Navigation of the American Explorers - 15th to 17th Centuries | Penobscot Bay History Online Navigation of the American Explorers - 15th to 17th Centuries Seventeenth century travelers to Maine’s coast such as Samuel ChamplainChamplain, Samuel de Samuel de Champlain 1567-1635. Between 1603 and 1635, Champlain made 12 voyages to what was to become Canada, establishing it as a French colony, founding Quebec, and exploring up the St. Lawrence River to the Great Lakes. Read More , George WaymouthWaymouth, George George Waymouth c.1585-c.1612. English ship captain and explorer, and student of mathematics, navigation and ship building. In 1602 he led an unsuccessful voyage in search of the Northwest Passage, exploring the area between Greenland and Labrador. After returning he wrote "The Jewell of Artes" a manuscript on navigation, shipbuilding and fortification presented to King James I. Read More , and John SmithSmith, Capt. John Captain John Smith c.1580-1631.

English soldier, sailor, and writer, who became the leader of the Jamestown Colony. Chiefly known in New England for his 1614 voyage in which he mapped much of the coast. Read More carried state-of-the-art navigation tools for both dead reckoningDead reckoning Navigating by applying courses and distances made through the water from the last known observed position. The term dead could be a form of "ded" from "deduced" reckoning. and celestial navigationCelestial navigation Using the sun, moon, stars, and planets to find your location.. Navigation Tools for Dead Reckoning and Piloting Invented in China in the 3rd century BC, the compassCompass Instrument which indicates true or magnetic north, enabling the mariner to guide a ship in any direction and to determine the direction of a visible object, such as another ship, heavenly body, or point of land. Read More did not come to Europe until the 12th century AD. By the time of Columbus'Columbus, Christopher Christopher Columbus 1451-1506. Italian explorer and navigator.

After finding Spanish backing for his plan to find a short way to the Orient, he sailed in 1492 and landed in the Bahamas, then explored a portion of the Caribbean before returning. Read More voyage it was common. Instead of degrees, the compass cardCompass card Magnetic marine compasses have the magnets fastened to a circular card which has the directions on it. Traditionally compasses were marked in points of 11 1/4 at every 11 1/4 degrees. The whole card rotates and a mark on the compass housing called a lubber line indicates direction. Read More , on which directions were drawn or printed, showed the points of the compass, including north, south, east, and west. There are 32 points of the compass, the four main quadrantsQuadrant An astronomical instrument for measuring angles with a quarter circle graduated arc. The seaman's quadrant, or simple quadrant, was the earliest instrument used by navigators for measuring the altitude of a heavenly body. Other instruments not strictly quadrants have also been called such. The backstaff may be called Davis Quadrant. The octant was originally called Hadley's quadrant. of the circle each divided into eight 11¼ ° points.

Columbus noticed that, as one sailed across the Atlantic Ocean, the variationMagnetic North and Variation Magnetic north is one of two points on the globe where the line of total magnetic force is vertical and towards which the magnetic needle points in all adjoining regions. Variation (also called magnetic declination) is the difference in direction between true north as determined by the earth's axis of rotation and magnetic north as determined by the earth's magnetism. Read More between magnetic north and true northTrue North The direction of the North Pole from any place on the earth's surface, through which all meridians of longitude pass on maps and charts. Read More changed. On future trips he used this to predict, roughly, his arrival in America. The next most important tool was the chip logChip log Device used in the past on sailing vessels for measuring the rate of speed of the vessel. A quarter circle quadrant of wood, or "chip," fastened to a line, was allowed to run out over the stern, and the amount of line run was measured in time with a half minute sand glass. The sand glass replaced counting the seconds. Read More , introduced in the late 16th century to measure speed.

The chip, a quarter circle of wood, was attached to a light line on a reel. Knots were tied at 47' 3" intervals, the distance the line would be pulled out in 28 seconds if the ship’s speed was one knotKnot A nautical measure of speed. One knot is equal to one nautical mile (6,080 feet) per hour. The term comes from the knots on the line of a chip log. or nautical mileNautical Mile One minute (or 1/60th of a degree) of latitude. Because the earth is not a perfect sphere, the length of a nautical mile varies somewhat according to latitude. Read More in an hour, when the chip was dropped overboard. With a 14- or 28-second sand glassSand glass sandglass A means of measuring time on board ship before the development of reliable clocks for ships. Two vacuum globes connected by a narrow neck, allowing sand to run from the top globe into the bottom one in a given period of time. Read More , navigators could see how fast the vessel was going by counting how many knots rolled out before the sandglass expired.

Before the chip log, navigators estimated speed by timing how long a chip of wood in the water would take to pass from bow to stern. Compass and log helped navigators keep track of position. They used a lead lineLead line A means of finding the depth of water near coasts and probably the earliest device used by coastal navigators to facilitate safe navigation. It consists of a hemp line with a lead weight attached (about 7 pounds). Read More to determine water depth and bottom type. A heavy piece of lead at the end of a long marked line had a cavity in its bottom, which, when coated with grease or tallow, brought up a bottom sample. Experienced navigators often could determine position based on whether the bottom was muddy, sandy, pebbly, rocky, or covered with vegetation or shell fragments. Crossing the Atlantic, navigators used the lead line to find the continental shelf, and, more importantly, find the Grand BanksGrand Banks A large shallow area, rich in fish, located in the North Atlantic off Newfoundland. Read More and other fishing grounds.

To record a vessel’s courses and speeds, the navigator used a traverse boardTraverse Board The traverse board is used to keep track of changes in a vessel's speed or course over the course of a four-hour watch. Read More . The board had a line of holes radiating from the center towards each of the 32 compass points. Sailors inserted pegs in the holes to show the vessel’s course and speed each half hour. The navigator then used traverse tables to add these and give an average course for a four hour watch. This result then was entered into a logbook along with information about the weather, changes in sails, and items concerning the crew. Guides for the Navigator The seventeenth century navigator had little published information. ChartsChart A nautical map giving navigation information, including: water depth; shoals, rocks, and other dangers; and aids to navigation such as lighthouses, buoys, and beacons. Charts use special symbols and abbreviations to convey information for mariners. were rare; some advanced navigators carried globes. MercatorMercator, Gerardus Gerardus Mercator 1512-1594.

Flemish mathematician and geographer who, after moving to Germany in 1552, invented the map projection named after him. His world map was first published in 1569. projection charts were far more useful than earlier charts. With its mathematical errors corrected by Edward WrightWright, Edward Edward Wright 1561-1615. English mathematician and cartographer. Edward Wright's world map of 1599, published in 1600, coupled with his 1599 book "Certaine Errors in Navigation" corrected errors in Mercator's projections so that rhumb lines appear as straight lines. Read More in 1599, the Mercator projection chart allowed mariners to draw a rhumb lineRhumb line Straight line compass course between two points on a Mercator chart. Formally a line of constant bearing that crosses all meridians at the same angle which displays as a straight line on a Mercator chart but as a spiral on a globe. This sprial line is called a loxodrome. between two points, get a bearing and sail that line.

The earliest sailing directionsSailing directions Originally called "routiers" or "rutters", written directions for navigation. originated in the Mediterranean as manuscripts called portolanosPortolano Italian manuscript sailing directions used in the Mediterranean between the 12th and 15th centuries. Contemporary with and backed up the portolan charts of the areas, hand drawn charts with compass or direction lines. which were first printed in the second half of the sixteenth century. The first important collection was published in 1584 by the Dutch pilot WaghenaerWaghenaer, Lucas Janszoon Lucas Janszoon Waghenaer c.1534-1606. Dutch pilot and cartographer. Based on 25 years of experience as a pilot and opportunities to examine numerous rutters and manuscript charts he was able to compile the first effective set of navigational charts published 1584-85 under the title Spiegel der Zeevaerdt or Mirror of the Sea. Read More .

These volumes, with charts, sailing directions, navigational instructions, and tables, became known in England as "Waggoners"Waggoner Waggoner, generic 17th century and early 18th century term for sea charts derived from the first publisher's name Waghenaer. In 1671, the first of four volumes of The English PilotThe English Pilot English sea atlas published from 1671 to 1803, initially by John Seller who was appointed Royal Hydrographer in 1671. These were intended as working charts. The Fourth Book, first issued in 1689 covered North America and Canada. appeared, based mostly on Dutch sources. These covered Europe, the Far East, and North America. Tools for Finding Latitude and Time The only way navigators could estimate a vessel’s longitudeLongitude Longitude lines around the globe run north-south, and measure position east or west of a reference line. Read More was by dead reckoning and measuring variation. Celestial navigational instruments were designed to help find a vessel’s latitudeLatitude Latitude is the measure of how far north or south one is from the equator.

This angular measurement is given in degrees, minutes (1/60th of a degree), and seconds (1/60th of a minute) of arc. Read More , the approximate time, and the direction of true southTrue South The direction of the South Pole from any place on the earth's surface, through which all meridians of longitude pass on maps and charts. It is the direction that the sun is from the observer at local noon or when the sun is highest in the sky.. The quadrant, the earliest device used to find latitude, was a quarter-circle of wood, marked in degrees, with a plumb line and a sight along one edge, first taken to sea around 1460. Another early latitude-measuring device is the astrolabeAstrolabe The seaman's astrolabe is a graduated ring or disc fitted with a sighting rule pivoted at the center. Read More . It is a disc with degrees and a movable arm with sights, first known to be at sea about 1481. In the 15th century, Portuguese Prince Henry the NavigatorHenry the Navigator 1394-1460.

Henry, Prince of Portugal, took the name Navigator because of his patronage of a succession of Portuguese seafarers who explored the Atlantic islands off Portugal and down the African coast, ultimately rounding the Cape of Good Hope. Read More pioneered nationally sponsored exploration and cartography. Portuguese navigators apparently took the cross staffCross staff cross-staff Early navigational device for measuring altitudes of heavenly bodies, also called a fore-staff, based on an Arab instrument called a Kamel. Read More to sea about 1515. It has two parts: a long graduated staff and a sliding crosspiece. The navigator holds one end of the staff near his eye, where both the sun and horizon may be sighted, and then moves the crosspiece along the staff until one end is lined up with the horizon and the other with the sun or star. The angle is read from the scale on the staff. The cross staff required the navigator to look directly into the sun, almost impossible in bright sunlight. But it could be used when the ship was moving, and it was simple and relatively inexpensive.

A variation of the cross staff is the backstaffBackstaff back staff, back-staff, Davis quadrant A navigational instrument for measuring the altitude of the sun, introduced in the 16th century. Read More , invented by John DavisDavis, John John Davis c.1550-1605. English navigator and explorer. In 1585, started his first expedition to search for a Northwest Passage around the top of North America to the Orient. Read More about 1594 and published in his Seaman’s Secrets in 1595. With it a navigator could measure angles accurately without looking directly at the sun. The backstaff, in its final form, was made of wood and was made up of two arcs, a larger 30° arc and a smaller 60° arc. VanesVane 1. Rotating device which is used to indicate direction of wind. Also called wind or weather vanes. Read More allowed accurate sighting of the horizon, while the sun showed a shadow on another vane. Also called a Davis quadrant, it could only be used for sun sightsSun sights Altitude observations of the Sun needed for celestial navigation.

"Sights" generally refers to taking a sun sight, but means also taking star sights at night.. At night, navigators could tell time using a nocturnalNocturnal An early instrument designed for measuring time of night by means of Polaris (the Pole Star) and other points in constellations., a device that measured the angle from the North Star to the pointer stars, either in Ursa Major (the Big Dipper or Big Bear) or in Ursa Minor (the Little Dipper or the Little Bear). It used the vertical as a reference, and required the month and date to be set. A sundialSundial A device that measures time by the position of the sun. Known from Ancient Egypt. For navigation special ones were developed that could tell local noon on board ship. could be used in daylight. By the middle of the 17th century, thanks to the invention of logarithms by John NapierNapier, John John Napier 1550-1617. Scottish mathematician and scholar, best known for his invention of logarithms and a calculator based on these. Napier's logarithms simplified calculations making trigonometric calculations possible for those with little mathematical training.

Kepler used them in his orbital calculations. which were transformed into a simple calculator by Edmund GunterGunter, Edmund Edmund Gunter 1581-1626. English mathematician who published trigonometric tables in 1620 with logarithms. Read More , navigators with little mathematical training could solve trigonometric navigational problems. By the end of the seventeenth century, navigators were able to tell time within a quarter of an hour and find their latitude within a few miles. Despite their relatively simple instruments, these mariners sailed the globe. NOVA - Official Website | Secrets of Ancient Navigators NOVA Oak and triple bronze must have girded the breast of him who first committed his frail bark to the angry sea... —Horace, Odes Among the many challenges, as implied in this classic line by Horace, that faced those who ventured onto the open sea was navigation. In the millennia before the 18th-century English clockmaker John Harrison invented a chronometer that enabled sailors to accurately determine their longitude—the last major hurdle in accurate location-finding at sea—how could mariners possibly know where they were, or where they were going, in the vast emptiness?

Well, find their way they did, using a host of ingenious methods. How did mariners of old navigate their way around the open ocean? Enlarge Photo credit: © Felix Mí¶ckel/iStockphoto.com Land and air The first seafarers kept in sight of land. That was the first trick of navigation—follow the coast. To find an old fishing ground or the way through a shoal, one could line up landmarks, such as a near rock against a distant point on land; doing that in two directions at once gave a more or less precise geometric location on the surface of the sea. Sounding using a lead and line also helped. "When you get 11 fathoms and ooze on the lead, you are a day's journey out from Alexandria," wrote Herodotus in the fourth century B.C. The Greeks even learned to navigate from one island to the next in their archipelago, a Greek word meaning "preí«minent sea." They may have followed clouds, which form over land, or odors, which can carry far out to sea. But what if land were nowhere nearby? The Phoenicians looked to the heavens.

The sun moving across the commonly cloudless Mediterranean sky gave them their direction and quarter. The quarters we know today as east and west the Phoenicians knew as Asu (sunrise) and Ereb (sunset), labels that live today in the names Asia and Europe. At night, they steered by the stars. At any one time in the year at any one point on the globe, the sun and stars are found above the horizon at certain fixed "heights"—a distance that mariners can measure with as simple an instrument as one's fingers, laid horizontally atop one another and held at arm's length. The philosopher Thales of Miletos, as the Alexandrian poet Kallimachos recorded, taught Ionian sailors to navigate by the Little Bear constellation fully 600 years before the birth of Christ: Now to Miletos he steered his course That was the teaching of old Thales Who in bygone days gauged the stars Of the Little Bear by which the Phoenicians Steered across the seas. Watching the direction a seabird traveled with food for its young was one reliable method to find the nearest land.

Enlarge Photo credit: © Ken Canning/iStockphoto.com Bird and wave The Norsemen had to have other navigational means at their disposal, for in summer the stars effectively do not appear for months on end in the high latitudes. One method they relied on was watching the behavior of birds. A sailor wondering which way land lay could do worse than spying an auk flying past. If the beak of this seabird is full, sea dogs know, it's heading towards its rookery; if empty, it's heading out to sea to fill that beak. One of the first Norwegian sailors to hazard the voyage to Iceland was a man known as Raven-Floki for his habit of keeping ravens aboard his vessel. When he thought he was nearing land, Raven-Floki released the ravens, which he had deliberately starved. Often as not, they flew "as the crow flies" directly toward land, which Raven-Floki would reach simply by following their lead. Like Eskimos study the snow, the Polynesians watched the waves. Heeding the flightpaths of birds was just one of numerous haven-finding methods employed by the Polynesians, whose navigational feats arguably have never been surpassed.

The Polynesians traveled over thousands of miles of trackless ocean to people remote islands throughout the southern Pacific. Modern navigators still scratch their heads in amazement at their accomplishment. Like Eskimos study the snow, the Polynesians watched the waves, whose direction and type relinquished useful navigational secrets. They followed the faint gleam cast on the horizon by tiny islets still out of sight below the rim of the world. Seafarers of the Marshall Islands built elaborate maps out of palm twigs and cowrie shells. These ingenious charts, which exist today only in museums, denoted everything from the position of islands to the prevailing direction of the swell. The chart and the compass were two early technological breakthroughs that benefited seafarers. Enlarge Photo credit: © DNY59/iStockphoto.com Chart and compass Charts have aided mariners ever since the Alexandrian astronomer Ptolemy created the first world atlas in the second century A.D. The redoubtable Ptolemy even plotted latitude and longitude lines on his atlas's 27 maps, though the farther one got from the known world centered on the Mediterranean, the dangerously less reliable they became.

Even before Ptolemy, there were sailing directions—the Greeks called them periplus or "circumnavigation"—that were compiled from information collected from sailors far and wide. One of these, The Periplus of the Erythraean Sea , a document written in the first century by a Greek merchant living in Alexandria, described trading routes as far east as India. By the 10th century, Italian-made portolans supplied detailed directions, distances, depths, and coastal descriptions, and by the 13th century, sea maps with scale and bearings began to appear. The greatest advance in navigation came with the compass. The Chinese apparently knew about the powers of magnetism as early as the third millennium B.C., when, historians tell us, one army defeated another after the battlefield had become enveloped in dense fog by using a device known as a "point-south carriage." This was a standard carriage for carrying royalty with a small, rotating figure mounted on the front, which by magnetism always pointed south. (The Chinese chose to have the arrow point south rather than north.) But no one seems to have manipulated the lodestone for sea navigation until early in the present millennium.

The first mention of the compass in the West comes from the Englishman Alexander Neckham, who wrote in 1187 that "sailors use a magnetic needle which swings on a point and shows the direction of the north when the weather is overcast." Despite its usefulness, the compass took a long time to come into wide use, as many seamen thought it operated by black magic. (Hence the invention of the binnacle, in which sea captains could hide their recondite instrument from the suspicious eyes of the crew.) In the meantime, sailors relied on natural forces they could readily comprehend. Currents may be invisible to the untrained, but not to seasoned mariners. Enlarge Photo credit: © Clicks/iStockphoto.com Current and wind One of these was currents. From time immemorial, journeys have been made or broken by these undersea winds. The western-trending currents of the Indian Ocean, for one, are likely responsible for the Indonesian-based race of Madagascar, an African island more than 3,500 miles from the nearest bit of Indonesia.

Similarly, the clockwise currents in the North Atlantic helped doom one of the greatest land scams in history: Erik the Red's colonization scheme for the island he cleverly dubbed "Greenland." Of the 25 ships that sailed west from Norway in the year 990, only 14 arrived. The father of those North Atlantic currents—the Gulf Stream—was named by none other than Benjamin Franklin. While deputy Postmaster-General of Great Britain in the 18th century, Franklin noticed that his mail ships to the American colonies took longer than whaling ships. Questioning whalers, he learned of a powerful current originating from the Gulf of Mexico—hence his name for it—and sweeping northeast into the North Atlantic (and, incidentally, giving the British Isles a climate positively balmy for such a northern latitude). The sextant allowed users to determine their latitude to within a sea mile or two, even from a swaying deck. Like currents, trade winds have always been important to mariners. Those blowing heads on yellowed old maps were not mere decoration.

In the Indian Ocean, for example, Indian traders over the ages have ridden the northeast monsoon to Africa in the cool, dry winter and taken the southwest monsoon back to the subcontinent in the hot, wet summer. To make their annual voyages from Tahiti to Hawaii, a journey of several thousand miles, the Polynesians hitched a ride on the prevailing south-easterly wind, setting a starboard tack and sailing northeast. With the sextant for determining latitude and the chronometer for longitude, sailors by the 1800s were able to navigate the high seas with great precision. Enlarge Photo credit: © Bruno Buongiorno Nardelli/iStockphoto.com Sun and star For millennia, as sailors from the Phoenicians to the Polynesians knew, the heavens remained the best way to find one's north-south position. Increasingly sophisticated devices were designed over the centuries to measure the height of the sun and stars over the horizon. The gnomon or sun-shadow disk operated like a sundial, enabling the user to determine his latitude by the length of the sun's shadow cast on a disk floating level in water.

The Arabian kamal was a rectangular plate that one moved closer or farther from one's face until the distance between the North star and the horizon exactly corresponded to the plate's upper and lower edges. The distance the plate lay away from the face—measured by a string tied to the center of the plate and held at the other end to the tip of the nose—determined the latitude. In the Middle Ages, sailors relied on the astrolabe, a disc of metal that one held suspended by a small ring. The disc had a scale with degrees and a ruler for measuring the height of an astronomical body. Other medieval mariners preferred the cross-staff, a T-shaped device whose base was held up to the eye. One measured the sun's height by pulling the slidable top of the T toward one's eye until the sun lay at the top and the horizon at the bottom. Since blindness resulted from frequent use, the explorer John Davis invented the back-staff in 1595, which enabled one to get the same measurement with one's back to the sun. The sextant was the most advanced of these devices, allowing users to determine their latitude to within a sea mile or two, even from a swaying deck.

Time In the years after the sextant was invented in 1731, many held out hope that it would aid in east-west navigation as well—that is, in finding longitude. Sailors could employ the sextant to figure longitude using the lunar-distance method, but with the astronomical tables of the 18th century, the process could take several hours to work out one's position—not remotely good enough for sea travel. In the end, it was the dogged clockmaker, John Harrison, who solved the longitude problem with his chronometers. And today, the precocious step-child of these highly accurate clocks, the Global Positioning System, has finally proved the Roman dramatist Seneca right, when he wrote in the first century: There will come an age in the far-off years When Ocean shall unloose the bonds of things, When the whole broad earth shall be revealed.... The History of the Sextant Peter Ifland The History of the Sextant Talk given at the amphitheatre of the Physics Museum under the auspices of the Pro-Rector for Culture and the Committee for the Science Museum of The University of Coimbra, the 3 October 2000. "Where are we?" Well, yes.

We're sitting here safe and dry in the Science Museum at the University of Coimbra. But the question has a different urgency when the ship is approaching a rocky coast and the life of the ship and its crew depends on a fast and accurate answer. It's the Navigator's job to provide the answer. So what do navigators need to find their position on the earth's surface by observing the stars? They need an Almanac prepared by the astronomers to forecast precisely where the heavenly bodies, the sun, moon planets and selected navigational stars, are going to be, hour by hour, years into the future, relative to the observatory that prepared the almanac, Greenwich, England in modern times. They need a chronometer or some other means of telling the time back at the observatory that was the reference point for the data in the almanac, It is the cartographer's job to provide accurate charts so that navigators can establish their position in latitude and longitude or in reference to landmasses or the hazards of rocks and shoals.

The navigators need a quick and easy mathematical method for reducing the data from their celestial observations to a position on the chart Finally, navigators need an angle-measuring instrument, a sextant, to measure the angle of the celestial body above a horizontal line of reference. How do navigators use the stars, including our sun, the moon, and planets to find their way? Well, for at least two millennia, navigators have known how to determine their latitude  their position north or south of the equator. At the North Pole, which is 90 degrees latitude, Polaris (the North Star) is directly overhead at an altitude of 90 degrees. At the equator, which is zero degrees latitude, Polaris is on the horizon with zero degrees altitude. Between the equator and the North Pole, the angle of Polaris above the horizon is a direct measure of terrestrial latitude. If we were to go outside tonight and look in the northern sky, we would find Polaris at about 40 degrees 13 minutes altitude - the latitude of Coimbra.

In ancient times, the navigator who was planning to sail out of sight of land would simply measure the altitude of Polaris as he left homeport, in todays terms measuring the latitude of home port. To return after a long voyage, he needed only to sail north or south, as appropriate, to bring Polaris to the altitude of home port, then turn left or right as as appropriate and "sail down the latitude," keeping Polaris at a constant angle. The Arabs knew all about this technique. In early days, they used one or two fingers width, a thumb and little finger on an outstretched arm or an arrow held at arms length to sight the horizon at the lower end and Polaris at the upper. Kamal In later years, they used a simple device called a kamal to make the observation. The kamal shown here actually is a modern piece that I made, but its very much like the ones used a thousand years ago, and probably much earlier. Notice the knots in the cord attached to the carved mahogany transom.

Before leaving homeport, the navigator would tie a knot in the cord so that, by holding it in his teeth, he could sight Polaris along the top of the transom and the horizon along the bottom. To return to homeport, he would sail north or south as needed to bring Polaris to the altitude hed observed when he left home, then sail down the latitude. Over time, Arab navigators started tying knots in the string at intervals of one issabah. The word issabah is Arabic for finger, and it denotes one degree 36 minutes, which was considered to be the width of a finger. They even developed a journal of different ports that recorded which knot on the kamal corresponded to the altitude of Polaris for each port they frequently visited. Throughout antiquity, the Greeks and Arabs steadily advanced the science of astronomy and the art of astrology. About a thousand years ago, in the 10th century, Arabs introduced Europe to two important astronomical instrumentsthe quadrant and the astrolabe. Astronomers Astrolabe. Arabic astronomer's astrolabe made by Hajji Ali of Kerbala around 1790.

Its about 3 and one-half inches in diameter. It was used to find the time of rising and setting of the sun and the altitude of the sun and selected stars. Importantly, it was used to find the direction of Mecca for the devout Moslem's morning and evening prayers. In the word "astrolabe" - "astro means star and "labe" roughly translates as to take or 'to find.' The astronomer's beautiful, intricate and expensive astrolabe was the grandfather of the much simpler, easy to use mariner's quadrant and astrolabe. The mariners quadranta quarter of a circle made of wood or brass--came into widespread use for navigation around 1450, though its use can be traced back at least to the 1200s. Mariners brass quadrant. The scale spans 90 degrees and is divided into whole degrees. A plumb bob establishes a vertical line of reference. The quadrant shown here is a replica of the type Columbus might have used on his voyages to the New World.

This one is marked off at the latitudes of Lisbon, Cabo Verde and Serra Leoa, down near the Equator where Columbus is known to have visited. The quadrant was a popular instrument with Portuguese explorers. Columbus would have marked the observed altitude of Polaris on his quadrant at selected ports of call just as the Arab seaman would tie a knot in the string of his kamal. Alternatively, the navigator could record the altura, or altitude, of Polaris quantitatively in degrees at Lisbon and at other ports to which he might wish to return. It wasnt long before lists of the alturas of many ports were published to guide the seafarer up and down the coasts of Europe and Africa. During the 1400s, Portuguese explorers were traveling south along the coast of Africa searching for a route to the orient. As a seafarer nears the equator heading south, Polaris disappears below the horizon. So, in southern seas, mariners had to have a different way of finding their latitude.

Under orders from the Portuguese Prince Henry, The Navigator, by 1480, Portuguese astronomers had figured out how to determine latitude using the position of the sun as it moved north and south of the equator with the seasons, what we now call its "declination." In simple terms, the navigator could determine his altura, his latitude, by using his quadrant to take the altitude of the sun as it came to its greatest altitude at local apparent noon, and then making a simple correction for the position of the sun north or south of the equator according to the date. The mariners quadrant was a major conceptual step forward in seagoing celestial navigation. Like the knots-in-a string method of the Arab kamal, the quadrant provided a quantitative measure, in degrees, of the altitude of Polaris or the sun, and related this number to a geographic positionthe latitude--on the earths surface. But for all its utility, the quadrant had two major limitations: On a windy, rolling deck, it was hard to keep it exactly vertical in the plane of a heavenly body. And it was simply impossible to keep the wind from blowing the plumb bob off line.

A beautiful mariners astrolabe made in Lisbon by J. de Goes in 1608, now in the Museum of the History of Science, Florence, Italy Mariner's astrolabes are now very rare and expensive - less than one hundred are known to survive and most of these are in poor condition having been recovered from ship wrecks. The seagoing astrolabe was a simplified version of the much more sophisticated Middle Eastern astronomers astrolabe that we saw a moment ago. All the complex scales were eliminated, leaving only a simple circular scale marked off in degrees. A rotatable alidade carried sighting pinnules. Holding the instrument at eye level, the user could sight the star through the pinnules and read the stars altitude from the point where the alidade crosses the scale. Astrolabe in use.For a sun sight, the astrolabe was allowed to hang freely and the alidade was adjusted so that a ray of sunlight passed through the hole in the upper vane and fell precisely on the hole in the lower vane. The astrolabe was popular for more than 200 years because it was reliable and easy to use under the frequently adverse conditions aboard ship.

A cross-staff. This one is a modern reproduction in the style popular with Dutch navigators in the eighteenth century. The next step in the evolution of celestial navigation instruments was the cross-staff, a device resembling a Christian cross. Interestingly, its operating principle was the same as that of the kamal. The vertical piece, the transom or limb, slides along the staff so that the star can be sighted over the upper edge of the transom while the horizon is aligned with the bottom edge. The Persian mathematician Avicenna wrote about a cross-staff in the eleventh century. The concept probably arrived in Europe when Levi ben Gerson, working in the Spanish school at Catalan in 1342, wrote about an instrument called a balestilla that he described as a being made from a "square stick" with a sliding transom. A cross-staff in use. This drawing, from a Spanish book on navigation published in 1552, shows how the cross-staff was used to determine the altitude of Polaris.

If youve ever heard the phrase "shooting the stars," it comes from the practice of holding a cross-staff up to the users eye with one hand, with the transom grasped in the other hand so that the person looks like an archer taking aim at the sun. Early cross-staffs had only two pieces - the staff and one transom. Over time they became more elaborate. After 1650, most "modern" cross-staffs have four transoms of varying lengths. Each transom corresponds to the scale on one of the four sides of the staff. These scales mark off 90, 60, 30, and 10 degrees, respectively. In practice, the navigator used only one transom at a time. The major problem with the cross-staff was that the observer had to look in two directions at once - along the bottom of the transom to the horizon and along the top of the transom to the sun or the star. A neat trick on a rolling deck! Davis quadrant. Made by an English craftsman named Walter Henshaw in 1711. Its made of rosewood with a diagonal scale on boxwood.

One of the most popular instruments of the seventeenth century was the Davis quadrant or back-staff. Captain John Davis conceived this instrument during his voyage to search for the Northwest Passage. It was described in his Seamans Secrets published in 1595. It was called a quadrant because it could measure up to 90 degrees, that is, a quarter of a circle. The observer determined the altitude of the sun by observing its shadow while simultaneously sighting the horizon. Relatively inexpensive and sturdy, with a proven track record, Davis quadrants remained popular for more than 150 years, even after much more sophisticated instruments using double-reflection optics were invented. One of the major advantages of the Davis back-staff over the cross-staff was that the navigator had to look in only one direction to take the sight - through the slit in the horizon vane to the horizon while simultaneously aligning the shadow of the shadow vane with the slit in the horizon vane. The major problem with back-sight instruments was that it was difficult if not impossible to sight the moon, the planets or the stars.

Thus, toward the end of the 1600's and into the 1700's, the more inventive instrument makers were shifting their focus to optical systems based on mirrors and prisms that could be used to observe the nighttime celestial bodies. The critical development was made independently and almost simultaneously by John Hadley in England and by Thomas Godfrey, a Philadelphia glazier, about 1731. The fundamental idea is to use of two mirrors to make a doubly reflecting instrumentthe forerunner of the modern sextant. Diagram of sextant How does such an instrument work? How many of you have ever held a sextant in your hand? Hold the instrument vertically and point it toward the celestial body. Sight the horizon through an unsilvered portion of the horizon mirror. Adjust the index arm until the image of the sun or star, which has been reflected first by the index mirror and second by the silvered portion of the horizon mirror, appears to rest on the horizon. The altitude of the heavenly body can be read from the scale on the arc of the instruments frame. Hadley's first doubly reflecting octants were made from solid sheets of brass. They were heavy and had a lot of wind resistance.

Lighter wooden instruments that could be made larger, with scales easier to divide accurately and with less wind resistance quickly replaced them. Early Hadley octant. This mahogany octant was made about 1760 by the famous London maker, George Adams. Hadley' octant of 1731 was a major advancement over all previous designs and is still the basic design of the modern sextant. It was truly a "point and shoot" device. The observer looked at one place - the straight line of the horizon sighted through the horizon glass alongside the reflected image of the star. The sight was easy to align because the horizon and the star seemed to move together as the ship pitched and rolled. We have seen how navigators could find their latitude for many centuries but ships, crews and valuable cargo were lost in shipwrecks because it was impossible to determine longitude. Throughout the seventeenth century and well into the eighteenth century, there was an ongoing press to develop techniques for determining longitude. The missing element was a way to measure time accurately. The clock makers were busy inventing ingenious mechanical devices while the astronomers were promoting a celestial method called "lunar distances".

Think of the moon as the hand of a clock moving across a clock face represented by the other celestial bodies. Early in the 18th century, the astronomers had developed a method for predicting the angular distance between the moon and the sun, the planets or selected stars. Using this technique, the navigator at sea could measure the angle between the moon and a celestial body, calculate the time at which the moon and the celestial body would be precisely at that angular distance and then compare the ships chronometer to the time back at the national observatory. Knowing the correct time, the navigator could now determine longitude. When the sun passes through the meridian here at Coimbra, the local solar time is 1200 noon and at that instant it is 1233 PM Greenwich Mean Time. Remembering that 15 degrees of longitude is equivalent to one hour of time gives us the longitude of 8 degrees, 15 minutes West of Greenwich. The lunar distance method of telling time was still being used into the early 1900s when it was replaced by time by radio telegraph. An octant measures angles up to 90 degrees and is ideally suited for observations of celestial bodies above the horizon.

But greater angle range is needed for lunar distance observations. It was a simple matter to enlarge Hadley's octant, an eighth of a circle, to the sextant, a sixth of a circle, that could measure up to 120 degrees. An early sextant by John Bird. The first sextant was produced by John Bird in 1759. This is a very early example of his work now in the Nederlands Scheepvaart Museum in Amsterdam. The frame is mahogany with an ivory scale. It is so large and heavy that it needed a support that fitted into a socket on the observers belt. A brass sextant by Dollond. Heres a fine brass sextant from the early nineteenth century by the master London instrument maker John Dollond. In the first half of the eighteenth century there was a trend back to wooden frame octants and sextants to produce lighter instruments compared to those made of brass. Ebony sextant. A very handsome example by H. Limbach of Hull of a sextant with an ebony frame. Ebony was used because of the dense wood's resistance to humidity. The scale and vernier were divided on ivory, or should we now say bone.

The design was not successful because the wood tended to split over the long arc of a sextant. - Examples of sextant frame designs. A sample of variations in frame design. The challenge was to produce sextant frames that were light weight, low wind resistance and with a minimum change is dimensions with changes in temperature. As you can see, some of them are quite esthetically pleasing. - Ramsden pentant. To be correct, the instrument should be called a pentant, a fifth of a circle, rather than a sextant. This jewel is only 4 1/2 inches radius. The scale is divided on silver from minus 5 degrees to 155 degrees with each degree further divided in three to 20 arc minutes. As you can see, the scale is beveled at 45 degrees. Why set the scale at an angle to the frame - perhaps just to show that he could do it! Probably the finest 18th century instrument maker was the Englishman Jesse Ramsden. His specialty was accurate scale division. Heres a small brass sextant that Ramsden made shortly before his death in 1800.

Ramsden's major achievement was to invent a highly accurate "dividing engine"the apparatus used to divide the scale into degrees and fractions of degrees. His design was considered so ingenious that the British Board of Longitude awarded Ramsden a prize of 615 poundsin 18th century terms, a small fortune. His "dividing engine" now resides in the Smithsonian Institution in Washington. The development of more precise scale division was a milestone in instrument development. Certainly, it permitted more accurate observations but it also permitted smaller, lighter, more easily handled instruments. The sextant you see here is my all-time favorite.

Question: What is a navigation instrument used until the 18th century to measure the angles between heavenly bodies, or between them and the horizon?
Answer: Astrolabe