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===Early history=== Humans first acquired knowledge of the waves and currents of the [[sea]]s and [[ocean]]s in pre-historic times. Observations on [[tide]]s were recorded by [[Aristotle]] and [[Strabo]] in 384–322 BC.<ref>{{Cite web|title=A History Of The Study Of Marine Biology ~ MarineBio Conservation Society|date=17 June 2018|url=https://www.marinebio.org/creatures/marine-biology/history-of-marine-biology/|access-date=2021-05-17|language=en-US}}</ref> Early exploration of the oceans was primarily for [[cartography]] and mainly limited to its surfaces and of the animals that fishermen brought up in nets, though depth soundings by lead line were taken. The Portuguese campaign of Atlantic navigation is the earliest example of a systematic scientific large project, sustained over many decades, studying the currents and winds of the Atlantic. The work of [[Pedro Nunes]] (1502–1578) is remembered in the navigation context for the determination of the loxodromic curve: the shortest course between two points on the surface of a sphere represented onto a two-dimensional map.<ref name="mathshistory.st-andrews.ac.uk">[https://mathshistory.st-andrews.ac.uk/Biographies/Nunes/ Pedro Nunes Salaciense] at the [[MacTutor History of Mathematics archive]] (retrieved 13/06/2020)</ref><ref>W.G.L. Randles, "Pedro Nunes and the Discovery of the Loxodromic Curve, or How, in the 16th Century, Navigating with a Globe had Failed to Solve the Difficulties Encountered with the Plane Chart", Revista da Universidade Coimbra, 35 (1989), 119–30.</ref> When he published his "Treatise of the Sphere" (1537), mostly a commentated translation of earlier work by others, he included a treatise on geometrical and astronomic methods of navigation. There he states clearly that Portuguese navigations were not an adventurous endeavour: ''"nam se fezeram indo a acertar: mas partiam os nossos mareantes muy ensinados e prouidos de estromentos e regras de astrologia e geometria que sam as cousas que os cosmographos ham dadar apercebidas (...) e leuaua cartas muy particularmente rumadas e na ja as de que os antigos vsauam"'' (were not done by chance: but our seafarers departed well taught and provided with instruments and rules of astrology (astronomy) and geometry which were matters the cosmographers would provide (...) and they took charts with exact routes and no longer those used by the ancient).<ref>Pedro Nunes Salaciense, Tratado da Esfera, cap. 'Carta de Marear com o Regimento da Altura' p.2 - https://archive.org/details/tratadodaspherac00sacr/page/n123/mode/2up (retrieved 13/06/2020)</ref> His credibility rests on being personally involved in the instruction of pilots and senior seafarers from 1527 onwards by Royal appointment, along with his recognized competence as mathematician and astronomer.<ref name="mathshistory.st-andrews.ac.uk"/> The main problem in navigating back from the south of the [[Canary Islands]] (or south of [[Boujdour]]) by sail alone, is due to the change in the regime of winds and currents: the [[North Atlantic gyre]] and the Equatorial counter current <ref>http://ksuweb.kennesaw.edu/~jdirnber/oceanography/LecuturesOceanogr/LecCurrents/LecCurrents.html (retrieved 13/06/2020)</ref> will push south along the northwest bulge of Africa, while the uncertain winds where the Northeast trades meet the Southeast trades (the doldrums) <ref>https://kids.britannica.com/students/assembly/view/166714 (retrieved 13/06/2020)</ref> leave a sailing ship to the mercy of the currents. Together, prevalent current and wind make northwards progress very difficult or impossible. It was to overcome this problem and clear the passage to India around Africa as a viable maritime trade route, that a systematic plan of exploration was devised by the Portuguese. The return route from regions south of the Canaries became the '[[Volta do mar|volta do largo' or 'volta do mar]]'. The 'rediscovery' of the [[Azores|Azores islands]] in 1427 is merely a reflection of the heightened strategic importance of the islands, now sitting on the return route from the western coast of Africa (sequentially called 'volta de Guiné' and 'volta da Mina'); and the references to the [[Sargasso Sea]] (also called at the time 'Mar da Baga'), to the west of the [[Azores]], in 1436, reveals the western extent of the return route.<ref name="ReferenceA">Carlos Calinas Correia, A Arte de Navegar na Época dos Descobrimentos, Colibri, Lisboa 2017; {{ISBN|978-989-689-656-0}}</ref> This is necessary, under sail, to make use of the southeasterly and northeasterly winds away from the western coast of Africa, up to the northern latitudes where the westerly winds will bring the seafarers towards the western coasts of Europe.<ref>{{cite web |url=https://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Map_prevailing_winds_on_earth.png/1200px-Map_prevailing_winds_on_earth.png |title=Map|website=upload.wikimedia.org |format=PDF|access-date=2020-09-15}}</ref> The secrecy involving the Portuguese navigations, with the death penalty for the leaking of maps and routes, concentrated all sensitive records in the Royal Archives, completely destroyed by the [[1755 Lisbon earthquake|Lisbon earthquake of 1775]]. However, the systematic nature of the Portuguese campaign, mapping the currents and winds of the Atlantic, is demonstrated by the understanding of the seasonal variations, with expeditions setting sail at different times of the year taking different routes to take account of seasonal predominate winds. This happens from as early as late 15th century and early 16th: [[Bartolomeu Dias]] followed the African coast on his way south in August 1487, while [[Vasco da Gama]] would take an open sea route from the latitude of [[Sierra Leone]], spending three months in the open sea of the South Atlantic to profit from the southwards deflection of the southwesterly on the Brazilian side (and the Brazilian current going southward - Gama departed in July 1497); and [[Pedro Álvares Cabral]] (departing March 1500) took an even larger arch to the west, from the latitude of Cape Verde, thus avoiding the summer monsoon (which would have blocked the route taken by Gama at the time he set sail).<ref>Carlos Viegas Gago Coutinho, A Viagem de Bartolomeu Dias, Anais (Clube Militar Naval) May 1946</ref> Furthermore, there were systematic expeditions pushing into the western Northern Atlantic (Teive, 1454; Vogado, 1462; Teles, 1474; Ulmo, 1486).<ref>Carlos Viegas Gago Coutinho, As Primeiras Travessia Atlanticas - lecture, Academia Portuguesa de História, 22/04/1942 - in: Anais (APH) 1949, II serie, vol.2</ref> The documents relating to the supplying of ships, and the ordering of sun declination tables for the southern Atlantic for as early as 1493–1496,<ref>Luís Adão da Fonseca, Pedro Álvares Cabral - Uma Viagem, INAPA, Lisboa, 1999, p.48</ref> all suggest a well-planned and systematic activity happening during the decade long period between [[Bartolomeu Dias]] finding the southern tip of Africa, and Gama's departure; additionally, there are indications of further travels by Bartolomeu Dias in the area.<ref name="ReferenceA" /> The most significant consequence of this systematic knowledge was the negotiation of the [[Treaty of Tordesillas]] in 1494, moving the line of demarcation 270 leagues to the west (from 100 to 370 leagues west of the Azores), bringing what is now Brazil into the Portuguese area of domination. The knowledge gathered from open sea exploration allowed for the well-documented extended periods of sail without sight of land, not by accident but as pre-determined planned route; for example, 30 days for [[Bartolomeu Dias]] culminating on [[Mossel Bay]], the three months Gama spent in the South Atlantic to use the Brazil current (southward), or the 29 days Cabral took from Cape Verde up to landing in [[Monte Pascoal]], Brazil. The [[Danish Arabia expedition (1761–67)|Danish expedition to Arabia]] 1761–67 can be said to be the world's first oceanographic expedition, as the ship [[HDMS Grønland (1756)|Grønland]] had on board a group of scientists, including naturalist [[Peter Forsskål]], who was assigned an explicit task by the king, [[Frederick V of Denmark|Frederik V]], to study and describe the marine life in the open sea, including finding the cause of [[Milky seas effect|mareel]], or milky seas. For this purpose, the expedition was equipped with nets and scrapers, specifically designed to collect samples from the open waters and the bottom at great depth.<ref>{{cite book |last1=Wolff |first1=Torben |title=Danish Expeditions on the Seven Seas |date=1969 |publisher=Rhodos |location=Copenhagen}}</ref> Although [[Juan Ponce de León]] in 1513 first identified the [[Gulf Stream]], and the current was well known to mariners, [[Benjamin Franklin]] made the first scientific study of it and gave it its name. Franklin measured water temperatures during several Atlantic crossings and correctly explained the Gulf Stream's cause. Franklin and Timothy Folger printed the first map of the [[Gulf Stream]] in 1769–1770.<ref name="NOAA_Franklin">{{Cite web|url=http://oceanexplorer.noaa.gov/library/readings/gulf/gulf.html|archive-url=https://web.archive.org/web/20051218185445/http://oceanexplorer.noaa.gov/library/readings/gulf/gulf.html|url-status=dead|title=1785: Benjamin Franklin's 'Sundry Maritime Observations'|archive-date=December 18, 2005}}</ref><ref>Wilkinson, Jerry. [http://www.keyshistory.org/gulfstream.html History of the Gulf Stream] 1 January 2008</ref> [[File:Rennel map 1799.png|left|thumb|1799 map of the currents in the [[Atlantic Ocean|Atlantic]] and [[Indian Ocean]]s, by [[James Rennell]]]] Information on the currents of the [[Pacific Ocean]] was gathered by explorers of the late 18th century, including [[James Cook]] and [[Louis Antoine de Bougainville]]. [[James Rennell]] wrote the first scientific textbooks on oceanography, detailing the current flows of the [[Atlantic Ocean|Atlantic]] and [[Indian Ocean|Indian]] oceans. During a voyage around the [[Cape of Good Hope]] in 1777, he mapped ''"the [[Agulhas Current|banks and currents at the Lagullas]]"''. He was also the first to understand the nature of the intermittent current near the [[Isles of Scilly]], (now known as Rennell's Current).<ref>{{cite DNB|wstitle=Rennell, James|volume=48}}</ref> The tides and currents of the ocean are distinct. Tides are the rise and fall of [[sea level]]s created by the combination of the [[Gravity|gravitational]] forces of the [[Moon]] along with the Sun (the Sun just in a much lesser extent) and are also caused by the [[Earth]] and [[Moon]] orbiting each other. An ocean current is a continuous, directed movement of [[Seawater|seawater generated]] by a number of forces acting upon the water, including wind, the [[Coriolis effect]], [[breaking wave]]s, [[cabbeling]], and temperature and [[Salinity|salinity differences]].<ref>{{Cite web |last=US Department of Commerce |first=National Oceanic and Atmospheric Administration |title=Tides and Currents |url=https://oceanservice.noaa.gov/navigation/tidesandcurrents/#:~:text=Tides%20involve%20water%20moving%20up,other%20facts%20that%20drive%20currents. |access-date=2024-04-25 |website=oceanservice.noaa.gov |language=EN-US}}</ref> Sir [[James Clark Ross]] took the first modern sounding in deep sea in 1840, and [[Charles Darwin]] published a paper on [[reef]]s and the formation of [[atoll]]s as a result of the [[Second voyage of HMS Beagle|second voyage of HMS ''Beagle'']] in 1831–1836. [[Robert FitzRoy]] published a four-volume report of ''Beagle''{{'}}s three voyages. In 1841–1842 [[Edward Forbes]] undertook dredging in the [[Aegean Sea]] that founded marine ecology. The first superintendent of the [[United States Naval Observatory]] (1842–1861), [[Matthew Fontaine Maury]] devoted his time to the study of marine meteorology, [[navigation]], and charting prevailing winds and currents. His 1855 textbook ''Physical Geography of the Sea'' was one of the first comprehensive oceanography studies. Many nations sent oceanographic observations to Maury at the Naval Observatory, where he and his colleagues evaluated the information and distributed the results worldwide.<ref>Williams, Frances L. ''Matthew Fontaine Maury, Scientist of the Sea.'' (1969) {{ISBN|0-8135-0433-3}}</ref>
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