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If It’s Tuesday, It Must Be Thursday

By Allan Eastman

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Some Time At Sea

Observers at the Royal Observatory (built 1675)

Illustrated above is a depiction of observers at the Royal Observatory, which was established in Greenwich, England, in 1675.


To get to the root of this story, we must go back to that era when Magellan and all the other explorer/entrepreneurs really opened up the World as one great big globe. It was the Age of Imperialism when the European Powers with their ocean going fleets and superior weaponry went out to exploit and colonize all the other continents of the planet. England and Holland soon challenged Spain and Portugal for overseas colonies and their riches. Vast fortunes were made and lost on a single voyage.

As these fleets sailed off into the waters of all corners of the planet, the map of the world as it really existed was finally being drawn. And with these epic voyages came the need for more accurate navigation to find your way from one side of the world to the other in pursuit of trade.

Mapmakers and navigators were soon using the Ptolemaic system of describing the globe as the 360 degrees of a circle with further divisions into minutes and seconds of arc.

Going North and South was no problem. Latitude had long been figured out. If you measure the height of the Sun at its apparent highest point (Noon), and calculate that angle to the visible horizon, from a set of tables you can derive the number of degrees you are above or below the equator. Sailors also learned to use the North Star and other celestial objects to calculate their latitude. Eventually, a highly accurate instrument was developed – the Sextant – to replace earlier tools like calibrated poles for making the Latitude calculations.

Longitude – your position East and West - was the problem. There was no astronomical observation that could give you the answer and it was impossible to accurately measure the distance traveled over the shifting surface of the Oceans. Accelerating losses of ships and crews in the growing worldwide commercial system gave urgency to solving what was soon officially called “The Longitude Problem.”

In 1675, the Royal Observatory was established at Greenwich, England. Its mandate was to be the central research facility for quantifying astronomical phenomena and related Time matters but its main focus was to find a way to derive Longitude.

All kinds of schemes were proposed by the natural philosophers of the day, ranging from clever but unworkable to the completely hare-brained. Gradually, as the true size of the world became obvious and mapmaking more advanced, it came to be understood that Longitude could be measured through the use of Timekeeping.


The world was conceived to be 360 degrees around its circumference. If you divided this by the 24 hours in a day, it meant that every hour of time related to 15 degrees of longitude.

If a ship’s Captain had an accurate clock on board telling the time back at his home location, he could compare that time with the local Solar Time at his own position – apparent Noon as measured by a sextant – and the difference between the 2 times would allow him to calculate the number of degrees he was east or west of his home port.

However, existing Time technology was too primitive to provide a really accurate timepiece for use at sea. The radical motions of ships on the bounding main defeated the regularity of pendulum clocks and the mechanisms of spring driven clocks were corrupted by temperature changes and salt corrosion. Many devices were tested but nothing was found to be accurate enough to do the job.

Desperate for a solution, the English parliament set up a special Board of Longitude in 1714 which offered a 20,000 Pound reward to anyone who could build an accurate enough clock, about 10,000,000 Dollars at current values.

One clockmaker, John Harrison, rose to the occasion. He worked on the problem for the next 20 years. His first effort was a meter square, weighed over 30 kg and utilized an incredibly complex clockwork but ended up being both too big to be practical at sea and not accurate enough for the job. But Harrison learned from the experience and his next 2 prototypes were progressively smaller and more technically advanced. A key breakthrough was combining steel and brass in the escapement mechanism to reduce the effects of temperature changes on board ship.

Finally, Harrison put all his learned experience into building his breakthrough masterpiece, the H4. This clock weighed a mere 3 kg and pioneered the use of jewels – diamonds and rubies – on the pivot points of the mechanism to reduce friction. During extended sea trials, the H4 was found to lose only about 5 seconds over a two and a half month period – accurate enough for close longitude calculations.

The British Navy and commercial shipping quickly adopted Harrison’s timepiece but inevitably, the Board of Longitude waffled on paying him the prize. He finally had to petition the King to claim his reward. Harrison eventually got the money and became a very rich man but in a general example of Life’s ironies, he died a few years later.

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