Since the time of Ptolemy in the second century, every seafaring nation has used its homeport as a starting point for measuring longitude. Through the centuries, a prime meridian has passed variously through the Azores, the Canary Isles, Cape Verde, Rome, Copenhagen, Jerusalem, St Petersburg, Pisa, Paris, and Philadelphia.
Why, you may ask, was it so important to have a baseline reference point for longitude? Unlike latitude, where the earth's axis of rotation determines two poles and a corresponding equatorial dividing line, there was no logical starting point for measuring longitude. Since latitude was measured in degrees starting from zero at the equator and ending at the poles with 90 degrees, it was decided that longitude could be measured in a somewhat similar fashion. Since the world revolves once every 24 hours, and there are 360 degrees to each rotation, then the world turns 15 degrees every hour and one degree for every four minutes.
Just as the equator was used to separate the northern and southern hemisphere, the zero degree meridian and its extension, the 180-degree meridian, separated the eastern and western hemispheres. Longitude was measured both eastward and westward from the prime meridian to the 180-degree meridian. Today the 180-degree meridian is also called the International Date Line because when the sun crosses this meridian, the date changes. Initially, the lines of longitude were not drawn out this far on charts as most of the world was yet to be discovered. It was this age of exploration that increased the demands for more accurate charts, better means of navigation and an accurate way to determine longitude at sea. On land it was easy enough to use the moons of Jupiter to determine longitude, but extremely difficult at sea as a telescope was needed.
As early as 1676, England, then the world's greatest seafaring power, used Greenwich, where the Royal Observatory was located, as the zero meridian. The publication of the first British Nautical Almanac in 1767 further entrenched Greenwich as the prime meridian. By 1883, Sweden, the US, and Canada had also adopted the Greenwich Meridian as the baseline for measuring longitude. With the growth of international commerce, a uniform, worldwide system for measuring longitude was needed. In 1884, an International Meridian Conference was held in Washington, D.C. to try to resolve the issue. The 25 participating countries put the matter to the vote and 22 of them voted for Greenwich (France and Brazil abstained). Greenwich was chosen for two reasons: First, the meridian had to lie on a major observatory. Second, because so much international shipping at the time was British, Greenwich was already the most widely used meridian in sea transport, having been adopted by a majority of the world's shipping companies.
From 1884 until 1911, only the French continued to use their local longitude as the Prime Meridian. They used the Paris Observatory as their zerp-degree meridian and could not bring themselves to refer to Greenwich Mean Time, instead they used Paris Mean Time retarded by nine minutes 21seconds. The original site of the observatory in Greenwich, England is where East meets West at the Greenwich Meridian (zero degrees Longitude) and world time is set. Long before the location of the prime meridian was agreed upon, the search for a method of determining one's longitude at sea was underway.
Columbus and others before and after him, sailed the ocean along parallels of latitude. They simply sailed north or south until they arrived at the latitude of their destination and then turned towards it. Latitude could be determined by the height of the sun at noon in conjunction with its declination, or by using the north star at night. However, for the mariner, longitude remained a matter of dead reckoning. For this method of navigation, the time a chip thrown overboard took to pass from stem to stern measured speed, and the ship's navigator set the ship's hour glass to "local" time by the sun at noon.
But when dead reckoning was wrong, men died. In late 1707, a British fleet of warships were homeward bound following a victorious battle with the French. The ships had sailed in fog for twelve days and the Admiral of the fleet was concerned about their position. He called the navigators from the five ships together. The consensus was they were west of the French Ile d'Ouessant, thus requiring them to continue north before turning east into the English Channel. Twenty four hours later, four of the ships ran onto the rocks of the Scilly Isles and 2,000 men drowned. This error in navigation was one of latitude determination, however, and not a longitude problem. If their dead reckoning had been more accurate, the squadron could have turned east into the English Channel hours earlier and not continued north to end up on the rocks. If not for the fog, they could have easily determined their latitude.
This tragedy brought about angry requests from the Royal Navy and other marine interests for the government to do something
that something was a decision to find a method for determining longitude. The British Longitude Act of 1714 promised a prize of 20,000 pounds for anyone who could provide a solution to the longitude problem with an accuracy of half a degree. This was an immense amount of money at the time, the equivalent of over $2 million today. The prize spurred some of the brightest minds in eighteenth century Europe to try and solve the "Longitude Problem." Men such as Edmund Halley (of Halley's Comet fame) and Sir Isaac Newton (the discoverer of Calculus) attempted, but failed to come up with an acceptable solution.
Centuries earlier the Greeks had observed that longitude could be regarded as a function of time. By carrying a clock on board, a sailor could find his longitude by reading the clock's time when the sun was at its highest point (local noon) and convert the difference in time to determine the ship's longitude relative to the starting longitude. Every four minutes of difference would indicate one degree of longitude. To make this process work, the sailor had to have a reliable and accurate clock. However, the Board of Longitude did not think that a sufficiently accurate clock could ever be built. The pendulum clock, even when gimbaled, simply could not keep accurate time at sea. The Board much preferred some sort of celestial solution to that of inventing an accurate timepiece.
At the time, John Harrison, a self-taught English clockmaker, didn't agree with the Board's assessment. His approach to winning the Longitude Prize was to construct a very precise clock that was stable against the movements of the sea and changes of temperature. In 1735, after six years of work, Harrison presented his first sea-going clock to the Royal Society in London. It was a large, awkward and heavy clock, as it was mounted on springs and enclosed in a gimbaled case. By 1763, three even more refined and smaller chronometers were constructed. The last one, his forth clock, was truly a masterpiece: not only did it far exceed the exactness required by the Board of Longitude, but it was much smaller in size than the previous models just a little larger than a pocket watch. Harrison's fourth innovative clock made its sea trial in late 1761 on an Atlantic crossing from England to Jamaica. During the two months at sea it lost nine seconds, or just over two minutes of longitude, which was well within the 30-minute requirement. The Board, however, was reluctant to award all the prize money and only gave him half. They also insisted on additional tests. In 1773, after 40 years of designing, building, and refining a series of sea clocks, the frail 80-year-old Harrison was finally awarded the remaining prize money, but only after the intervention of the King. The explorers of the world finally had a way to determine their longitude at sea.
After Harrison showed the way, many other clockmakers started work on improving the design. New escapements and balance wheels were designed until finally the cost of an accurate chronometer was reduced tenfold. With the increased availability of inexpensive chronometers in the 1800s, a navigator needed only a sextant to determine his position at sea. Of course he also needed an Almanac with tables of sunrises, sunsets, and the declination of the sun for the different dates. This almanac was provided by the Royal Observatory at Greenwich. Celestial navigation using a sextant, clock, and Nautical Almanac took the world well into the twentieth century. During this time, the only changes were improvements in accuracy to these instruments and the Almanac. A working knowledge of celestial navigation was the undisputed hallmark of all sea captains. Even surveyors used celestial to map the world, and it wasn't until satellite photography that we realized there were some small errors in the mapped latitude and longitude of major cities in the world. These discrepancies were mainly due to the assumption that the earth was a perfect sphere and partly to small inaccuracies in the instruments.
Once the problem of telling time accurately at sea was solved, ships at anchor in Greenwich used a very special means of setting their marine chronometers. On top of the Royal Observatory, a mast was built with a moveable red ball attached called the time ball. Each day, at 12:55, the time ball rose halfway up its mast; three minutes later it rose all the way to the top. At 13:00 exactly, the ball fell and provided a time signal to all who could see it. This red ball continues to rise and fall to this very day. Today we also use a ball in Times Square to signify the start of the New Year.
The establishment of a worldwide system to measure longitude brought with it a notion of worldwide time. Since there are 24 hours in a day and 360 degrees in a circle, each 15 degrees of longitude represented one hour. Thus, by wrapping a 360-degree longitudinal grid around the earth, we divide the planet into 24 time zones, each one hour different from its two neighbors. Each zone is 15 degrees of longitude in width and uses its central median to determine the Local Mean Time (LMT) for the zone. These zones are designated by numbers starting from 0 at Greenwich to -12 and +12 either side of the International Date Line (IDL) at the 180-degree meridian. Each of these numbers indicates the hours to be added to or subtracted from Local Mean Time to obtain Greenwich Mean Time (GMT). Because the time is earlier west of Greenwich, these zones are plus, while the zones east of Greenwich are minus, since they are later times. The zones also are designated by letters of the alphabet for addition reference. Greenwich is designated Z and that is why GMT is often called Zulu time, as in 0800 Zulu. The other time zones rarely, if ever, use their alphabet designation.
Over the years, the zone boundaries have been modified to conform with geographical, national, and political borders for greater convenience. Only rarely will a zone boundary be allowed to divide a city, state, or nation into different time zones. Of course this convenience often adds to the temporal confusion of converting from one time zone to another, or from GMT to LMT. The standard procedure of using GMT for navigation and keeping LMT on board a vessel for everything else leads to many interesting and sometimes confusing situations as the boat crosses time zones during its voyage. The most confusing part is crossing the IDL, when the date also changes. Can you imagine being the navigator while crossing the IDL at 2400 GMT on December 31, 2000? For a moment in time, as you crossed the 180-degree meridian, you would have been in both milleniums.
Incidentally, if you are one of those people who think the new millenium started January 1, 2000, you are wrong. Just because the media thinks so doesn't make it correct.
Dead but Not Deceased by John Rousmaniere
Dead Reckoning Calculations by Jim Sexton
Plotting Equipment by Jim Sexton
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