diamond geezer

 Saturday, July 14, 2018

À PARIS: la Méridienne

The equator may be well-defined, but where to place the line of zero longitude is a subjective, contentious matter. Today the world draws its line through the Royal Observatory at Greenwich, but the (slightly older) Paris Observatory was nearly chosen instead, and the French would have been far happier if it had. The Paris meridian's story began in 1667, on the day of the summer solstice, at noon, when members of the Academy of Sciences assembled to mark a north/south line on a patch of land gifted to them by King Louis XIV. L'Observatoire de Paris was built across this line, which duly became France's prime meridian. [map]

The most formal representation of the Paris meridian can be found on the floor of the observatory's central Cassini Room, but that's private, so staff and guests only. More publicly, the line is marked by a brass strip across the lawn of a small public park immediately to the south of the observatory, which descends briefly from a flowerbed towards the park's iron gates. There's not a lot around to explain precisely what the line is, and on my visit a small shrieking girl was using the strip to roll a ball downhill towards her less enthusiastic brother.

A more prominent monument can be found by continuing the imaginary line a few metres further south, to the other side of the main road. The plinth situated here is a memorial to the scientist, metrologist and astronomer François Arago, one of whose (many) roles was as chair of the Bureau des Longitudes, the French equivalent of the Ordnance Survey. The plinth is empty because Arago's statue was removed by the Germans in 1941 to melt down to make weapons. But at its foot is a more recent commemorative plaque surrounding a small bronze medallion, twelve centimetres in diameter, bearing his name and a pair of compass markings. 135 of these medallions were placed across Paris in 1994 to mark the line of the meridian, a marvellously fitting tribute, although a large number have alas since disappeared.

This blog loves nothing more than a walk along an imaginary line, so I thought I'd try tracking at least one of the remaining medallions down. The meridian's initially easy to follow because it follows the main axis of the Jardin du Luxembourg for over a kilometre to the north of the Observatory, but slices less obviously across the Rive Droite, including a direct hit on the Louvre. I resigned myself to a frustrating walk looking fruitlessly for tiny black circles... then walked round the back of Arago's plinth and saw one stuck to the rear, which saved me from what could have been a very long hike.

Paris lost out to Greenwich at the International Meridian Conference in 1884, following a vote in which only France and Brazil abstained. The French continued to use their own meridian until 1911, after which they reluctantly switched to the international standard, which they described as "Paris mean time, retarded by 9 minutes and 21 seconds". But the British proved equally stubborn, refusing to implement Resolution 6 urging adoption of a metric system of units. And of course the metre was originally defined as one ten-millionth of the distance from the equator to the north pole along the Earth's meridian through Paris, and so the line lives on.

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