Those Useful Imaginary Lines
BY AWAKE! CORRESPONDENT IN BRITAIN
TAKE a look at a world map or a globe. Do you notice the network of perpendicular and horizontal lines drawn all over it? No doubt you quickly recognize the one running horizontally across the middle of the map as the equator. But what about the other lines? What are they?
These lines are the so-called lines of latitude and longitude. Lines of latitude, or parallels, running horizontally on your map, join points on the surface of the earth that are the same distance from the equator. Lines of longitude, or meridians, on the other hand, are drawn from north to south, running from one pole to the other. That much you may remember from your geography lessons in school. But what is the purpose of this system of lines? How does it work? And how did it originate?
Pinpointing Where You Are
With such an interlocking grid of lines of latitude and longitude, every point on the surface of the earth can be precisely located by means of two measurements, called coordinates. For example, you can find New York City on a map by the reference lat. 40°42’ N and long. 74°0’ W, meaning that the city is located 40 degrees 42 minutes north of the equator and 74 degrees west of the internationally accepted prime meridian, the line of longitude that runs through Greenwich, a borough of London, England.a If seconds are added to these coordinates, even buildings within a city can be located. For instance, the city hall in New York City is at lat. 40°42’45” N and long. 74°0’23” W.
Distances also are reckoned with reference to these lines. The length of a nautical mile, for example, is one minute of latitude measured along a meridian. Since a pole is at 90 degrees, or 5,400 minutes (90 x 60 = 5,400), latitude from the equator, one nautical mile is 1/5,400 the distance from the pole to the equator. Thus, the average nautical mile is 6,080 feet [1.8532 km].
The ability to pinpoint any location accurately is certainly a tremendous boon, especially for navigators. For such a system to work, though, it must have certain reference points. The equator is a logical choice as the baseline from which measurements of latitude are made. But why was Greenwich chosen as the location of the prime meridian, the reference point for east-west longitudinal measurements? In fact, how did this whole idea of these imaginary lines imposed by man on his maps come about?
Lines With a History
As early as the second century B.C.E., the Greek astronomer Hipparchus used the concept of the imaginary lines to locate places on the surface of the earth. He chose a line through the Greek island of Rhodes as the reference from which to calculate positions to the east and to the west. The Greek astronomer Claudius Ptolemy of the second century C.E. is generally credited as the first one to come up with a system similar to the one that is in use today. His latitude lines were traced parallel to the equator. For longitude his starting point was a line through the western extremity of the world of his day, the Fortunate Isles, as the Canary Islands were then called.
It was not until 1884 that worldwide agreement was reached on the choice of a prime longitude line from which to measure positions to the east and to the west. In that year the International Meridian Conference at Washington, D.C., brought together 41 delegates from 25 countries. For the necessary astronomical observations to be made at the prime meridian, the delegates favored a line passing through a well-equipped observatory. By an overwhelming majority, they selected the line passing through Greenwich, England.
Travel and Time Zones
The choice of Greenwich as the location of the prime meridian was no accident. Since the 18th century, sea captains embarking from the busy port of London had been noting that as they sailed westward across the Atlantic, the sun reached its zenith later each day. They knew that because the earth rotates through 360 degrees every 24 hours, a time difference of one hour represented 15 degrees of longitude from Greenwich. Thus, using chronometers set by the master clock at the Greenwich observatory, they could fix their position on open sea simply by noting the difference between Greenwich time and their local time. For example, if they were at a spot where the sun reached its zenith (12:00 noon local time) at 3:30 p.m. Greenwich time, then by a simple calculation, they could fix their position as 52.5 degrees (15 x 3.5) west of Greenwich, that is, off the east coast of Newfoundland, provided they had stayed on the same latitude.
Staying on the same latitude, or parallel sailing, was a simple task. For centuries sailors in the Northern Hemisphere observed that the polestar, or Polaris, appeared virtually stationary when compared with the nightly movement of most other stars. They began to estimate how far north and south they were by measuring the height of that star above the horizon. Out on the open sea, they knew they were sailing due east or west as long as that star kept the same height.
The choice of Greenwich as a reference had other benefits for England. With the advent of railway travel there, a system of standardizing time within the country was needed. Too bad for the traveler who on arriving at Exeter railway station to catch the 11:33 found that it had departed some 14 minutes earlier! The problem? He used Exeter time; the rail system used London time. Acceptance of Greenwich Mean Time throughout the land put an end to those difficulties.
Even greater problems existed in the United States. Different railroads kept to different times. This situation led to a General Time Convention of the railroads, held in 1883. Four time zones, each spanning some 15 degrees of longitude, or one hour in time, and covering the continental United States, were adopted. All the towns within a zone were to keep the same time.
Eventually this zoning arrangement won worldwide acceptance. The world was divided into 24 time zones. The center of the system was Zone 0, extending 7 1/2 degrees each side of the Greenwich meridian. As someone traveled east, he would set his watch one hour later as he moved through each zone. To the west he would set his watch one hour earlier.
Halfway around the world from Greenwich, an interesting situation arises. Here, at the 180-degree meridian, there is a 24-hour difference in time from one side of the line to the other. Consequently, the 180-degree meridian, with minor variations to accommodate national boundaries, became the international date line. In crossing this line in a westerly direction, a traveler loses one day. Conversely, going eastward across the line, the traveler gains one day.
The days of chronometers checked at Greenwich and taken out to sea for calculating longitude have passed. Modern technology has replaced all of that. Radio beacons, radar, and international telecommunications provide more accurate information. Yet, pinpointing your location on a chart or map still depends on those imaginary lines of latitude and longitude. We can be grateful for those very useful imaginary lines.
a In angular measurement, one degree (°) is divided into 60 minutes (’), and each minute is divided into 60 seconds (”).
[Box/Pictures on page 20]
GREENWICH MEAN TIME
In 1675, King Charles II of England commissioned that “a small observatory” be constructed in what is now the London borough of Greenwich “in order to the finding out of the longitude of places for perfecting navigation and astronomy.” Two newly invented timepieces, with pendulums 13 feet [4 m] long, were installed to make accurate calculations of the earth’s rotation.
Scientists at the Royal Observatory soon discovered that the rotation of the earth is not isochronal, or at a constant speed. This is because the earth’s orbit around the sun is not a perfect circle and the earth’s axis is tilted. Thus, the solar day—the interval from noon to noon—varies in length throughout the year. With the Greenwich clocks in motion, calculations were possible that established a mean, or average, length for the day.
Noon Greenwich Mean Time is the moment when the sun reaches its zenith over any point on the Greenwich longitude line, or meridian (Latin, meridianus, of midday). Based on this Latin word, time before noon became known as ante meridiem (a.m.), or before midday; time after noon came to be post meridiem (p.m.).
Above: Greenwich Royal Observatory. Right: Prime meridian line on cobbled courtyard
[Map on page 18]
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WORLD TIME ZONES