Finding Your Position on the
Globe Was Not Always Easy
STEVE EMBER: This is Steve Ember.
BOB DOUGHTY: And this is Bob Doughty
with the VOA Special English program EXPLORATIONS. Today, we tell about how
people learned an important piece of information necessary for safely sailing on
the oceans. It is called longitude.
STEVE EMBER: On a
foggy October night in seventeen seven, four English navy ships hit rocks in the
Atlantic Ocean and sank. Two thousand men drowned. The ships had been sailing in
the thick fog for twelve days. There was no sure way to know where they
The commander of the ships had been worried that they could
hit rocks if they were not careful. He asked his navigators for their opinion on
their location in the ocean.
The navigators did not really know. They
told the commander they thought they were west of a small island near the coast
of northwestern France.
They were wrong. Instead, they sailed onto rocks
near a small group of islands southwest of England's Atlantic coast. The
navigators' lack of knowledge led to the loss of four ships and two thousand
BOB DOUGHTY: When people began sailing out of sight of land,
sailors did not know how to tell where they were on the open sea. Land travelers
can look at a mountain, or a river, or an object that shows them where they are
in relation to where they came from. On the ocean, however, there is no sign to
tell a sailor where he is.
The most important device for knowing
directions on the ocean is a compass. A compass is a device containing a metal
object that points toward the magnetic north pole. This shows navigators the
direction of north, and therefore also south, east, and west. But sailors need
more information to sail safely on the open sea.
Most maps of the world show lines that are not on the Earth's surface. One line
is the equator. It is an imaginary line around the widest part of the
There are similar lines both north and south of the equator.
These circles become smaller and smaller toward the north pole and the south
These lines, or circles, are parallel - meaning that they are
equally distant from each other at any point around the world. These lines show
what is called latitude.
A navigator can know the latitude of his
ship by observing the location of stars, where the sun rises in the morning and
sets in the evening, and what time of year it is. With this information he knows
where his ship is in relation to the north or south pole and the
BOB DOUGHTY: Still, there is one more important piece of
information necessary for safely sailing the oceans.
centuries, scientists, astronomers and inventors searched for a way to tell
longitude. The lines of longitude go the other way from latitude lines. They
stretch from the north pole to the south pole, and back again in great circles
of the same size. All of the lines of longitude meet at the top and bottom of
In her book, “Longitude,” writer Dava Sobel tells the
story about longitude and how the problem of knowing it was solved.
STEVE EMBER: For centuries, the great
scientists of the world struggled to develop a way to learn longitude. To learn
longitude at any place 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 one 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
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.
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.
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
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 make it possible to find longitude
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. Mr. Harrison had to develop a clock that was not affected by the
movement of a ship on the ocean or changes in temperature or atmospheric
He began developing his clock in seventeen thirty. It took
five years to complete. The complex device weighed thirty-four kilograms.
Several years later, Mr. Harrison built a second clock. It was smaller, but
weighed more than the first. Mr. Harrison was not satisfied and began working on
yet another device.
Twenty years later, he completed a device that was
smaller than the first two, and weighed less. But still Mr. Harrison was not
Two years later, in seventeen fifty-seven, he produced a
small clock that he could hold in his hand. The clock could tell the correct
time in two places, meeting the requirements for learning longitude on the
STEVE EMBER: For many years after Mr. Harrison's work was
completed, the idea of using a clock to learn longitude was rejected. However,
that opinion changed when manufacturers learned how to make better and less
costly versions of Mr. Harrison's clocks.
The clocks became known
as chronometers. By eighteen fifteen, five thousand chronometers were in use on
ships sailing the world's oceans. The complex documents and mathematical work
were no longer necessary. Almost any sailor could tell what his longitude was by
simply looking at a clock. The world had changed.
BOB DOUGHTY: John
Harrison's clocks can be seen today at the Old Royal Observatory in Greenwich,
England. The first three are still operating, showing the correct time.
To look at them is to see the simple solution to a problem that worried people
for many centuries.
Today, the solution to the problem is so common that
it is difficult to understand that there was a problem at all.
STEVE EMBER: This program was written by Oliver Chanler and
produced by Paul Thompson. Our studio engineer was Al Alaby. This is Steve
BOB DOUGHTY: And this is Bob Doughty. Join us again next week for
EXPLORATIONS, a program in Special English on the Voice of America.