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Latitude and Longitude

By Ted Andros

Inset close-up. A chart of the Gulf Stream, 1786.
Inset close-up. A chart of the Gulf Stream, 1786.

Latitude and longitude are the basic tools of mapmaking. They are the language used by mapmakers to communicate accurately about the locations of the various places on planet earth.

Latitude and longitude are imaginary lines traced on the surface of the earth for the purpose of locating a specific place. The lines of latitude run east and west, and are parallel to each other. For convenience, the earth has been divided into 360 degrees of latitude. Measure of latitude starts at the equator and is measured as 90 degrees north to the North Pole and 90 degrees south to the South Pole. Each degree represents 60 nautical miles on the earth's surface. The lines of longitude run north and south and are not parallel to each other. Consecutive degrees of longitude are 60 miles apart at the equator, and all come together at the North Pole and the South Pole. These are 360 degrees of longitude - 180 degrees lie east, and 180 degrees lie west of Greenwich, England, an arbitrary starting point used by all mapmakers.

All lines of longitude are called "Great Circles" because they are all representative of the circumference of the earth. Only one latitude line is a great circle, the one lying on the equator. All other latitude lines are smaller than the circumference of the earth.

Since earliest times, astronomers and mapmakers have used the equator to divide the world into northern and southern hemispheres. The starting point for longitude, however, has varied many times over the ages. It has been located in Egypt, Greece, Spain, France, and other places, depending on which country was preeminent at the time in the study of location by celestial observation. In 1884, the international community adopted Greenwich as the mean in order to standardize world mapmaking.

The use of grid lines (latitude and longitude) was first suggested by the Greek astronomer Hipparcus about 300 years before Christ. Shortly after, Hipparcus devised the method of fixing the location of places on earth by observation of the celestial bodies--the sun, moon and stars. He worked out the mathematics of spherical trigonometry, which allowed the results of these observations to be plotted on an earth that he perceived to be a sphere.

Some 75 years later (around 225 B.C.) Eratosthenes, another Greek mathematician and astronomer, measured the circumference of the earth (accurate to within 300 miles) raising the art of mapmaking to new standards of accuracy.

From that point on, the Greeks could find latitude quite easily. They knew the position of the sun north or south of the equator. By using spherical trigonometry they could measure the sun's angle at noon relative to the equator. Noon was easy to determine since it occurred when the sun was at its highest point in the heavens.

Longitude, however, was not so easy. The distance from one place to another on the earth in an east-west direction could be plotted from the position of the stars and sun, but only if the time difference between the two places was known. The measurement of longitude was to remain inexact until the invention of the telescope and the accurate clock, almost two thousand years later.

A way of longitude by measuring the time lapse in observing the eclipses of the satellites of Jupiter, became possible with the invention of the telescope. While this was difficult, it did provide some progress toward the measurement of longitude. This solution, however, was restricted to use on land since the eclipses could not be accurately observed on the deck of a moving ship.

Accurate clocks, which began to appear in the 16th century, made longitude measurements on land a reality. It was not until 1760, however, that an Englishman named John Harrison invented an exact chronometer, making possible the measurement of longitude at sea.

Today, with the aid of satellites, someone with the proper instruments on even a small yacht can measure latitude and longitude at sea to within an accuracy of 50 feet.


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