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In September 1522 the survivors of the Magellan and Elcano expedition, the first to circle the globe, reached the Cape Verde Islands with logs saying Wednesday, July 9, while locals said Thursday, July 10. Sailing west had cost them a day, as the 14th century geographer Abu'l-Fida had predicted. The International Date Line roughly follows the 180 degree meridian, but the 1884 International Meridian Conference in Washington, which set the prime meridian at Greenwich, refused to fix time zones or a date line, so the line has no treaty behind it and bends around national choices, swerving east around Russia's Chukotka and passing between the Diomede Islands, 1.5 kilometers apart and 21 hours apart.

Kiribati, independent from the UK in 1979, straddled the line and shared only four business days a week until 1994, when it pushed the line east and created UTC+14, making one atoll, renamed Millennium Island, first to see 2000. With Baker Island at UTC minus 12, the clock now spans 26 hours, so three calendar days coexist for two hours daily. The Philippines, tied to Mexico by the Manila Acapulco galleons, stayed a day behind its Asian neighbors for over 300 years until it deleted Tuesday, December 31, 1844. Samoa repeated Monday, July 4, 1892 to align with American traders, then skipped Friday, December 30, 2011 to align with Australia and New Zealand.

• When Alaska passed from Russia to the US in 1867, it crossed the line and left the Julian calendar, so Friday, October 6 was followed by Friday, October 18.
• Many Seventh-day Adventists in Tonga keep the Sabbath on the local Sunday, and Samoa's 2011 switch split its Adventist congregations.
• Islamic months begin with a physical sighting of the new crescent moon, sidestepping the line.
• Jewish refugees who reached Japan and China in World War II sparked debate over where a religious date line should fall.
• A 1917 Anglo-French conference set a rigid nautical date line, so a ship off Kiribati can sit in Sunday while the island is in Monday.

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Portolan charts are medieval nautical maps of the Mediterranean whose coastlines are startlingly accurate. The name, applied in the 1890s, comes from the Italian portolano, a book of sailing directions, but surviving charts from before 1500 are mostly drawn on vellum and covered in rhumb lines radiating from compass roses in 16 and later 32 directions. Sailors held one bearing along a line to their destination. Because the lines follow magnetic north, researchers can date undated charts from shifts in magnetic declination. Place names run perpendicular to the coast so a navigator could rotate the chart, and symbols marked hazards: crosses for submerged rocks, black dots for visible rocks, and red dots for shoals.

The oldest surviving chart, the Carta Pisana of around 1280, is already as accurate as those made 200 years later. One theory says medieval sailors compiled dead reckoning data, though compounding errors make that hard to credit. Another, based on cartometric analysis, argues the charts are tracings of older maps assembled as a mosaic of regional charts drawn on something like the Mercator projection, which Gerardus Mercator did not publish until 1569. Many surviving charts were status pieces owned by aristocrats and merchants, while working charts rotted at sea.

• Every chart before 1600 was traced by hand, and the first printed woodcut chart appeared in 1472.
• Copper plate printing, developed around 1477, allowed far finer lines and lettering.
• Mallorcan Jewish cartographer Abraham Cresques made the 1375 Catalan Atlas for the King of Aragon, drawing on Marco Polo.
• Arabic portolan charts from the 15th century and al-Umari's 14th century encyclopedia show Arab cartographers' role.
• Portuguese mapmakers added the equator and latitude lines as their ships lost sight of the North Star.

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When a great whale dies and its lungs deflate, its body, slightly denser than seawater, sinks fast enough to pass the midwater scavengers, and cold and pressure keep gases dissolved so it reaches the deep seafloor intact. The first clue came in 1854, when a new mussel species was taken from floating whale blubber, and in the 1960s trawlers hauled up Osteopelta limpets attached to whale bone. On February 19, 1977, Navy pilots in the bathyscaphe Trieste II spotted a stripped gray whale skeleton off Santa Catalina, California, and in 1987 oceanographer Craig Smith's Alvin team found a living whale fall community 1,240 meters down in the Catalina Basin.

The ecosystem runs in four stages. Hagfish, sleeper sharks, and eelpouts strip 40 to 60 kilograms of tissue a day for months up to a year and a half, then opportunists clean up for up to about four and a half years. For 50 to 100 years, bacteria digest the lipids in the bones and release hydrogen sulfide, which sulfur loving bacteria use for chemosynthesis, feeding clams and mussels, before bare bones become a reef. Eocene and Oligocene whale falls lack these communities because older whales had less fatty bones.

• Osedax, bone eater, has no mouth or stomach and uses root-like structures and acid to bore into bone.
• Sulfur reducing bacteria and methane producing archaea coexist at whale falls instead of competing.
• Whale shark and mobulid ray carcasses stall after the scavenger stage because cartilage skeletons lack the fat.
• A 40 ton carcass holds about two tons of carbon, equal to 2,000 years of normal carbon flux to that spot.
• Models estimate that industrial whaling cut deep sea biomass by more than 30 percent.

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The diving bell rests on a principle Aristotle recorded in the 4th century BC: an upturned cauldron forced straight down traps its air. In 1535 Guglielmo de Lorena used a bell to explore Roman barges in Lake Nemi, staying down over an hour until cold and exhaustion drove him up. The 1600s brought an underwater gold rush: Edward Bendall cleared a wreck from Boston Harbor in 1642, Albrecht von Treileben salvaged bronze cannons from the Vasa in 1658, and William Phips recovered jewels, gold, and about 30 tons of silver from a Spanish galleon off Haiti in 1687. Carbon dioxide was the enemy, and in 1691 Edmund Halley, of comet fame, lowered lead lined barrels of fresh air whose water pressure pushed air through a leather hose into his bell.

In 1783 Edinburgh confectioner Charles Spalding and his nephew suffocated in his improved bell off Dublin. Modern wet bells stay open at the bottom and are fed gas from the surface, and divers ascending in them must decompress for hours. Closed bells sealed the problem, carrying saturation divers between pressurized deck chambers and the seabed for weeks of work. Submarine rescue bells reverse the idea, clamping onto a sunken sub's hatch so sailors ride up at surface pressure, while airlock bells like the British Admiralty's 1902 Gibraltar bell let workers walk down a tube in street clothes.

• A Type 1 wet bell runs divers' umbilicals to the surface, while a Type 2 feeds them from a panel inside the bell.
• A closed bell's bottom hatch opens inward, so internal pressure presses it tighter against its seals.
• Divers pinned to the bottom can cut their own umbilical from inside and float up on emergency gas and soda lime scrubbers.
• Launch systems guide bells down tensioned clump weight cables, and aerated moon pools soften their entry into the water.
• The diving bell spider, Argyroneta aquatica, stocks an underwater web dome with air carried in its body hairs.

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In October 844, 80 longships sailed up the Guadalquivir River in the Emirate of Cordoba and besieged Seville, a reach made possible by one technology. The longship's roots run from Bronze Age Swedish petroglyphs around 1800 BC and the Hjortspring boat of about 350 BC to the Nydam ship of around 320 AD, which introduced clinker building with overlapping planks fastened by thousands of iron rivets smelted from bog ore. Shipwrights split freshly cut green oak with axes instead of saws, choosing naturally curved trees so the fibers stayed intact, and as the green wood dried it pulled tighter against the rivets, sealed with tarred wool or animal hair. The result was a light, flexible hull that twisted with the waves and could float in a meter of water.

The sail, adopted around 780, reshaped Scandinavian society: one 100 square meter wool sail needed about 180 kilometers of hand spun yarn and more than a year of women's labor, and a spar called the beitass let ships work into the wind. Navigators relied on the sun, the North Star, seabirds, and swells. Thorkild Ramskou's 1969 sunstone theory is unproven, since no sunstone has been found in a wreck and astronomer Bradley Schaefer found Iceland spar performed poorly in real fog. Longships dominated for centuries until tall Hanseatic cogs made boarding impossible, and in 1429 a Norwegian levy fleet was destroyed by the Victual Brothers.

• The word starboard comes from steerboard, the side where the steering oar was mounted.
• Rigging was made from linden bark or spiral cut walrus hide that resisted saltwater rot.
• A wooden disk found at Uunartoq in Greenland appears to be a sun compass.
• Roskilde 6, from around 1025 AD, is nearly 37 meters long and carried about 100 men, including 78 rowers.
• Olaf Tryggvason's Long Serpent was a dragon ship with 34 pairs of rowing benches.

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Duke Kahanamoku was born in Honolulu in 1890, three years before the overthrow of the Hawaiian Kingdom. Duke was his given name, inherited from his father, who was named for Prince Alfred, Duke of Edinburgh, and his family were of alii descent and served the Kamehameha royals as kahu. He left the Kamehameha Schools to help support his family and grew up on Waikiki Beach, surfing a 16 foot, 114 pound finless koa board he called his Papa Nui. In 1911 he swam the 100 yard freestyle in Honolulu Harbor in 55.4 seconds, 4.6 seconds under the record, and the Amateur Athletic Union refused to believe it, blaming alarm clocks and currents.

He answered with gold in the 100 meter freestyle at the 1912 Stockholm Olympics, gold again in 1920 in Antwerp, and silver in 1924 in Paris at age 34, behind Johnny Weissmuller and ahead of his brother Samuel. His flutter kick replaced the draggy scissors kick. Touring as a swimmer, he brought surfing to Southern California in 1912 and to Australia on December 24, 1914 at Freshwater Beach in Sydney. He later acted in Hollywood films including Mister Roberts, married Nadine Alexander in 1940, and served as Sheriff of Honolulu from 1935 to 1961.

• For the Australian exhibition he carved a new board from sugar pine bought at a local hardware store.
• On June 14, 1925 at Newport Beach, California, he pulled eight men from a capsized fishing boat using his surfboard.
• That rescue helped make the surfboard a standard lifeguard tool.
• His name is on Duncan v. Kahanamoku, the 1946 Supreme Court case that ruled against trying civilians in military tribunals in wartime Hawaii.
• His 1990 bronze statue in Waikiki faces the street rather than the ocean, arms open to welcome visitors.

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The astrolabe, from the Greek for star taker, is a handheld brass model of the sky that tells time, finds latitude, surveys heights, and predicts the solstice. Its base, the mater, holds interchangeable plates called tympans engraved for specific latitudes using stereographic projection, and over them turns the rete, an open star map whose pointers were shaped like leaves, hands, dogs' heads, or dragons. You hang it plumb, sight a star such as Sirius with the alidade on the back, turn the rete to match its altitude, and read the time off the rim. The idea grew from Greek armillary spheres, and the popular claim that Hypatia invented it is a myth: her pupil Synesius only wrote that she taught him to build one, and Ptolemy's Almagest describes a 3D armillary sphere instead.

The earliest manuals are John Philoponus's 6th century Greek treatise and Severus Sebokht's Syriac text describing brass instruments. In the Islamic world, Muhammad al-Fazari built the first, al-Sufi wrote 386 chapters on over a thousand uses, and it was used for prayer times, the qibla, and Ramadan. It reached India, where the Jain astronomer Mahendra Suri wrote a Sanskrit manual in 1370 calling it the king of instruments, and Europe through Gerbert of Aurillac, later Pope Sylvester II. Mariners stripped it to a heavy, open ring for sighting latitude at sea.

• Sharaf al-Din al-Tusi built a linear astrolabe that was just a marked rod and plumb line.
• Ibn al-Sarraj's 14th century universal astrolabe worked at any latitude without swapping plates.
• Geoffrey Chaucer wrote an astrolabe treatise in plain English for his 10 year old son Lewis.
• Early astronomical clocks such as Richard of Wallingford's and the Prague Orloj are mechanized astrolabes.
• Ulysse Nardin and Christiaan van der Klaauw still make wristwatches with astrolabe dials.

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The HMS Wager began around 1734 as an East Indiaman, a wide cargo hauler, and the Royal Navy bought her for a little over 3,900 pounds and spent about 7,000 more at Deptford Dockyard to make her a 28 gun sixth rate. Assigned as the supply ship for Commodore George Anson's 1,800 man expedition against Spain's Pacific coast of South America, she was crewed with aged, sick, and invalid men while the best sailors went to the warships. Her experienced captain, Dandy Kidd, died, and Lieutenant David Cheap took command. Separated from Anson in a storm off Cape Horn, Cheap pushed on toward Chile, and on May 13 and 14, 1741, in the Gulf of Penas, the carpenter glimpsed land but said nothing, Cheap dislocated his shoulder in a fall, and the ship struck rocks.

Invalids drowned in their hammocks, drunken crewmen looted the stores, and the survivors on Wager Island discovered that naval pay and a captain's authority ended when the ship was lost. Gunner John Bulkeley led 81 men away in the boats while about 20 stayed with Cheap, among them midshipman John Byron, grandfather of the poet Lord Byron. More than five years later only a handful of Bulkeley's group and four of Cheap's reached England.

• Under the Articles of War, a warrant officer like the carpenter risked a charge of mutiny by questioning officers about navigation.
• The pay rule was meant to make crews fight to save a ship, but the stranded men used it to reject Cheap's command.
• Fearing a British base, Spain fortified the region, including a new fort town at Ancud on Chiloe Island.
• A 2006 Scientific Exploration Society expedition found a 1740s hull section in a riverbed well inland, scorched by Spanish scavengers seeking iron and copper.
• The region has had 94 earthquakes since the wreck, lifting Wager Island about seven meters and turning the survivors' inlet by Mount Misery into a lake.

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In 1768 the British Board of Customs asked Benjamin Franklin why mail packets from Falmouth, Cornwall took weeks longer to reach New York than American merchant ships. His cousin Timothy Folger, a Nantucket whaling captain, explained that the British sailed straight into a fast current that whalers read through water temperature, color, and whale behavior, and in 1769 Franklin sent the British a chart of it, which their captains mostly ignored. Spanish pilot Anton de Alaminos had logged the same force in 1513 on Juan Ponce de Leon's expedition, when the current dragged ships backward against a fair wind. In 1958 oceanographer Henry Stommel showed little of the water comes from the Gulf of Mexico: the Gulf Stream is the western edge of the North Atlantic Subtropical Gyre, pinned against North America by western intensification.

It carries 30 sverdrups through the Florida Straits and 150 south of Newfoundland, when all rivers entering the Atlantic total about 0.6. Wind drives the fast jet, while cold, salty water sinking in the north pulls the wider circulation along. The warmth keeps Tromso, Norway ice free, though climatologists call the current only partly responsible for Europe's mild climate. Two 2018 studies in Nature found it at its weakest in at least 1,600 years, and the IPCC says the broader AMOC is weakening as Greenland meltwater dilutes the salt, though wind will keep the Gulf Stream itself from shutting down.

• One sverdrup equals one million cubic meters of water moving every second.
• Nantucket stays warmer in winter than the mainland 30 miles north and marks a boundary for northern and southern plants.
• Hurricane Sandy in 2012 tracked up the Gulf Stream's axis and fed on its heat.
• The current generates about 75 percent of the region's subtropical cyclones and spins eddies as deep as 3,500 meters.
• A slower current would let water banked offshore slump back and raise sea levels on the US East Coast.

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A letter of marque and reprisal was the paper that separated a privateer from a pirate: carry it and you were a prisoner of war with a lawyer, lose it and you were hostis humani generis, an enemy of mankind bound for the gallows. Henry III issued early commissions around 1243 and the first documented letter appeared under Edward I in 1295, letting a robbed merchant recover his exact losses from the offending nation. By the 16th century governments used them to outsource naval war, with private owners supplying ships, powder, and crews in exchange for prizes. The system ran on bureaucracy: applicants listed tonnage and every cannon, British captains posted bonds of around 1,500 pounds, and every captured ship had to be sailed to an admiralty court and legally condemned before auction.

Profits were huge. Guernsey privateers took French and American ships worth over 125 million pounds in today's money during the American Revolution, Elizabethan voyages by Francis Drake and Walter Raleigh were funded by plunder, and the East India Company armed ships such as the Royal Charlotte and Triton with letters of marque. The 1856 Declaration of Paris abolished privateering, but the United States refused to sign, and Article 1, Section 8 of the Constitution still lets Congress issue letters of marque. Lawmakers keep reviving the idea against terrorists after September 11, Somali pirates in 2009, Russian oligarchs' yachts in 2022, and Mexican cartels in a December 2025 bill from Senator Mike Lee.

• A letter of marque also named a slow armed cargo ship that carried the paperwork in case an enemy crossed its path.
• Captain Luke Ryan and his lieutenants sailed under the flags of three countries in a single conflict.
• The Lafitte brothers of New Orleans bribed unstable new governments for commissions to cover a smuggling empire.
• William Kidd held a legitimate commission yet was hanged for piracy and left in a gibbet over the Thames.
• The rumor that World War II Goodyear blimps like the Resolute flew as privateers was false.