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lace requires knowledge about time. A navigator needs to know what time it is on his ship and also the time at another place of known longitude - at the very same moment.

The Earth takes twenty-four hours to complete one full turn or revolution of three-hundred-sixty degrees. One hour marks on twenty-fourth of a turn, or fifteen degrees. So each hour's time difference between the ship and the starting point marks a ship's progress of fifteen degrees of longitude to the east or west. Those fifteen degrees of longitude mark a distance traveled.

At the equator, where the Earth is widest, fifteen degrees stretches about one thousand six hundred kilometers. North or south of that line, however, the distance value of each degree decreases. One degree of longitude equals four minutes of time all around the world.

But in measuring distance, one degree shrinks from about one hundred nine kilometers at the equator to nothing at the north and south poles.

BOB DOUGHTY: For many centuries, navigators hoped they could find longitude by observing the movement of stars at night.

During the day, the sun provided information about the time on a ship, and its direction. However, it did not provide necessary information about the time somewhere else.

In the sixteenth century, one astronomer suggested that navigators could observe the moon as it passed in front of different known stars to tell longitude.

But, there was not enough information about the stars to use this method effectively. Astronomers could not tell exactly where the moon would be from one night or day to the next.

Yet it seemed to those seeking to solve the longitude problem that the only solution was in the moon and stars.

(MUSIC)
STEVE EMBER: During the Seventeenth Century, English astronomers began a major effort to map the stars and their relationship to the moon as it passed across the sky.

Royal Astronomer John Flamsteed worked at this task for forty years. The next Royal Astronomer, Edmund Halley, spent another forty years gathering information about the moon's orbit.

After many years of gathering the necessary information, it became possible to learn longitude by observing the stars and the moon.

In seventeen sixty-six, Royal Astronomer Nevil Maskelyne published the Nautical Almanac and Astronomical Ephemeris.

It contained all the necessary information about the moon and stars that sailors would need to help them learn their longitude.

This new method was not simple. A navigator had to use complex observing instruments to note the position of the moon and stars.

Then he had to seek the correct information in the Nautical Almanac about the moon and stars at that time of night or day. The final step in the process was to take the mathematical information from the book, link it to the current information and solve the resulting problem. This took an average of four hours to do.

John Harrison's clocks helped sailors find longitude accurately.

BOB DOUGHTY: While scientists were studying the stars and moon to solve the longitude problem, a man named John Harrison was working on another project. He was trying to build a clock that would help sailors learn longitude.

His task also was difficult and complex. Mister Harrison had to develop a clock th