Ferro-Meridian

The meridian of Ferro, on the Canary island of Ferro ( El Hierro ) located, was from ancient times until 1884 in Europe most widely used prime meridian, most recently linked to the meridian of Paris, and - by the prevalence of the meridian of Greenwich on nautical charts - limited to maps. After the meridian of Ferro, the coordinates of numerous navigation and maps are particularly aligned from the 16th to the 19th century.

For the first time to put 100 Marinos of Tyre, the insulae fortunatae (Canary Islands), the western end of the known world, as a reference point fixed. Their implementation, however, this definition owes its acquisition by the major astronomer Claudius Ptolemy in the year 150. She was, however, of more theoretical importance, as Ptolemy was referring in his work on his native city of Alexandria, whose position he fixed to 60.5 °, which in no way reflect the correct position of the Canary Islands. With the Middle Ages and its cycle maps meridians were meaningless, the Canary Islands disappeared from the known world of Europe. The later portolan gave no geographical coordinates again.

With the rediscovery of the Canaries in the 14th century and the subsequent onset of Renaissance that attacked the Ptolemaic ideas on again and put the westernmost island of Ferro (now El Hierro ) as the zero meridian fixed. In connection with the voyages of discovery and the establishment of observatories in Europe, initially without lens instruments, the number of competing zero meridians rose from the 16th century. However, most acquired only local or national importance. In support for the retention of Ferro also had that the meridian of the magnetic pole, which appeared first as a possible natural definition of the prime meridian, apparently was only slightly west. Another advantage of this length count were positive values ​​for all of Europe. In April 1634 it was confirmed by a scholarly conference of all seafaring nations of the prime meridian of Ferro and at the same time again more closely at the Punta Orchilla, the western tip of the island, set.

The error in the length determination had previously located in several degrees: Ptolemy determined the longitudinal extension of the Mediterranean Sea to 62 °, Mercator she gave in 1554 with 52 ° back, actually it is 42 °. The well-known from about 1610 telescope made ​​possible by observation of astronomical events, especially the Jupitermondumläufe, templates for the 1668 tables, reducing the error to 10 to 12 minutes, with simultaneous observation at the reference to 2 to 3 minutes. Thus, the importance of well-built observatories at reference meridian began to increase further. Precursor Brahe Uraniborg had been gained international importance especially the Paris Observatory from 1666 to the Royal Greenwich Observatory from 1676, all of which have also led to new Meridian systems for practical reasons. In the ferro construction of a large observatory was unthinkable. Since the Ferro meridian nevertheless should not be abandoned, the definition of Ferro was after determination of the difference in length of the Paris Observatory in 1724 by Louis Feuillée to between 19 ° 52 'and 20 ° 06' west at exactly 20 ° from the Paris Observatory consensus. Thus, the zero meridian of Ferro had become an undercover prime meridian of Paris. Until then, was calculated from 22.5 ° with a length difference of Paris - Ferro.

As a result, many national zero meridians were replaced by ferro- or reduced to Ferro, so that from the mid-18th century, only Ferro and Paris as well as on the one hand, first to a lesser extent, Greenwich on the other hand had more important. Shortly thereafter, from the last third of the 18th century, the reliable length determination was possible at sea by taking a sufficiently accurate lunar distances in lunar tables and sufficiently accurate chronometer. This also precise length settings at the charts were required. Since the naval power, Britain was a leader here, sat on nautical charts of the Greenwich meridian through. Only in 1884 it was agreed due to its Greenwich as the reference point internationally as a recommendation and then sat down very quickly, it was binding on the International Card Conference 1913.

Even after the adoption of the maps of the prime meridian of Ferro within the survey administrations was mostly used further. In Germany the prime meridian of Ferro in the preparations was to Gauss Krueger coordinates replaced in 1923 with the transfer of land surveying by the Greenwich. To this end, the rounded value of 17 ° was 40 ' deducted in order to maintain the existing sheet section of the topographic maps can range from the previous length values. The Austrian National Survey was based until 2010 on the zero meridian of Ferro. The conversion to Greenwich lengths was performed using the

• Internationally standardized value of 17 ° 40 '00 "(see also Hermannskogel and Rauenberg )
• While the older value of 17 ° 39 ' 46.02 "from the European Längenausgleichung of Theodor Albrecht ( 1890 ) had shown. Due to an absolute vertical deflection of 13-14 " in Vienna and Berlin but the round value was ultimately determined.