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John Harrison and Longitude

Until the 17th century, seamen had difficulty finding their coordinates at sea. They could easily find their latitude by measuring the angle between Polaris (the North Star) and the horizon in the northern hemisphere and the Crux (or the Southern Cross) and the horizon in the southern hemisphere.  The measurement of latitude was found by measuring this angle. Measuring the angle was no problem with the invention of the sextant. A sextant allows for the measurement of an angle of a celestial object relative to the horizon.


The problem was finding the longitude. The longitude is an arbitrary index we created while the latitude is a physical property of the earth (the actual angle of one object to another). Another factor which makes measuring longitude seem more artificial is that while along each latitude, like 0° (the equator) or 90° (the north pole), you have a very specific climate; that is not the case for longitude. Longitude is an artificially created measurement made by men so that we can all communicate our locations. For example, the coordinates of New York City is 40.6700° N, 73.9400° W. By principles of geometry the angle from the center of the earth to NYC will be the same as NYC to the north star (see drawing for explanation). Latitude is a circle. If you stand anywhere on the latitude circle which goes through NYC, which is on the surface of the earth, and you measure the angle from the horizon to the North Star, it will be the same as this measurement in NYC. The longitude is more complicated to measure; for this we must introduce the concept of The Prime Meridian.

 

The Prime Meridian is an arbitrary semi-circular line that passes from the North Pole to the South Pole going through Greenwich, England, which represents the zero longitude line. For years there was a debate between France and England in regards to who would have the honor of having the zero longitude run through it. The French wanted it in Paris and the British wanted it in Greenwich, near London. The reason the British wanted the location to be Greenwich and not in the center of London was because the Royal Observatory was in Greenwich and it was very convenient to determine local noon by celestial observation. At the time the British far exceeded the French in their political influence internationally, and so they won the honor of hosting the Prime Meridian, the zero longitude. Part of the reason the British were so influential was because they were especially strong as a maritime power during the 19th century, this agreement being internationally accepted in 1884. We know New York City is located at 40.67° latitude and 73.94° longitude. In order to find this longitude we would need a very accurate clock set to Greenwich Mean Time (GMT), another clock displaying the local time, and a sextant, or instead of all of those we could use a GPS. The clock displaying GMT needs to be very precise, which in the watch industry is called a chronometer, the reason being that it is able to withstand differences in temperatures and humidity during long voyages (this was more important in the days when cross-ocean travelling was done by boat). It also needs to be less affected by gravity since ships could rock during storms. The difference between local time and GMT can determine the longitude. To see this think about longitude 180° west or east (it doesn’t matter which). It is exactly 12 hours ahead (or behind) Greenwich Time. At 90° west the time is 6 hours behind Greenwich Mean Time (GMT). The local time was very easy to determine with the second clock that did not need to be as accurate. One could set this clock daily to local noon whenever the sun was at the highest point in the sky. Comparing your two clocks, one showing the local time and one showing GMT, will give you the difference. This difference can be easily used to make a conversion to the angle which represents the longitude. The problem during the 17th and early 18th century was that their clocks were not accurate enough to maintain a reliable GMT time for a long voyage by sea. This was the biggest scientific challenge of the time. Newton and other famous scientists did not believe it would ever be possible to produce a clock with the level of accuracy necessary to find the longitude.

 

To the surprise and awe of many, John Harrison, after investing his whole life, tackled this challenge in 1765 when he was 68 years old. The original clock that was his prize invention is currently displayed in the Maritime Museum in London. Today, pilots still use GMT whenever they communicate with Air Traffic Control (ATC). Airplanes arriving from different time zones need a common language when talking about ETAs, circling times, etc… GMT is a very convenient common denominator.

 

Torgoen watches produce various models that can display a second time zone in a 24 or 12 hour formats. The T05, Torgoen’s first GMT watch is still today one of the best sellers, both due to an affordable price, great quality, and its simple and bold design. It has two time zones, one of which could be set to local time and the other (the red hand with the big triangle) can be set to GMT, or any time zone for that matter, with a 24 hour format.