The Phoenicians were the world’s first great navigators. They determined the location of their ships in terms of the length and width of the Mediterranean, the sea on which they sailed. The Phoenicians taught their navigational methods to the Greeks, from whom the Romans learned.

Eventually translated into Latin, the sailors’ two perpendicular directions became the longitude (longus, or long) and latitude (latus, or wide). Eratosthenes drew a map of the known world around the third century BCE that included lines of latitude and longitude, but his lines were determined by large cities; that is, they were not equally spaced. In the second century BCE Hipparchus of Nicaea greatly improved on this by spacing the lines equally around the globe. By assigning 360° to the circumference of Earth, and by using Eratosthenes’ nearly correct calculation for the size of Earth, Hipparchus was able to obtain good distances for a degree. Hipparchus’s system continues in use today.

Sailors of classical times, however, used only half the system –– the latitude. Early travelers observed the changes in the constellations as one travels north or south. One star, Polaris, is visible every clear night in the Northern Hemisphere, but its position dips closer to the horizon as one travels south. At the North Pole, Polaris is directly overhead; it disappears below the horizon at the equator. Until sailors began to travel in the Southern Hemisphere, all that was needed to find the latitude was an instrument for measuring how high Polaris, the pole star, was above the horizon. The Portuguese sailors who rounded the Cape of Good Hope were terrified because they had lost their main navigational tool on the trip down the side of Africa.

Sailors did not use the longitude because they had no way to measure it. The historian of Magellan’s circumnavigation reported that Magellan himself spent long hours trying to find ways to measure the longitude, but those under him were too proud of their navigational skills to speak of it. Common sailors and their pilots believed that they could navigate by a combination of charts, dead reckoning, and the latitude. Governments, however, realized that this was not good enough. The governmental view was particularly brought home to the English when in 1691 and again in 1707 large parts of the British navy were lost because of navigational errors.

Much earlier, in 1598, Philip III of Spain offered the first of several prizes by seagoing nations for the person who could find the longitude. Among the schemes suggested to Philip was one from Galileo. Having discovered the four largest satellites of Jupiter, he proposed that they could be used to locate the longitude. Galileo’s idea was based using charts that showed the relative positions of the satellites at different times. By observing the positions, one can determine the time. From the time, one can find the latitude. By telling the time exactly, one can determine the longitude. As a clock is carried from place to place, it will be off by one hour for each 15° of longitude. This is why there are four time zones in the lower 48 states of the United States. This part of the United States is approximately 4 × 15 or 60° of longitude wide. Comparing universal time with local time helps one to obtain the longitude. Local noon can be obtained easily by determining when the Sun reaches its daily zenith. It is universal time that is hard to obtain.

For example, if you get a long-distance phone call while you are in New York City and you do not know where the call is from, you can ask “What time is it?” If it is noon in New York, and the caller says 11 a.m., then you know the caller’s longitude is approximately between 90° W and 107° W, the approximate longitude of Central time in the United States. To determine the exact longitude, however, you need to forget about time zones and compare Sun times between two places. Specifically, compare local Sun time with Universal Time (UT), formerly known as Greenwich Mean Time (GMT), the time at longitude 0°. If you know it is 1:00 p.m. UT and you observe the sun time where you are to be noon, your longitude is exactly in 15° W.

Astronomical events that repeat frequently are a good way to obtain Universal Time. Eclipses of the Moon could be used, for example, if they were not too infrequent to be of much use to sailors. The positions of Jupiter’s moons are more useful because one or another is frequently eclipsed. Although Galileo’s suggestion was ignored by Philip, it was taken up in the 17th century by French astronomers, led by Giovanni Cassini. Using Jupiter’s moons, the French were able to establish correctly the longitude of cities in Europe and of islands in the Atlantic. The observations required, however, were too difficult and too time-consuming to be done by sailors at sea.

In 1530, Gemini Frisius suggested that the easy way to solve the problem would be to carry a good clock, set to some universal time, with the ship. Others had the same idea but clocks of the time were inadequate for the degree of precision needed. Even after Huygens developed a pendulum clock that theoretically kept good time, it was too imprecise to find the longitude and too delicate to be used on ships. Christopher Columbus noted on his first voyage that the compass needle changed its deviation from true north as he sailed across the Atlantic. Many expeditions were launched in the 18th century –– notably that of Edmond Halley –– to chart these magnetic deviations in the hopes that they would lead to the secret of the longitude. It was discovered, however, that the deviations vary in such an unpredictable manner that they are unsuitable for this purpose.

In 1714, the English Parliament provided a reward of £20,000 for anyone who could find the longitude and demonstrate the method on a voyage to the West Indies. Many of the leading scientists of the eighteenth century worked on the problem in competition for the prize, but the achievement was accomplished by a self-taught watchmaker, John Harrison. His clock Number Four –– or marine chronometer, as it came to be called –– made the West Indies trip in 1761 and passed the test with flying colors. All along the way Harrison’s son, who was in charge of the chronometer, predicted landfalls with greater accuracy by far than the ship’s pilots. Unfortunately for Harrison, the board governing the prize included a number of scientists who still hoped to get the money themselves. The board gave him half the prize after four years, and then stalled him again for seven more years. They would probably have stalled longer if King George III had not intervened on Harrison’s behalf and obtained the remainder of the prize for him.