Though I am quite acquainted with the perils of transatlantic jet lag and have by now conquered it, it is always a bit disconcerting to be in the air westbound for ten hours and arrive just an hour past departure Paris time, never mind the old Concorde trick of arriving before you left which I sadly never had a chance to experience. I can't even begin to imagine the dislocation produced by the effect of crossing the International Date Line.

The International Date Line is an arbitrary line that has been drawn over the centuries 180° opposite the meridian of Greenwich, which is conveniently, mostly Pacific Ocean or you would have the odd paradox of being able to have one side of you on Sunday and the other on Monday as you straddled the line. However, without the line, a circuit of the earth would produce the circumnavigator's paradox: an eastbound circumnavigator would cross twenty four time zones, dutifully move his watch an hour ahead even though the time he spent on his journey did not increase and accrue an entire day's ahead worth of error by the time he returned to his point of origin, reverse the concept for the westbound traveller.

Before anybody had actually gone around the globe, as a matter of fact, even before europeans believed the earth was anything but flat, several scholars had postulated the circumnavigator’s paradox that would cause a westbound traveller to lose a day after a full circuit of the globe and an eastbound traveller to gain one. An early mention of the paradox is found in the works of the Syrian prince and geographer-historian Isma‘il ibn ‘Ali ibn Mahmud ibn Muhammad ibn Taqi ad-Din ‘Umar ibn Shahanshah ibn Ayyub al Malik al Mu’ayyad ‘Imad ad-Din Abu ’l-Fida (1273 - 1331) in his Taqwin al-Buldan (‘The ??? of the Lands’).

The first real life example of the paradox was documented by one of the few survivors of Ferdinand Magellan's first circumnavigation, Antonio Pigafetta:

“On Wednesday, the ninth of July 1522, we arrived at one these islands named Santiago, where we immediately sent the boat ashore to obtain provisions. ... And we charged our men in the boat that, when they were ashore, they should ask what day it was. They were answered that to the Portuguese it was Thursday, at which they were much amazed, for to us it was Wednesday, and we knew not how we had fallen into error. For every day I, being always in health, had written down each day without any intermission. But, as we were told since, there had been no mistake, for we had always made our voyage westward and had returned to the same place of departure as the sun, wherefore the long voyage had brought the gain of twenty-four hours, as is clearly seen.”
Almost a hundred years later, in 1612 the French historian Nicholas Bergier (1567 - 1623) published a work entitled Archemeron (Le point du jour), ou traicté du commencement des jours et de l’endroit où il est étably sur la terre in which he proposed to adopt the meridian opposite to the prime meridian of the cartographer Gerardus Mercator namesake of the Mercator Projection, as a suitable date line. It took another two hundred years though for the line to be established and to come into general use. Even late into the 19th century, the line is still being debated and does not appear in any maps or globes of the period, which is not unusual considering that folks at the time could not even agree on the prime meridian.

At the end of the day, the International Date Line continues to be a misnomer as it has never been ratified by any international treaty or convention. The closest it came was when the attendees to the International Meridian Conference held in Washington, DC in 1884 that ratified Greenwich as the official prime meridian remarked how convenient it was that the opposite 180° merididian passed mostly over water. Hence the IDL has been set by merchant marines, the hydography departments of the US and British Navies, and the vagaries of national whims and it snakes across the Pacific like a drunken sailor. The last big movement was the Kiribati adjustment of 1995 which created a 1,500 mile jog in the line to place all its islands in the same day. The IDL has also featured prominently in a number of works of literature by Edgar Allan Poe, Arthur C. Clarke and most famously by Jules Verne in Around the World in 80 Days

Rudolf Wolf, Handbuch der Astronomie, Ihrer Geschichte und Literatur (Zurich, 1890), vol 1, pp. 465-466
Robert H. van Gent, A History of the International Date Line,, 6/1/2004
Prime Meridian,, 6/2/2004
Millenium Firsts,, 6/2/2004

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