## 2.2 Prime meridians

There is a natural origin in the latitudes: the position of the rotation axis of the Earth provides the natural zero to start counting the latitudes from: the equator. However, in case of the longitudes, the cylindrical symmetry of the system does not offer a similar natural starting meridian therefore we have to define one.

The meridian of the fundamental point of a triangulation network (see Point 3.3) is usually selected as zero or prime meridian. Ellipsoidal longitudes of all points in the network are given according to this value. If we’d have just one system, it could work well. As we have several different networks and different prime meridians, we need to know the angular differences between them. Instead of handling the differences between all prime meridian pairs, it is worth to choose just one, and all of the others can be described by the longitude difference between it and the chosen meridian.

Fig. 3. The cover page of the protocol of the 1884 Washington conference that decided to use Greenwich as the international prime meridian.

The use the Greenwich prime meridian was proposed by the 1884 Washington Conference on the Prime Meridian and the Universal Day (Fig. 3). It was accepted by 22 votes, while Haiti (that time: Santo Domingo) voted against, France and Brazil abstained. France adapted officially the Greenwich prime meridian only in 1911, and even nowadays, in many French maps, we can find longitude references from Paris and in new degrees. It is interesting that the question of the international prime meridian was discussed in that time: the newly invented telegraph enabled to accomplish the really simultaneous astronomical observations at distant observatories. Table 2 shows the longitude difference between Greenwich and some other important meridians that were used as local or regional zero meridians.

 Prime meridian Longitude from Greenwich Paris 2° 20’ 14,025” Rome 12° 27’ 8,04” Madrid –3° 41’ 16,48” Oslo 10° 43’ 22,5” Pulkovo 30° 19’ 42,09” Ferro1 –17° 40’ Ferro2 –17° 39’ 46,02” Ferro3 –17° 39’ 45,975” Vienna, Stephansdom4 34° 02’ 15” (from Ferro) Vienna, Stephansdom5 16° 22’ 29” Budapest, Gellérthegy6 36° 42’ 51,57” (from Ferro) Budapest, Gellérthegy7 36° 42’ 53,5733” (from Ferro) Budapest, Gellérthegy8 19° 03’ 07,5533”

Table 2. Longitude values of some prime meridians. 1Used in Germany, Austria and Czechoslovakia. 2The’Albrecht difference’, used in Hungary, Yugoslavia and in the Habsburg Empire. 3According to the Bureau International de l’Heure. 4From Ferro, in the system 1806. 5Applying the Albrecht difference. 6From Ferro, according to the 1821 triangulation. 7From Ferró, according to the system1909. 8The 1909 value, applying the Albrecht difference.

Fig. 4. „Östlich von Ferro” = East of Ferro: indication to the old Ferro prime meridian in a sheet of a Habsburg military survey.

As we see in the Table 2, some prime meridians are described by more longitude differences from Greenwich. For example, this is the situation of the Ferro meridian, which was widely, almost exclusively used in Central Europe prior to the first part of the 20th century. Ferro (Fig. 4; nowadays it is called El Hierro) is the westernmost point of the Canary Islands. The meridian ’fits to the margin of the ancient Old World’ (the one without the Americas; Fig. 5). In fact, the longitude of Ferro refers to the Paris prime meridian. The longitude difference between Ferro and Paris is, according to the French Bureau International de l’Heure (BIH), 20 degrees, in round numbers (Fig. 6). The Ferro prime meridian itself was proposed as a commonly used one also by a – mostly forgotten – ’international conference’, brokered by the French Cardinal Richelieu in the 17th century.

Fig. 5. Ferro, now El Hierro, Canary Islands, in the Google Earth. As the Ferro prime meridian is cca. 17º 40’ west of Greenwich, it is quite surprising that Ferro is ’west of Ferro’ indeed. This prime meridian was artificially selected and not connected to the island at all.

About the given three different values of Ferro in Table 2: the value of the BIH refers to the exact 20 degree west from Paris. The ’Albrech-difference’ between Ferro and Greenwich differs from that by about one meter. Later, this difference was modified by Germany, and later by two successor states of the Monarchy. The cause was an error in the longitude observation at the old observatory tower of Berlin; this error was 13,39 arc seconds. Adding this value to the Albrecht-difference, it is 17° 39’ 59.41”, which can be substituted by the round number of 17° 40’ with an error around 1.5 meters. So, this figure was used in Germany, Austria and Czechoslovakia, which enabled to further use the sheet system of the topographic maps.

Fig. 6. The ’Cassini meridian’ of the old Paris observatory. The Ferro prime meridian was indeed defined as a meridian that is west of this line by 20 degrees in round numbers (Wikipedia).

At the Gellérthegy, the fundamental point of the old Hungarian networks, there are also several figures indicated: similarly to the latitude, the coordinates of the point are the functions of the (different) geodetic datum(s).

We can find maps, e.g. in Spain and Norway, at which the Greenwich prime meridian used, but their sheet system, remained to connected to the old, in this examples to the Madrid or Oslo meridians (Fig 7).

Fig. 7. The sheet frames of the modern 1:50,000 map of Norway follows the old Oslo meridian, however the longitudes are give from Greenwich.

Prime meridians are also applied at mapping of celestial bodies. In case of the Mars, the prime meridian is defined at the crater ’Airy-0’ (named after the former director and Royal Astronomer of Greenwich). At the moon, this longitude is fixed at the Bruce Crater, in the middle of the visible part. Differently from the terrestrial coordinate system, in the sky there is a unique prime meridian, which is a good one for the celestial system. The longitude of the vernal equinox, the ascending node of the Sun’s apparent orbit, is a natural possibility. The only problem is that the vernal equinox is slowly moves because of the luni-solar precession of the earth, so the celestial prime meridian should be connected to an epoch of that.

Nowadays, our terrestrial coordinate systems are not connected to the physical location of the Greenwich Observatory anymore: they are derived from the celestial system (the ICRF, the International Celestial Coordinate Frame) via the epoch of the vernal equinox and the Earth’s rotation parameters. That’s why in the WGS84 (see point 3.3) used by the GPS units and also by the Google Earth, the longitude of the historical Airy meridian in Greenwich is 5.31 seconds west ’from itself’, indeed from the new prime meridian (Fig. 8).

Fig. 8. Surprisingly enough, the Airy meridian of the Greenwich observatory is ’west of Greenwich’ by cca. 150 meters in the WGS84 datum of the Google Earth. The WGS84 is connected to the celestial reference system, not to the traditional Greenwich meridian.