For measurements in the east-west direction, the earth's surface has been divided into lines of longitude, or meridians; one complete circuit around the earth equals 360 degrees of longitude, and all longitude lines intersect at the North and South Poles. By international agreement, the line of longitude that runs through Greenwich, England, has been labeled the zero meridian; and longitude is measured east and west from this meridian to the point where the measurements meet at 180 degrees, on the opposite side of the earth from the zero meridian.

At any point on earth, the sun travels across the sky from east to west at the rate of 15 degrees in one hour, or one degree in four minutes. So if a navigator has a very accurate clock aboard his ship-one that can tell him very accurately the time at Greenwich or the zero median-he can easily figure his longitude. He simply gets the time where he is from the sun. For every four minutes that his clock, showing Greenwich time, differs from the time determined locally from the sun, he is one degree of longitude away from Greenwich.

At night he can get his position by observing the location of two or more stars. The method is similar to obtaining latitude from the North Star. The difference is that whereas the North Star appears suspended in the sky, the other stars appear to move in circular paths around the North Star. Because of this, the navigator must know the time in order to find out where he is. If he does not know the time, he can read his location with respect to the stars, as they "move" around the North Star, but he has no way at all to tell where he is on earth! His navigation charts tell him the positions of the stars at any given time at every season of the year; so if he knows the time, he can find out where he is simply by referring to two or more stars, and reading his charts.

The principle of the method is shown in the illustration. For every star in the sky there is a point on the surface of the earth where the star appears directly overhead. This is Point A for Star #1 and Point B for Star #2 in the illustration. The traveler at Point 0 sees Star #1 at some angle from the overhead position. But as the illustration shows, all travelers standing on the black circle will see Star #1 at this same angle. By observing Star #2, the traveler will put himself on another circle of points, the blue circle; so his location will be at one of the two intersection points of the blue and the black circles.

He can look at a third star to choose the correct intersection point; or, as is more usually the case, he has at least some idea of his location, so that he can pick the correct intersection point without further observation.

The theory is simple. The big problem was that until about 200 years ago, no one was able to make a clock that could keep time accurately at sea.

BUILDING A CLOCK THAT WOULDN'T GET SEASICK

During the centuries of exploration of the world that lay thousands of miles across uncharted oceans, the need for improved

navigation instruments became critical. Ship building improved, and larger, stronger vessels made ocean trade as well as ocean warfare increasingly important. But too often ships laden with priceless merchandise were lost at sea, driven off course by storms, with the crew unable to find out where they were or to chart a course to a safe harbor.

Navigators had long been able to read their latitude north of the equator by measuring the angle formed by the horizon and the North Star. But east-west navigation was almost entirely a matter of "dead reckoning." If only they had a clock aboard that could tell them the time at Greenwich, England, then it would be easy to find their position east or west of the zero meridian.

It was this crucial need for accurate, dependable clocks aboard ships that pushed inventors into developing better and better timepieces. The pendulum clock had been a real breakthrough, and an enormous improvement over any timekeeping device made before it. But it was no use at all at sea. The rolling and pitching of the ship made the pendulum inoperative.

ROLLING SEA
TEMPERATURE
SALT SPRAY

In 1713 the British government offered an award of £20,000 to anyone who could build a chronometer that would serve to determine longitude to within 12 degree. Among the many craftsmen who sought to win this handsome award was an English clock maker named John Harrison, who spent more than 40 years trying to meet the specifications. Each model became a bit more promising as he found new ways to cope with the rolling sea, temperature changes that caused intolerable expansion and contraction of delicate metal springs, and salt spray that corroded everything aboard ship.

When finally he came up with a chronometer that he considered nearly perfect, the men of the government commission were so afraid that it might be lost at sea that they suspended testing it until Harrison had built a second unit identical with the first, to provide a pattern. Finally, in 1761 Harrison's son William was sent on a voyage to Jamaica to test the instrument. In spite of a severe storm that lasted for days and drove the ship far off course, the chronometer proved to be amazingly accurate, losing

less than 1 minute over a period of many months and making it possible for William to determine his longitude at sea within 18 minutes of arc, or less than 1% of one degree. Harrison claimed the £20,000 award, part of which he had already received, and the remainder was paid to him in various amounts over the next two years-just three years before his death.

For more than half a century after Harrison's chronometer was accepted, an instrument of similar design-each one built entirely by hand, of course, by a skilled horologist-was an extremely valuable and valued piece of equipment-one of the most vital items aboard a ship. It needed very careful tending, and the one whose duty it was to tend it had a serious responsibility.

Today there may be almost as many wrist watches as crew aboard an ocean-going ship-many of them as accurate and dependable as Harrison's prized chronometer. But the ship's chronometer, built on essentially the same basic principles as Harrison's instrument, is still a most vital piece of the ship's elaborate complement of navigation instruments.