David’s Voyage Blog

Publish Date: Wednesday, March 16, 2011
I am in London this week on business, and on Sunday I took the opportunity to visit Greenwich, England, where the National Maritime Museum, the Royal Observatory, and the prime meridian are all located. It was a fun field-trip, and very relevant to our travels aboard Three@Sea.

You probably know that a “meridian” is a line of longitude, running from the North Pole to the South Pole, and longitude is measured in degrees east or west of the prime meridian. The location of the prime meridian is an arbitrary choice (versus a geographical characteristic like the equator), and the current prime meridian in Greenwich was established by international agreement in 1884.

One of the most challenging problems in the history of seafaring navigation was formulating a way to accurately determine your longitude when at sea. Latitude is relatively easy because it can be determined by direct measurement of the angle between the observer and a known celestial body (like the Sun) using a sextant. Longitude, on the other hand, is more difficult because the Earth is always rotating. Solving the longitude problem was considered so important that in 1714 the British Parliament offered a prize of up to 20,000 pounds to any person who could produce a solution.

There were several proposed approaches to solving the problem, but keeping accurate time while at sea was one of the most promising: If you know the current time in Greenwich by carrying an accurate clock aboard that is set to Greenwich Mean Time, and you accurately determine the local time by observing a celestial body (like the Sun) at its apex, then you can compute your longitude from the difference between the two times. Last winter Ayla and I wrote a blog about our experience learning to use a sextant, and this was one of the measurements we made. The Earth spins at 15 degrees per hour, so if your local time is 2 hours behind that of Greenwich, you are at 30 degrees west longitude. We are about 4 hours and 20 minutes behind GMT in Fajardo, Puerto Rico, which means our longitude is about 65 degrees west.

Although the time method is simple in principle, the main challenge in the 17th century was keeping accurate time while at sea: no clock or watch existed that was capable of doing so in such a harsh and unstable environment. Motivated by Parliament’s challenge, a self-taught English clock maker named John Harrison set out to build an accurate “marine chronometer”, and he stayed focused on the problem for more than thirty years. He produced three very accurate seafaring clocks (H1, H2, and H3), and in 1761 he produced the H4 “sea watch” that was capable of keeping accurate time on a trans-Atlantic voyage. Captain James Cook carried a replica of H4 on board during his second and third voyages in the 1770s, and the charts he made of the Pacific Ocean are extremely accurate because of this chronometer. The story of the longitude problem and Harrison’s achievements are chronicled in the book “Longitude“, which is an enjoyable and enlightening read.

The main reason I went to Greenwich is because the original Harrison clocks are housed at the Royal Observatory. I stood in front of them and tried to let the significance of their contribution to navigation sink in. In our modern world of GPS I think it is important to remember that we can reach as high as we do only because we are “standing on the shoulders of giants.”