The Long and the Short of Longitude, GMT and the IDL

Royal Observatory

Several people took the time to thank me for my article on John Harrison, so I thought we would look a little further at the Longitude Problem and the measurement of time itself. Longitude is the old school British spelling of longitude and as I was taught using it, I prefer it.

Let's recap for a moment for our new readers: In order to calculate their longitude, sailors had to measure the local time, wherever they were, by observing the Sun. But navigation required that they also know the time at some reference point, which was the problem that John Harrison solved. His portable clock allowed the voyagers to know the time at that reference point. Once both times were known, the rest was just simple mathematics. So what reference point should be used? Well, in one of those strange quirks of fate, the cart preceded the horse by about one hundred years.

"Whereas, in order to the finding out of the longitude of places for perfecting navigation and astronomy, we have resolved to build a small observatory within Our Park at Greenwich" said Charles II in 1675

Actually, the longitude problem, described by the king as being for perfecting navigation and astronomy was something of an understatement. I came across a review of what looks like a wonderful book on Harrison, "Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time", by Dava Sobel, 1995, published by Penguin Books. Not the catchiest of titles but a good read nonetheless if the review is anything to go by.

In the review, published in the Washington Monthly in 1996, Gregg Easterbrook says:

"Serious pursuit of a means of fixing longitude began in 1707, after four British frigates ran aground in fog near their home port owing to total east-west disorientation, with the loss of nearly 2,000 men. Sobel recounts, in a horrifying passage, how after the vessels became lost in a week-long mist, a sailor approached Admiral Sir Clowdisley Shovell to declare that a private navigational logbook he had been keeping indicated the squadron was about to founder on the dangerous rocks of Sicily Isles southwest of England. British fleet rules then forbade any study of navigation by non-officers, because navigators had a wizard's status no enlisted personnel were allowed to challenge. Sir Clowdisley immediately had the sailor hanged for questioning the judgment of an Officer. Shortly afterward, his flagship ploughed into the Sicily rocks, the three following ships faithfully smashing in as well".

Sadly, Sobel does not know the name of this lost sailor, who seemingly hit upon a significant idea and might himself have become an important figure had he not lived in a society that discriminated ruthlessly against the low-born. At any rate, the English government's response to the event was not to redress the outrage of executing a common man for telling an officer the truth, but to create a new bureaucracy--the Board of Longitude, which was to supervise research and award a prize of £20,000 (equivalent then to $12 million now) for the first reliable means of longitude fixation.

So it was a little more serious than perfecting navigation and astronomy. The thinking of the day was that if an accurate catalogue of the positions of the stars could be made, and if the position of the Moon could then be measured accurately relative to the stars, the Moon's motion could be used as a natural clock. Sailors at sea could measure the Moon's position relative to bright stars and use tables of the Moon's position, compiled at the Royal Observatory, to calculate the time at Greenwich. This means of finding Longitude was known as the "Lunar Distance Method" and like the work of most committees, suffered from its own self-made complexity.

Prime Meridian

The brilliance of John Harrison's work was not just that he engineered an incredible timepiece, but that his solution flew in the face of the wisdom of the establishment (which was betting the farm on its Lunar Distance Method). However, the concept of Greenwich being used as the reference longitude was already established, hence we have Greenwich Mean Time (GMT). Quite an oddity that the basis of our time measurement should have been chosen in this manner.

The Prime Meridian is denoted by the Meridian Line at the Royal Observatory in Greenwich. Tourists straddling this line have a foot in each hemisphere. So what? you ask. Both feet are in the same time zone and only a fraction apart in time. The real trick is to straddle the International Date Line (IDL).

The IDL, according to Wikipedia, is an imaginary line on the surface of the Earth opposite the Prime Meridian which offsets the date as one travels east or west across it. Crossing the IDL traveling East results in a day or 24 hours being subtracted, and crossing West results in a day being added. Straddle this line and your two feet are a whole day apart! That's much more impressive, right?

Now here is the second oddity in our story about the Greenwich Meridian: The International Date Line is the least populated meridian on the planet in terms of land mass. It only has to wiggle around a few islands in the Pacific. How fortunate are we that Greenwich was chosen for the Prime Meridian? Imagine if it had been chosen as what is now 90° E. Then the IDL would run through what is now the Central Time Zone and people across town from each other in say, Memphis , would be a day apart! So I guess we should be glad that Britain was the naval power it was back then, and that King Charles II chose to put his observatory in Greenwich.

Just in case you are feeling a little mind-boggled, take heart in the fact that this conundrum of times and dates has been going on for some time. Wikipedia notes that the first date-line problem occurred in association with the circumnavigation of the globe by Magellan's expedition (1519 to 1522). The surviving crew returned to a Spanish stopover sure of the day of the week, as attested by various carefully maintained sailing logs. Nevertheless, those on land insisted the day was different. Although now readily understandable, this phenomenon caused great excitement at the time, to the extent that a special delegation was sent to the Pope to explain this temporal oddity to him.

I quite like this story. I can imagine the heated arguments going on over an ale or two in the local taverns. As for me, you can take it to the bank that TickTockTony is extremely grateful that antique grandfather clocks don't have to handle longitude or even leap years!