French cartography and the figure of the Earth in 1682 In the year 1634, France, then ruled by
Louis XIII and
Cardinal Richelieu, decided that the
Ferro Meridian through the westernmost of the
Canary Islands should be used as the reference on maps, since
El Hierro (Ferro) was the most western position of the
Ptolemy's world map. French cartographers would use it as their prime meridian for more than 200 years. Old maps from continental Europe often have a common grid with Paris degrees at the top and Ferro degrees offset by 20 at the bottom. The application of the telescope to angular instruments was an important step. He was the first who in 1669, with the telescope, using such precautions as the nature of the operation requires, measured a precise arc of meridian (
Picard's arc measurement). He measured with wooden rods a
baseline of 5,663
toises, and a second or base of verification of 3,902 toises; his
triangulation network extended from Malvoisine, near Paris, to
Sourdon, near
Amiens. The angles of the triangles were measured with a quadrant furnished with a telescope having cross-wires. The difference of latitude of the terminal stations was determined by observations made with a sector on a star in
Cassiopeia, giving 1° 22′ 55″ for the amplitude. The terrestrial degree measurement gave the length of 57,060 toises, whence he inferred 6,538,594 toises for the Earth's diameter. Four generations of the Cassini family headed the
Paris Observatory. In 1783 the
French Academy of Science presented his proposal to
King George III. Between 1792 and 1798
Pierre Méchain and
Jean-Baptiste Delambre surveyed the Paris meridian arc between
Dunkirk and
Barcelona (see
meridian arc of Delambre and Méchain). They extrapolated from this measurement the distance from the North Pole to the
Equator which was 5,130,740
toises. As the metre had to be equal to one ten-millionth of this distance, it was defined as 0,513074 toises or 443,296
lignes of the Toise of Peru (see below) and of the double-toise N° 1 of the apparatus which had been devised by
Lavoisier and
Borda for this survey at specified temperatures. In the early 19th century, the Paris meridian's arc was recalculated with greater precision between
Shetland and the
Balearic Islands by the astronomer
François Arago, whose name now appears on the plaques or medallions tracing the route of the meridian through Paris (see below).
Biot and
Arago published their work as a fourth volume following the three volumes of by
Delambre and
Méchain's "''Bases du système métrique décimal ou mesure de l'arc méridien compris entre les parallèles de Dunkerque et Barcelone''" (Basis for the decimal
metric system or measurement of the meridian arc comprised between
Dunkirk and
Barcelona). , through Great Britain, France and Spain to
El Aghuat in Algeria, whose parameters were calculated from surveys carried out in the mid to late 19th century. It yielded a value for the equatorial radius of the Earth
a = 6 377 935 metres, the ellipticity being assumed as 1/299.15. The radius of curvature of this arc is not uniform, being, in the mean, about 600 metres greater in the northern than in the southern part. The
Greenwich meridian is depicted rather than the Paris meridian. In the second half of the 19th century,
Carlos Ibáñez e Ibáñez de Ibero directed the survey of Spain. From 1870 to 1894 the Paris meridan's arc was remeasured by
Perrier and Bassot in France and Algeria. The triangulation of France was then connected to those of Great Britain, Spain and Algeria and thus the Paris meridian's arc measurement extended from
Shetland to the
Sahara. The fundamental co-ordinates of the
Panthéon were also obtained anew, by connecting the
Panthéon and the
Paris Observatory with the five stations of Bry-sur-Marne, Morlu, Mont Valérien, Chatillon and Montsouris, where the observations of latitude and azimuth were effected. To combine the measurements it was necessary to compare the geodetic standards of length used in the different countries.
Alexander Ross Clarke and Henry James published the first results of the standards' comparisons in 1867. The first president of the
International Committee for Weights and Measures was the Spanish geodesist
Carlos Ibáñez e Ibáñez de Ibero. He also was the president of the Permanent Commission of the
Europäische Gradmessung from 1874 to 1886. In 1883 the General Conference of the
Europäische Gradmessung proposed to select the
Greenwich meridian as the prime meridian in the hope that
Great Britain would accede to the
Metre Convention.
From the Paris meridian to the Greenwich meridian The United States passed an Act of Congress on 3 August 1882, authorizing President
Chester A. Arthur to call an international conference to fix on a common prime meridian for time and longitude throughout the world. Before the invitations were sent out on 1 December, the joint efforts of Abbe, Fleming and William Frederick Allen, Secretary of the US railways'
General Time Convention and Managing Editor of the ''Travellers' Official Guide to the Railways
, had brought the US railway companies to an agreement which led to standard railway time being introduced at noon on 18 November 1883 across the nation. Although this was not legally established until 1918, there was thus a strong sense of fait accompli'' that preceded the
International Meridian Conference, although setting local times was not part of the remit of the conference. In 1884, at the
International Meridian Conference in Washington DC, the
Greenwich meridian was adopted as the
prime meridian of the world. San Domingo, now the
Dominican Republic, voted against. France and Brazil abstained. The United Kingdom acceded to the
Metre Convention in 1884 and to the
International Geodetic Association in 1898. In 1911,
Alexander Ross Clarke and
Friedrich Robert Helmert stated in the
Encyclopædia Britannica : In 1891, timetabling for
its growing railways led to standardised
time in France changing from
mean solar time of the local centre to that of the Paris meridian: 9 minutes 20.921 seconds ahead of
Greenwich Mean Time (GMT). In 1911 the country switched to GMT for timekeeping; in 1914 it switched to the Greenwich meridian for navigation. Despite this, French cartographers continue to indicate the Paris meridian on some maps.
From wireless telegraphy to Coordinated Universal Time With the arrival of wireless telegraphy, France established a transmitter on the
Eiffel Tower to broadcast a time signal. The creation of the
International Time Bureau, seated at the
Paris Observatory, was decided upon during the 1912 ''Conférence internationale de l'heure radiotélégraphique''. The following year an attempt was made to regulate the international status of the bureau through the creation of an
international convention. However, the convention wasn't ratified by its member countries due to the outbreak of
World War I. In 1919, after the war, it was decided upon to make the bureau the executive body of the
International Commission of Time, one of the commissions of the then newly founded
International Astronomical Union (IAU). From 1956 until 1987 the
International Time Bureau was part of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS). In 1987 the bureau's tasks of combining different measurements of
Atomic Time were taken over by the
International Bureau of Weights and Measures (BIPM). Its tasks related to the correction of time with respect to the
celestial reference frame and the
Earth's rotation to realize the
Coordinated Universal Time (UTC) were taken over by the
International Earth Rotation and Reference Systems Service (IERS) which was established in its present form in 1987 by the
International Astronomical Union and the
International Union of Geodesy and Geophysics (IUGG). == The Arago medallions == . In 1994 the Arago Association and the city of Paris commissioned a Dutch conceptual artist,
Jan Dibbets, to create a memorial to
Arago. Dibbets came up with the idea of setting 135 bronze medallions (although only 121 are documented in the official guide to the medallions) into the ground along the Paris meridian between the northern and southern limits of Paris: a total distance of 9.2 kilometres/5.7 miles. Each medallion is 12 cm in diameter and marked with the name ARAGO plus N and S pointers. Another project, the
Green Meridian (
An 2000 – La Méridienne Verte), aimed to establish a plantation of trees along the entire length of the
meridian arc in France. Several missing Arago medallions appear to have been replaced with the newer 'An 2000 – La Méridienne Verte' markers. == Unfounded speculation ==