If the Earth were cut through at the equator, the section would pass through the center of the Earth. That makes the equator a "great circle." Every sphere has an infinite number of great circles, but the equator is the only parallel of latitude that is one of them.

It early became customary to measure off the parallels of latitude in degrees. There are 360 degrees, by coilven 40 tion, into which the full circumference of a sphere can be divided. If you travel from the equator to the North Pole, you cover a quarter of the Earth's circumference and therefore pass over 90 degrees. Consequently, the parallels range from O' at the equator to 90' at the North Pole (the small ' representing "degrees"). .If you continue to move around the Earth past the North Pole so as to travel toward the equator again, you must pass the parallels of latitude (each of which encircles the Earth east-west) in reverse order, traveling from 90' back to O' at the equator (but at a point directly opposite that of the equatorial beginning). Past the equator, you move across a second set of parallels circling the southern half of the globe, up to 90' at the South Pole and then back to O', finally at the starting point on the equator.

To differentiate the O' to 90' stretch from equator to North Pole and the similar stretch from equator to South Pole, we speak of "north latitude" and "south latitude."

Thus, Philadelphia, Pennsylvania is on the 400 north latitude parallel, while Valdivia, Chile is on the 40' south latitude parallel.

Parallels of latitude, though excellent as references about which to build a map, cannot by themselves be used to locate points on the Earth's surface. To say that Quito, Ecuador is on the equator merely tells you that it is some where along a circle 25,000 miles in circumference.

For accurate location one needs a gridwork of lines-a set of north-sbuth lines as well as east-west ones. These north-south lines, running up and down the conventionally oriented map (longways) would naturally be called "longi tude."

Whenever it is midday upon some spot of the Earth it is midday at all spots on the same north-south line, as one can easily show if the Earth is considered to be a rotating sphere. The north-south line is therefore a "meridian" (a corruption of a Latin word for "midday"), and we speak of "meridians of longitude."

Each meridian extends due north and south, reaching the North Pole at one extreme and the South Pole at the other. All the meridians therefore converge at both poles and are spaced most widely apart at the equator, for all the world like the boundary lines of the segments of a tangerine. If one imagines the Earth sliced in two along any meridian, the slice always cuts through the Earth's center, so that all meridians are great circles, and each stretches around the world a distance of approximately 25,000 miles.

By 200 B.C. maps being prepared by Greeks were marked off with both longitude and latitude. However, making the gridwork accurate was another thing. Latitude was all right. That merely required the determination of the average height of the midday sun or, better yet, the average height of the North Star. Such determinations could not be made as accurately in ancient Greek times as in modem times, but they could be made precisely enough to produce reasonably accurate results.

Longitude was another matter. For that you needed the time of day. You had to be able to compare the time at which the Sun, or better still, another star (the sun is a star) was directly above the local meridian, as compared with the time it was directly,above another meridian. If a star passed over the meridian of Athens in Greece at a certain time, and over the meridian of Messina in Sicily 32 minutes later, then Messina was 8 degrees of longitude west of Athens. To determine such matters, accurate time pieces were necessary; timepieces that could be relied on to maintain synchronization to within fractions of a minute over long periods while separated by long distance; and to remain in synchronization with the Earth's rotation, too.

In ancient times, such timepieces simply did not exist and therefore even the best of the ancient geographers managed to get their meridians tangled up. Eratosthenes of Cyrene, who flourished at Alexandria in 200 B.c., thought that the meridian that passed through Alexandria also passed through Byzantium (the modern city of Istanbul, Turkey). That meridian actually passes about 70 miles east of Istanbul. Such discrepancies tended to increase in areas farther removed from home base.

Of course, once the circumference of the earth is known (and Eratosthenes himself calculated it), it is possible to calculate the east-west distance between degrees of longi tude. For instance, at the equator, one degree of longitude is equal to about 69.5 miles, while at a latitude of 40' (either north or south of the equator), it is only about 53.2 miles, and so on. However, accurate measurements of distance over mountainous territory or, worse yet, over stretches -of open ocean, are quite difficult.

In early modem times, when European nations first began to make long ocean voyages, this became a horrible problem. Sea captains never knew certainly where they were, and making port was a matter of praying as well as sailing. In 1598 Spain, then still a major seagoing nation, offered a reward for anyone who would devise a timepiece that could be used on board ship, but the reward went begging.

In 1656 the I)rutch astronomer Christian Huygens in vented the pendulum clock-the first accurate timepiece.

It could be used only on land, however. The pitching, roll ing, and yawing of a ship put the pendulum off its feed at once.

Great Britain was a major maritime nation after 1600, and in 1675 Charles 11 founded the observatory in Green wich (then a London suburb, now Part of Greater Lon don) for the express purpose of carrying through the necessary astronomical observations that would make the accurate determination of longitude possible.

But a good timepiece was still needed, and in 1714 the British Government offered a large fortune (in those days) of 20,000 pounds for anyone who could devise a good clock that would work on shipboard.

The problem was tackled by John Harrison, a Yorkshire mechanics self-trained and gifted with mechanical genius.

Beginning in 1728 he built a series of five clocks, each better than the one before. Each was so mounted that it could take the sway of a ship without being affected. Each was more accurate at sea than other clocks of the time were on land. One of them was off by less than a minute after five months at sea. Harrison's first clocks were per haps too large and heavy to be completely practical, but the fifth was no bigger than a large watch.

The British Parliament put on an extraordinary display of meanness in this connection, for it wore Harrison out in its continual delays in paying him the money he had earned and in demanding more and ever more models and tests. (Possibly this was because Harrison was a provincial mechanic and not a gentleman scientist of the Royal So ciety.) However, King George III himself took a personal interest in the case and backed Harrison, who finally re ceived his money in 1765, by which time he was over 70 years old.

It is only -in the last two hundred years, then, that the latitude-longitude gridwork on the earth became really accurate.

Even after precise longitude determinations became pos sible, a problem remained. There is no natural reference base for longitude; nothing like the equator in the case of latitude. Different nations therefore used different systems, usually basing "zero longitude" on the meridian passing through the local capital. The use of different systems was confusing and the risk was run of rescue operations at sea being hampered, to say nothing of war maneuvers among allies being stymied.

To settle matters, the important maritime nations of the world gathered in Washington, D.C. in 1884 and held the "Washington Meridian Conference." The logical de cision was reached to let the Greenwich observatory serve as base since Great Britain was at the very height of its maritime power. The meridian passing through Greenwich is, therefore, the "prime meridian" and has a longitude of 00.

The degrees of longitude are then marked off to the west and east as "west longitirde" and "east longitude."

The two meet again at the opposite side of the world from the prime meridian. There we have the 180' meridian which runs down the middle of the Pacific Ocean.

Every degree of latitude (or longitude) is broken up into 60 minutes ('), every minute into 60 seconds ("), while the seconds can be broken up into tenths, hun dredtbs, and so on. Every point on the earth can be located uniquely by means of latitude and longitude. For instance, an agreed-upon reference point within New York City is at 40' 45' 06" north latitude and 73' 59' 39" west longi tude; while Los Angeles is at 34' 03' 15" north latitude and 118' 14' 28" west longitude.

The North Pole and the South Pole have no longitude, for all the meridians converge there. The North Pole is defined by latitude alone, for 90' north latitude represents one single point-the North Pole. Similarly, 90' south lati tude represents the single point of the South Pole.

It is possible to locate longitude in terms of time rather than in terms of degrees. The complete day of 24 hours is spread around the 360' of longitude. This means that if two places differ by 15' in longitude, they also differ by 1 hour in local time. If it is exactly noon on the prime meridian, it is 1 P.m. at 15' east longitude and 1 1 A.M. at 151 west longitude.

If we decide to call prime meridian 0:00:00 we can assign west longitude positive time readings and east longi tude negative time readings. All points on 15' west longi tude become +1:00:00 and all points on 15' east longi tude become - 1: 00: 00.

Since New York City is at 73' 59' 39" west longitude it is 4 hours 55 minutes 59 seconds earlier than London and can therefore be located at +4:55:59. Similarly, Los Angeles, still farther west, is at +8:04:48.

In short, every point on Earth, except for the poles, can be located by a latitude and a time. The North and South Poles have latitude only and no local times, since they have no meridians. This does not mean, of course, that there is no time at the poles; only that the system for measuring local times, which works elsewhere on Earth, breaks down at the poles. Other systems can be used there; one pole might be assigned Greenwich time, for instance, while the other is assigned the time of the 180' meridian.