Longitude | The Daily Star
12:00 AM, August 09, 2014 / LAST MODIFIED: 01:53 AM, March 08, 2015



Birds know their way. Photo: Ihtisham Kabir
Birds know their way. Photo: Ihtisham Kabir

One day last winter I was watching a flock of ducks flying high overhead as they migrated a long distance. Nature had armed them with the means to navigate: knowing where they were and where they should head. They did this effortlessly.

Months later, I was on a long-haul flight and found myself following the airplane's trajectory on the flight-path display. Thanks to the wonder of Geographical Positioning Systems (GPS), the passengers, like those flying ducks, knew where we were and our heading.

But if this has been easy for birds, it was not always so for humans.

Centuries ago, when explorers travelled by ship across the oceans, navigation was challenging. The ocean has no landmark and looks the same day after day, mile after mile. To know their position, sailors needed the ship's latitude and longitude.

Latitudes and longitudes are imaginary lines that go around the earth. Latitudes are parallel and encircle the earth horizontally, starting with the equator. Longitudes encircle the earth vertically, going through the poles.

On a ship in the middle of the ocean, knowing the longitude told you the distance travelled eastward (or westward) from your start-point. Latitude told the northward (or southward) distance travelled.

Latitude is easier to measure because it has a starting point (Latitude 0) at the equator. From a ship, latitude could be derived from measuring the angle of the sun.

But calculating longitude turned out to be a much harder scientific problem. Dava Sobel's book Longitude tells the story of this puzzle. One reason was that there was no natural origin for counting longitudes. Greenwich in England was chosen arbitrarily as zero longitude.

Because longitude could not be calculated, sailors could not know exactly how far east or west they had travelled. Obviously this was a major cause of unnecessary detours and  shipwrecks. In 1701, a shipwreck of an English ship off the coast of Italy cost two thousand lives. This moved the British Parliament to announce a “Longitude Prize.” Announced in 1714, the prize was worth GBP 20,000 (well over ten million pounds today) to be awarded to the person who solved this problem.

How was the conundrum solved? In an unexpected way that involved timekeeping.

It was well-known that there is a time difference between places east or west of each other. A change of fifteen degrees longitude results in a time difference of one hour. So, onboard a ship, if you knew the current time and the time at your start-point, you could calculate the ship's longitude.

The current time on the ship could be measured from the sun's angle. But a watch that showed the accurate time at the start-point did not exist. Pendulum-based timekeeping did not do well onboard a rocking ship.

The problem was solved by John Harrison, a relatively uneducated – but brilliant - watchmaker. He made four progressively improved clocks over several years; each was taken on long voyages and proved its worth.

A beneficiary of Harrison's clock was Captain Cooke, the first European to reach Australia, Hawaii and New Zealand.

Incidentally, the notion of using satellites for GPS was first predicted by science fiction writer Arthur C. Clarke in 1956.



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