French cartography and the figure of the Earth

Paris Observatory

Line of the Paris meridian

In the year 1634, France 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.^[3] It was also thought to be exactly 20 degrees west of Paris.^[3] The astronomers of the French Academy of Sciences, founded in 1666, managed to clarify the position of El Hierro relative to the meridian of Paris, which gradually supplanted the Ferro meridian.^[3] In 1666, Louis XIV of France had authorized the building of the Paris Observatory. On Midsummer's Day 1667, members of the Academy of Sciences traced the future building's outline on a plot outside town near the Port Royal abbey, with Paris meridian exactly bisecting the site north–south.^[4] French cartographers would use it as their prime meridian for more than 200 years.^[3] 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.^[3]

A French astronomer, Abbé Jean Picard, measured the length of a degree of latitude along the Paris meridian (arc measurement) and computed from it the size of the Earth during 1668–1670.^[1] 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.^[2][5]

Four generations of the Cassini family headed the Paris Observatory.^[6] They directed the surveys of France for over 100 years.^[6] Hitherto geodetic observations had been confined to the determination of the magnitude of the Earth considered as a sphere, but a discovery made by Jean Richer turned the attention of mathematicians to its deviation from a spherical form. This astronomer, having been sent by the Academy of Sciences of Paris to the island of Cayenne (now in French Guiana) in South America, for the purpose of investigating the amount of astronomical refraction and other astronomical objects, observed that his clock, which had been regulated at Paris to beat seconds, lost about two minutes and a half daily at Cayenne, and that to bring it to measure mean solar time it was necessary to shorten the pendulum by more than a line (about ¹⁄₁₂th of an in.). This fact, which was scarcely credited till it had been confirmed by the subsequent observations of Varin and Deshayes on the coasts of Africa and America, was first explained in the third book of Newton’s Principia, who showed that it could only be referred to a diminution of gravity arising either from a protuberance of the equatorial parts of the Earth and consequent increase of the distance from the centre, or from the counteracting effect of the centrifugal force. About the same time (1673) appeared Christiaan Huygens’ De Horologio Oscillatorio, in which for the first time were found correct notions on the subject of centrifugal force. It does not, however, appear that they were applied to the theoretical investigation of the figure of the Earth before the publication of Newton's Principia. In 1690 Huygens published his De Causa Gravitatis, which contains an investigation of the figure of the Earth on the supposition that the attraction of every particle is towards the centre.

Between 1684 and 1718 Giovanni Domenico Cassini and Jacques Cassini, along with Philippe de La Hire, carried a triangulation, starting from Picard's base in Paris and extending it northwards to Dunkirk and southwards to Collioure. They measured a base of 7,246 toises near Perpignan, and a somewhat shorter base near Dunkirk; and from the northern portion of the arc, which had an amplitude of 2° 12′ 9″, obtained 56,960 toises for the length of a degree; while from the southern portion, of which the amplitude was 6° 18′ 57″, they obtained 57,097 toises. The immediate inference from this was that, with the degree diminishing with increasing latitude, the Earth must be a prolate spheroid. This conclusion was totally opposed to the theoretical investigations of Newton and Huygens, and accordingly the Academy of Sciences of Paris determined to apply a decisive test by the measurement of arcs at a great distance from each other – one in the neighbourhood of the equator, the other in a high latitude. Thus arose the celebrated French Geodesic Missions [fr], to the Equator and to Lapland, the latter directed by Pierre Louis Maupertuis.^[2]

In 1740 an account was published in the Paris Mémoires, by Cassini de Thury, of a remeasurement by himself and Nicolas Louis de Lacaille of the meridian of Paris. With a view to determine more accurately the variation of the degree along the meridian, they divided the distance from Dunkirk to Collioure into four partial arcs of about two degrees each, by observing the latitude at five stations. The results previously obtained by Giovanni Domenico and Jacques Cassini were not confirmed, but, on the contrary, the length of the degree derived from these partial arcs showed on the whole an increase with increasing latitude.^[2]

The West Europe-Africa Meridian-arc

Cesar-François Cassini de Thury completed the Cassini map, which was published by his son Cassini IV in 1790.^[1] Moreover, the Paris meridian was linked with international collaboration in geodesy and metrology. Cesar-François Cassini de Thury (1714–1784) expressed the project to extend the French geodetic network all around the world and to connect the Paris and Greenwich observatories.^[7] In 1783 the French Academy of Science presented his proposal to King George III.^[7] This connection and a proposal from General William Roy led to the first triangulation of Great Britain.^[7] France and Great Britain surveys' connection was repeated by French astronomers and geodesists in 1787 by Cassini IV, in 1823–1825 by François Arago and in 1861–1862 by François Perrier.^[7][8]

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.^[9][10]

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 "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) by Delambre and Méchain.^[11]

In the second half of the 19th century, Carlos Ibáñez e Ibáñez de Ibero directed the survey of Spain.^[12] From 1870 to 1894 the Paris meridan's arc was remeasured by Perrier and Bassot in France and Algeria.^[6][8] In 1879, Ibáñez de Ibero for Spain and François Perrier for France directed the junction of the Spanish geodetic network with Algeria.^[13] This connection was a remarkable enterprise where triangles with a maximum length of 270 km were observed from mountain stations over the Mediterranean Sea.^[6][14] 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.^[13]

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.^[2]

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.^[21] 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).