Earth Science: Navigation
Earth Science: Navigation
Maritime navigation began when vessels from ancient Greece, China, and Phoenicia first ventured along the coastline, lining up landmarks (such as a near outcrop of rock against a distant point on land) to plot a course. Heeding the flight paths of birds, wind direction, Pole Star, and the path of the sun allowed sailors to make ever more ambitious voyages in open waters. The Chinese and Europeans independently invented the compass around the eleventh century, and European sailors also developed navigational instruments such as the cross-staff and quadrant in the twelfth and thirteenth centuries, making it possible for medieval mariners to find their latitude (the angular distance between an imaginary line around a heavenly body parallel to its equator and Earth's equator).
While determining latitude was relatively straight-forward, figuring out longitude was much trickier. Englishman John Harrisons (1693–1776) invention of the chronometer, a reliable clock whose spring mechanism was not affected by the pitching of the waves, permitted an accurate reading of local time, making determination of longitude possible. This allowed sailors to determine how far north or south of the equator they were, as well as east or west of the prime meridian, the imaginary half circle running pole to pole though Greenwich, England. In other words, a ship could now determine its coordinates on Earth's map grid.
By the end of the twentieth century, radio technology was beginning to be used to determine directions, and in 1904, time signals were sent to ships via radio to check their chronometers for errors. Radar was installed for the first time on an American warship in 1937, and in the twentieth century, satellite navigation systems and the development of global positioning systems (GPS) permitted ships and air traffic to navigate the oceans and the atmosphere.
Historical Background and Scientific Foundations
Early navigators stayed in sight of land, steering from point to point and creating coastal charts using dead reckoningan estimate of current position based on a previously determined position. Sailors based their calculations on their known speed, elapsed time, and the direction of their course via the formula Time = Distance / Speed. The elapsed time between two points was computed via counting or an hourglass, and speed was determined by an instrument called a chip log, a wooden board, reel, and attached log line with uniformly spaced knots. The board was tossed overboard and floated while the ship moved past while the log line ran out for a fixed period of time. The ships speed was indicated by the length of the line (and number of knots) that had passed over the stern. In areas like ancient Greece, which are dominated by chains of islands, navigating with fixed land reference points was fairly reliable.
The Role of the Pole Star
In open waters however, it was also necessary to steer by the sun and the stars. The sun's path or ecliptic through the sky gave early Phoenician and Arab sailors their direction. The Pole Star, or Polaris, always remains in a fixed position in the sky, and its distance above the horizon is a direct measure of terrestrial latitude. So if one is at the equator, the Pole Star will be zero degrees above the horizon; at the North Pole it is 90 degrees directly overhead.
The ancient Greeks, Phoenecians, and Arabs likely used one or two finger widths to measure Polaris's distance from the horizon. To return home, ancient sailors only had to sail south or north to bring Polaris to thealtitude of their home port, and then turn as appropriate to keep Polaris at a constant angle, “sailing down” the latitude.
By the eighth century, Arabs used an instrument called a kamal, a rectangular plate attached to a string, to make their observations of latitude. The sailor moved the plate closer or farther from his face until the distance between the Pole Star and the horizon corresponded to the plate's top and bottom. The plate's distance from the face was measured by the string, with the lengthmarked by tying a knot. Soon, Arab sailors created almanacs of different ports that recorded which knot on a kamal corresponded to the height of the Pole Star for each port they visited.
The Arabs also invented the astrolabe approximately in the first century AD. Adopted by medieval European navigators, the astrolabe was a simple metal disc engraved with marks to measure the height of a star or the sun over the horizon at different latitudes. A more complex planispheric astrolabe is a two-dimensional model of the skies as they appear in relation to Earth, including a rete or star pointer to indicate stellar positions. It was used to tell time via solar altitude, to calculate the duration of the night and day, and of course, to track the stars.
Another popular medieval tool for determining latitude was the cross-staff, an adaptation of the kamal. It was a T-shaped device with a base and slidable cross-bar to measure the sun's or star's height. The base was held to the eye, while the sliding top was pulled back until the desired celestial object was at the top and the horizon at the bottom. This made the navigator look like an archer taking aim at the sky, the origin of the phrase “shooting the stars”. In the seventeenth century the cross-staff was embellished with mirrors and prisms for celestial observations at night, developing into the sextant.
Chinese texts indicate that the compass was used in marine navigation by the eleventh century, perhaps as much as a century ahead of its development in Europe. (The first European compass was described by Peter Peregrinus [fl. thirteenth century] in a book about magnets in 1269, but it may have been used before this date.) The compass not only permitted the development of more accurate nautical charts (called portolan charts), but accounted for increasing economic prosperity for the people of the Mediterranean area in the late thirteenth century. Italian city-states were particularly engaged with
Near East trade. The compass extended the ability of the Venetians, Florentines, and Genoese to embark on trade voyages during the winter months when the sky was cloudy and it was difficult to navigate by the sun and stars. Mediterranean voyages around Spain to England and the Netherlands also became safer. The sponsoring nations' ensuing prosperity may have contributed to the cultural and artistic achievements of the Renaissance, which were funded by monied merchants such as the Medici.
Improved navigational techniques also led to a renaissance of exploration by the Portuguese in the fifteenth century under the leadership of Henry the Navigator (1394–1460), prince of Portugal. He founded institutes dedicated to cartography, where maps that the Portuguese created of the African coastlines were held as state secrets. By the sixteenth century, the Spanish, English, and French were using improved instruments to explore the New World. Christopher Columbus (1451–1506), however, in his attempt to find a western passage to the Orient mainly used dead reckoning—little wonder he did not reach his initial goal and discovered Cuba instead.
Although it was relatively easy to find latitude at sea with navigational instruments by the mid eighteenth-century, it was still not possible to discern a ship's longitude. To figure out longitude at sea, one has to know time at both the home port (or another place where the longitude is known), and time aboard ship. The differences in time are used to calculate geographical position. Earth takes 24 hours to revolve 360 degrees, so one hour is 1/24 of the revolution—and 1/24 of 360 degrees is 15 degrees. Each hour's difference from the home port is therefore 15 degrees of longitude to the east or west, depending on which way the ship is sailing.
The secret of finding longitude was in making a clock that was accurate at sea, something that had not been possible in an era of pendulum clocks—the bob was simply useless in a ship rolling and pitching on the waves. The damp sea air also meant that traditional lubricants would not work on the metal parts of clocks, and their wooden components would swell.
In 1707 several of British Admiral Sir Cloudesley Shovell's (1650–1707) ships, on a return voyage from France, miscalculated their position in a deep fog and were wrecked on rocks near the Scilly Islands. The resulting deaths of 1,400 men, including Shovell, prompted the British government to sponsor the longitude prize in 1714. The act promised a prize of 20,000 to anyone who could solve the longitude problem to an accuracy of 1/2 degree.
John Harrison (1693–1776), a skilled instrument maker, devised a series of brass and steel chronometers that would not rust or unduly expand and contract with temperature, enclosed in tropical wood (that self-lubricated) and utilized a spring mechanism rather than a pendulum. The last and best clock (1759) also had a caged roller bearing and ran independently of outside gravitational force, which was perfect for a moving ship. One of his instruments, made into a pocket watch, was carried by James Cook (1728–1779) on his voyages from the tropics to the Antarctic; it was never off by more than 8 seconds per day. Harrison's chronometers finally allowed the calculation of accurate longitude at sea.
Navigation by Radio and Radar
After Harrisons invention, nineteenth-century navigation instruments such as the marine chronometer and sextants were improved and refined. The advent of electronics technology in the twentieth century, particularly radio, raised navigation to a new level. Radio technology maintains links between ships (and, eventually, aircraft) and known locations on Earth via the use of electromagnetic radiation, which has wavelengths that are longer than visible light. Bad weather, distance, and darkness thus do not interfere with radio direction finders on aircraft and marine vessels.
The ship or plane receives synchronized pulse signals from transmitting stations, often located in light-houses or radio control towers. By measuring the time difference between the arrivals of signals from a pair of these transmitting stations, a curved line of position can be plotted on a LORAN (long-range navigation) chart. A second set of signals from another pair of stations produces another curved line of position, and crossing the two lines gives a fixed position for the ship or aircraft.
In the 1930s Scottish physicist Robert Alexander Watson-Watt (1892–1973), Arnold F. Wilkins, and Alan Blumlein (1903–1942) developed radar systems as part of the war effort to detect incoming German aircraft. Radar developed out of a British belief that some sort of visible death ray could be used as an offensive weapon against the Nazis. While ultimately that line of inquiry came to naught, British Post Office engineers did notice that an airplane flying through an experimental high frequency beam caused the beam to jump on an oscilloscope. Watson-Watt and Wilkins concluded it would be possible to develop a form of aircraft-detection system, in which a plane would return a radio signal aimed at it back to its source.
Between 1935 and 1937 an experimental radar system called Chain Home proved that it could detect enemy aircraft. Radar uses wavelengths that are shorter than radio but longer than light waves; from a single landmark it can give a direct indication of distance. Radar does this by measuring the time lag between the plane or boat sending and receiving signal pulses to the landmark. The distance to the reflecting surface of
the landmark is given in terms of the velocity of light. It was used to great effect in the Battle of Britain in 1940 to guide the Royal Air Force (RAF) pilots toward the incoming German bombers. Radar was also installed on British and American warships.
Radar technology, developed in wartime, became a mainstay of commercial navigation, forming the basis of radiation-contact guidance equipment called the Doppler system. In this type of radar system, continuous waves rather than pulses are transmitted from the plane or ship to the landmark. There is a measurable shift in frequency between reflected and transmitted waves. Called a Doppler shift, this can be quantified to measure the distance between the ship or plane and the landmark.
The launch of the Soviet satellite Sputnik in 1957 gave American scientists the inspiration for the global positioning system (GPS). Dr. Richard Kershner (1913–1982), from the Johns Hopkins Applied Physics Laboratory, was monitoring Sputnik s radio transmissions. He noticed that due to the Doppler shift, he measured a different frequency as Sputnik approached than as it traveled away from them. By measuring the Doppler shift, they could pinpoint where the satellite was in its orbit.
Using this principle, the U.S. Navy began to develop a satellite navigation system in 1960; there are presently 31 broadcasting satellites in the global positioning system. Though administered by the U.S. Department of Defense, the GPS system is free for civilian use and navigation. Using Doppler shift calculations, GPS receivers calculate their position by measuring the distance between themselves and three or more GPS satellites. The increasingly low cost of GPS chips means that GPS satellite signals are used to power remote door openers, automotive navigation systems in cars, cell phones, systems that track vehicles carrying dangerous goods, and the location of utility lines. Several governments are studying how the loss of GPS signals (for example from a downed satellite) would affect industry and communications infrastructure.
Modern Cultural Connections
Navigation tells us where we are and where we are going. Our sense of place now has a global context, and using navigational systems we have made the first strides into outer space. From dead reckoning to GPS, navigation permits the transmission of goods and knowledge, and its increasing efficiency means the expansion of our world and our greater freedom to travel and learn.
See Also Earth Science: Exploration.
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Anna Marie Eleanor Roos