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History of Navigation
Navigation is the art of getting from one place to another, safely and efficiently. The first sailors stayed close to shore and navigated by sighting landmarks. They did not have charts but instead used lists of directions. When they did venture out of sight of land, the navigator was able to determine his latitude (north/south direction) by observing the height of the sun during the day and the North Star at night.
Early mariners plotted their course by observing major constellations, though this was not an exact science. Estimates of distance travelled were made based upon multiplying the time underway by the speed of the vessel. Since time was measured with a sandglass and speed was estimated by watching pieces of seaweed pass by the hull, these early calculations were often less than accurate. One of the earliest man-made navigation tools was the mariner’s compass, which was invented by the Chinese in the twelfth century though the first versions were very crude. The navigator would rub an iron needle against a lodestone, place it in a piece of straw, and float it in a bowl of water. The needle would point in a northerly direction. Early mariners found the compass inconsistent – most likely because they did not understand that it pointed to the magnetic north pole, not true north.
After mariners began to keep detailed records of their voyages, land-based mapmakers used these sailing directions to create the first nautical charts called Portolan Charts (c. 13th Century). The charts were rare and very expensive, often kept secret so that competing mariners would not have access to this knowledge.
The charts did not have latitude or longitude lines but did have compass roses indicating bearings between major ports. They were, however, unreliable because the distance travelled at sea could not be measured accurately and mapmakers had not yet developed a good technique for depicting the contours of the earth on a two-dimensional plane.
Mariners at this time also used the cross-staff and the astrolabe to measure the angle above the horizon of the sun and stars to determine latitude. The forerunner of the much more portable (and accurate) sextant, the astrolabe was used to measure the altitude of a sun or star. Heavy and clumsy, it was very difficult to use aboard a rolling ship.
A major advance was the invention of the chip log (c.1500-1600). Essentially a crude speedometer, a line was knotted at regular intervals and weighted to drag in the water. It was tossed overboard at the stern as the pilot counted the knots that were let out during a specific period of time. From this he could determine the speed the vessel was moving. Interestingly, the chip log has long been replaced by equipment that is more advanced but we still refer to speed on the water as “knots.”
The key to determining longitude, or how far east or west they were located, lay in the invention of an accurate time-keeping device. Navigators knew that the sun reached its maximum altitude at noon, no matter where on earth they were. If they could determine what that exact time was on the longitude of 0° they could easily calculate the longitude of their present position by the difference in the two times since one hour equals 15° of longitude. This was considered so important that countries offered prizes for the invention of an accurate chronometer, or time piece. The British prize was won by John Harrison in 1764 for his seagoing chronometer, which was accurate to one-tenth of a second per day. James Cook used Harrison’s chronometer to circumnavigate the globe, and when he returned in 1779 his calculations of longitude based upon the chronometer proved correct to within 8 miles.
John M. Luykx Collection of Navigational Instruments
The Houston Maritime Museum is fortunate to have on display the outstanding collection of navigational instruments assembled by Lieutenant Commander John M. Luykx of the U.S. Navy. He was renowned as an expert in navigation, a teacher of navigation and seamanship, and a designer of navigational equipment. A member of the Navigation Institute, the Foundation for the Preservation of the Art of Navigation, and the U.S. Naval Institute, Commander Luykx was buried in Arlington National Cemetery following his death on August 22, 2003.
The Mariner’s Astrolabe
To find the latitude of a ship at sea, the noon altitude of the Sun was measured during the day or the altitude of a star of known location was measured when it was on the meridian (due north or south) at night. A number of devices evolved over time for these measurements, including the quadrant, cross staff, the back staff, and the mariner’s astrolabe. The Mariner’s Astrolabe, which was popular in the late 15th and early 16th centuries, was a simple brass ring, graduated in degrees with a rotating alidade used for sighting the sun or a star and measuring its distance.
In the early days of exploration, it was difficult for navigators to determine the exact time down to the second, which was necessary in order to determine longitude. Aboard ship, this was accomplished for the first time with the spring-driven marine chronometer. By compensating for varying temperature and maintaining its balance despite the movement of the ship, the chronometer could maintain the exact time at 0° Longitude, or Greenwich Mean Time. The navigator could then determine his position by calculating their time based on a 15° difference per hour.
The quadrant is a quarter-circle panel usually of wood or brass with markings of graduating degrees along the edge. Examples of marine navigational quadrants date as early as 1460. Along one edge there were two sights, and a plumb bob was suspended by a line from the center of the arc at the top. The navigator would view the star to be measured through the sights while holding the quadrant so that the plumb bob hung vertically. The plumb line indicated the calculations for the star’s altitude on the arc’s graduations.
Developed in the early 1700s, the sextant became a critical piece of navigation equipment for sea captains. A sextant is primarily used to measure the height of a celestial object like the sun, but it can also be used horizontally to measure the angle between two stationary objects such as lighthouses or rocks. The navigator looks through the eyepiece and observes the horizon through a slightly angled and silvered mirror that allows some light to penetrate. The light from the sun then reflects through a second mirror, the “index mirror.” The navigator moves the index bar until the light reflects off the horizon mirror making the sun appear to be on the horizon. The index bar is clamped into place marking the angle measurement, which can then be used to determine latitude and, if you also have a chronometer, longitude. Though the sextant has been replaced in most situations by Global Positioning Systems (GPS), it remains a useful backup tool in case human controlled systems such as GPS and electricity fail.
The cross-staff is a long staff, with sliding cross pieces perpendicular to the main staff, which was marked with graduated measurements. Observers could measure the angle of the sun or a star by sliding the cross pieces until the ends lined up with the object being measured. Introduced into England in the mid-sixteenth century, the staff originally had only one vane, was very long, and was consequently very difficult to use on a rocking ship. This problem was solved by adding more vanes which reduced the length of the staff to about 2 1/2 feet.
The cross-staff was used to find the latitude by measuring the angle of the North Star or the sun above the horizon. However, the observer had to look directly into the blinding sun for a solar measurement. In the early seventeenth century, the back-staff was developed to avoid this problem.
The Back Staff
In about 1594, John Davis, an English captain, developed a simple back-staff intended to be an improvement on the mariners’ quadrants, astrolabes, and cross-staffs. The Davis back-staff consisted of a graduated staff, an arc of a circle attached to the staff with a fixed vane, and a brass horizon vane with a slit in it at the fore-end of the staff.
The observer placed the staff on his shoulder and stood with his back to the sun. With the horizon vane lined up with the horizon, he slid the arc back and forth until the shadow of its vane fell across the slit in the bottom vane while the horizon was visible through the slit. By doing this the observer was able to sight both the sun and the horizon while his back was towards the sun.