For the purpose of finding an relative location on the earth’s surface with precision, geographers have drawn certain circles to which the position of any point may be referred. The art of navigation may have begun approximately 6,000 to 8,000 years ago; eventually it became a science of vast importance, particularly during Europe’s age of discovery, when knowing one’s exact position at sea saved both ships and the lives of sailors. With longitude being determined by time, and latitude by the angle of the sun at noon, mariners were able to find their exact position.

The earth, not a perfect orb, is an oblate spher­oid, as discovered by A. R. Clarke in 1866. It is known that the earth rotates on its polar axis, or a line between the north and south poles. From this axis great circles can be contrived, such as the equa­tor or 0° latitude, which has a 90° angle to the axis, and divides the earth in half, making the equator the largest or great circle. With the equator at 0°, the north and south poles are at 90° north and south latitude, respectively, with various angles from the axis in between. Latitude, then, is a line or parallel on the earth’s surface that runs east and west at various angles from the axis and measures distance north and south on the earth’s surface. Thus, the only latitude being a great circle is the equator with all other latitudes being smaller circles.

But, a number of great circles can be drawn on the earth depending on the angle from the axis, such as longitude. Longitude is a line or meridian that runs north and south but measures distance east and west (see the entry on John Harrison in this encyclopedia). Using an accurate timepiece such as a ship’s chronometer, longitude can be calculated, and using a sextant the latitude can be found so that an exact position on the earth’s sur­face can be plotted. Depending upon the accuracy required, latitude and longitude are measured by degrees, minutes, and seconds of an angle. The distance between lines of longitude decreases as one proceeds from the equator to the poles or as latitude increases.

It was Pythagoras, around 529 BCE, who first advanced the concept of the earth being a sphere with a moon evolving around the sun along with five known planets at that time. Later, Eratosthenes, around 300 BCE, was the first to place parallels on a sphere, but he named them after places. He also calculated the earth’s cir­cumference very accurately for that time, using the distance between two latitudes in Egypt. It was Ptolemy who first wrote about latitude and longitude in his Geographia, about the time that Alexander the Great was expanding Greek civilization.

The tools of navigation are a chart, a chro­nometer, a compass, and a sextant. Probably the first tool was the chart. The first charts, perhaps appearing as early as 2,700 BCE, were quite inac­curate. But, as time passed and technology improved, charts have become very accurate, so that contemporary charts actually a margin of error of less than one tenth of 1%. For measuring latitudinal position, the common quadrant was invented. From this crude instrument the astro­labe developed, first appearing about the 3rd century BCE. The astrolabe, like the common quadrant, was a device to measure the angle of the sun and various stars from the zenith. It was a cumbersome procedure involving several mari­ners. Later, the cross-staff was developed, with which the visible horizon was used rather than the zenith. Then, the back-staff or sea quadrant was invented by John Davis in 1590. Finally, in 1730, John Hadley perfected the sextant that is still in use today, commonly to measure latitude at noon using the angle between the horizon and the sun.

Another important tool is the compass. Crude though it may have been at first, it certainly decreased the chance of being lost at sea or in the desert. Perhaps the first compasses came into use about around the 11th century and were used by Norsemen. But, it wasn’t until almost the 20th century that an acceptable compass was developed by Lord Kelvin for the British Admiralty. An accu­rate timepiece for use at sea, the chronometer, was developed by John Harrison in the 18th century for measuring longitude; this was the last tool to be developed for accurate navigation at sea. Today, the global positioning system (GPS) is used to find accurate location or position.

Latitudes of significance, besides the equator and the poles, include the Arctic and Antarctic Circles at 67.5° north latitude and 67.5° south latitude, respectively, and the Tropic of Cancer and the Tropic of Capricorn at 23.5° north latitude and 23.5° south latitude, respectively. These smaller circles represent the relationship of the sun’s illu­mination at the summer and winter solstices. During the vernal equinox and the autumnal equi­nox, the sun’s rays are directly on the equator, but during the solstices the sun’s direct rays are over the tropics of Cancer and Capricorn, while the poles are in either total darkness or sunshine. These relationships between the earth and the sun are directly related to our weather patterns, sea­sons, and climate.

Richard A. Stephenson

See also Astrolabes; Chronometry; Harrison, John;

Presocratic Age; Timepieces

Further Readings

Bowditch, N. (2002). American practical navigator.

Washington, DC: National Imagery and Mapping Agency. (Original work published 1802) Christopherson, R. W. (2004). Elemental geosystems

(4th ed.). Upper Saddle River, NJ: Prentice Hall. Dunlap, G. D., & Shufeldt, H. H. (1972). Dutton’s

navigation and piloting (12th ed.). Annapolis, MD: Naval Institute Press.

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