by Wire: Finding North America |date=2009 |publisher=University of South Carolina Press |location=Columbia |isbn=978-1-57003-801-3}}</ref>{{rp|98–107}}

The "telegraphic longitude net"<ref name="Schott 1897">{{cite journal |last1=Schott |first1=Charles A. |title=The telegraphic longitude net of the United States and its connection with that of Europe, as developed by the Coast and Geodetic Survey between 1866 and 1896 |journal=The Astronomical Journal |date=1897 |volume=18 |pages=25–28 |doi=10.1086/102749 |bibcode=1897AJ.....18...25S |doi-access=free}}</ref> soon became worldwide. Transatlantic links between Europe and North America were established in 1866 and 1870. The US&nbsp;Navy extended observations into the West Indies and Central and South America with an additional transatlantic link from South America to Lisbon between 1874 and 1890.<ref name="Green1877">{{cite book |last1=Green |first1=Francis Matthews |title=Report on the telegraphic determination of differences of longitude in the West Indies and Central America |date=1877 |publisher=US Hydrographic Office |location=Washington |url=https://archive.org/details/reportontelegra00greegoog}}</ref><ref name="Green1880">{{cite book |last1=Green |first1=Francis Matthews |title=Telegraphic determination of longitudes on the east coast of South America embracing the meridians of Lisbon, Madeira, St. Vincent, Pernambuco, Bahia, Rio de Janeiro, Montevideo, Buenos Ayres, and Para, with the latitude of the several stations |date=1880 |publisher=US Hydrographic Office |location=Washington |url=https://archive.org/details/telegraphicdete00norrgoog}}</ref><ref name="Davis1885">{{cite book |last1=Davis |first1=Chales Henry |last2=Norris |first2=John Alexander |title=Telegraphic Determination of Longitudes in Mexico and Central America and on the West Coast of South America: Embracing the Meridians of Vera Cruz; Guatemala; La&nbsp;Libertad; Salvador; Paita; Lima; Arica; Valparaiso; and the Argentine National Observatory at Cordoba; with the Latitudes of the Several Sea-coast Stations |date=1885 |publisher=US Hydrographic Office |location=Washington |url=https://archive.org/details/davis-1885-telegraphic-determination-of-longitudes}}</ref><ref name="Laird1891">{{cite book |last1=Norris |first1=John Alexander |last2=Laird |first2=Charles |last3=Holcombe |first3=John H.L. |last4=Garrett |first4=Le&nbsp;Roy M. |title=Telegraphic determination of longitudes in Mexico, Central America, the West Indies, and on the north coast of South America, embracing the meridians of Coatzacoalcos; Salina Cruz; La&nbsp;Libertad; San Juan del Sur; St.&nbsp;Nicolas Mole; Port Plata; Santo Domingo; Curacao; and La&nbsp;Guayra, with the latitudes of the several stations |date=1891 |publisher=US Hydrographic Office |location=Washington |url=https://archive.org/details/telegraphicdete00garrgoog}}</ref> British, Russian and US observations created a chain from Europe through Suez, Aden, Madras, Singapore, China and Japan, to Vladivostok, thence to Saint Petersburg and back to Western Europe.<ref name="Green1883">{{cite book |last1=Green |first1=Francis Mathews |last2=Davis |first2=Charles Henry |last3=Norris |first3=John Alexander |title=Telegraphic Determination of Longitudes in Japan, China, and the East Indies: Embracing the Meridians of Yokohama, Nagasaki, Wladiwostok, Shanghai, Amoy, Hong-Kong, Manila, Cape St.&nbsp;James, Singapore, Batavia, and Madras, with the Latitude of the Several Stations |date=1883 |publisher=US Hydrographic Office |location=Washington |url=https://archive.org/details/in.ernet.dli.2015.177254}}</ref>

Australia's telegraph network was linked to Singapore's via Java in 1871,<ref name="Martinez2017">{{cite journal |last1=Martinez |first1=Julia |title=Asian Servants for the Imperial Telegraph: Imagining North Australia as an Indian Ocean Colony before 1914 |journal=Australian Historical Studies |date=2017 |volume=48 |issue=2 |pages=227–243 |doi=10.1080/1031461X.2017.1279196 |s2cid=149205560 |url=https://ro.uow.edu.au/cgi/viewcontent.cgi?article=4008&context=lhapapers}}</ref> and the net circled the globe in 1902 with the connection of the Australia and New Zealand networks to Canada's via the [[All Red Line]]. The two determinations of longitudes, one transmitted from east to west and the other from west to east, agreed within one second of arc ({{frac|1|15}}&nbsp;second of time – less than 30&nbsp;metres).<ref name="Stewart1924">{{cite journal |last1=Stewart |first1=R.Meldrum |title=Dr. Otto Klotz |journal=Journal of the Royal Astronomical Society of Canada |date=1924 |volume=18 |pages=1–8 |bibcode=1924JRASC..18....1S |url=https://ui.adsabs.harvard.edu/abs/1924JRASC..18....1S}}</ref>