A meridian is a great circle, from pole to pole, along which the longitude is the same. The prime meridian is the meridian of zero longitude, the base line from which longitude is measured; although usually called the first or prime meridian it is the zeroeth. Today it is defined by the position of the Airy transit telescope at the old Royal Observatory, Greenwich; it is shifting a little as the earth's crust moves. The prime meridian has not always been based on Greenwich, at some periods there have been multiple prime meridians. The Greenwich Meridian was chosen for international use at the International Meridian Conference, Washington, United States, 1884.
BEWARE: the notes that follow are based on a variety of secondary sources, which I have had to take on trust.
On the Ground
There is no line on the ground showing the prime meridian! only in our collective minds. There are places where it is marked; most conspicuously at the old observatory at Greenwich, in brass, glass, lights, etc.
At the edge of Epping Forest there is the Chingford Pillar, an obelisk marking the Bradley meridian, set up 1824. It has a plaque indicating another obelisk 5.79 metres east, for the Airy meridian. There are other Bradley markers in England; and more recent Airy markers.
|Ptolemy 2nd century||
... Hence the length of the known
earth, that is, from the meridian drawn through or
terminated by the Fortunate Islands in the extreme west, to
Sera in the extreme east is 177 degrees 15 minutes.
The Fortunate Islands are the Canaries. Ptolemy lists Venta:-
It is desirable that all the nations
of Europe, in place of arranging geographical longitude from
their own observatories, should agree to compute it from the
same meridian, one indicated by nature herself, in order to
determine it for all time to come.
Good idea, but there is no natural prime meridian. Piazzi Smyth 1870s: in an ironic response to some argument about the prime meridian the Astronomer Royal for Scotland suggested the Great Pyramid in Egypt.
The FIRST MERIDIAN is that from
which geographers begin to count the longitude of places. On
English maps and globes the first meridian is a semicircle
supposed to pass through London, or the royal observatory at
Greenwich. Foreigners count the longitude from the
observatories of their respective nations.
The longitude of a place is the
number of degrees (reckoned upon the equator) that the
meridian of the said place is distant from the meridian of
any other place from which we reckon, either eastward or
westward, for 180 degrees, or half round the globe. The
English reckon the longitude from the meridian of London,
and the French now reckon it from the meridian of Paris. The
meridian of that place, from which the longitude is
reckoned, is called the first meridian.
'What's the good of Mercator's North
Poles and Equators; Tropics, Zones and Meridian Lines?' So
the Bellman would cry: and the crew would reply; 'They are
only conventional signs!'
from the Hunting of the Snark.
Past and Present
The list below does not bother much with foreign national meridians. I am concerned to understand maps of Hampshire, and perhaps of England and Wales.
The first person to give the position of places by
latitude and longitude was Hipparchos, astronomer of Rhodes,
which place he used as his prime meridian. Hipparchos was
born about 160BC, died 125BC. On the Island of Rhodes:-
Rhodes = 28d 2m E approximately
The Canary Islands were used by Ptolemy, Claudius
Ptolomaeus, born about AD90 died 168, who called them the
Fortunate Islands, in the 2nd century. He was following the
usage of Marinus of Tyre. Ptolemy regarded these islands as
the western edge of the inhabitable world; he listed the
latitude and longitude of places in The Geography.
Canaries, centred = 16d 0.0m W approximately The western edge of Ferro, Ile de Fer or Hierro:-
Ile de Fer = 18d 10m W approximately
In a cartouche on the globe he published in 1622, John Blaeu said:-
... But in our days a good many think this starting point ought to be based on nature itself, and have taken the direction of the magentic needle as their guide and placed the prime meridian where that points true north. But that these are under a delusion is proved ... for itself [the needle] varies along the same meridian according as it is near one land mass or another ... ... following in the steps of Ptolemy, have chosen the same islands and in them Juno, commonly called Tenerife, whose lofty and steep summit covered with perpetual cloud, called by the natives El Pico, shall mark the prime meridian. In that way we have differed barely a quarter of a degree from the longitude of the Arabs who chose the extreme western shore of Africa [Cape Verde], and I thought it well to point this out.
El pico = 16d 45m W approximately
The Canary Islands were used by various early cartographers. Cardinal Richelieu convened a meeting of mathematicians and astronomers in Europe, 1634, to consider a prime meridian to be recognised by all nations. They chose the westernmost coast of the westernmost island, Ferro, in the Canaries. Louis XIII decreed, 1634:-
... In order that this first meridian may be more clearly known than it has for some time been, the Admiral of France has consulted persons of knowledge and experience in navigation. The King in consequence forbids all pilots, hydrographers, designers or engravers of maps or terrestrial globes to innovate or vary from the ancient meridian passing through the most westerly of the Canary Islands, without regard to the novel ideas of those who recently fixed it in the Azores on the supposition that there the compass does not vary, for it is certain that this happens also in other places that have never been taken for the meridian.
Determinations by Louis Feuillee from the Paris Academy, 1724, published 1742, gave:-
Paris = 20d 02m E
London, St Paul's = 17d 37.5m E
The Island of St Mary in the Azores was used to define
the prime meridian by Christopher Saxton, 1584. St Michael's
Island was used by John Davis, 1594; because he [mistakenly]
thought there was no compass deviation at that place. The
islands are the western edge of Europe; they are part of
Azores, centre = 28d W approximately
Azores, western edge = 31d W approximately
|W of the Azores||
Pope Alexander VI, 1493, declared a line of demarcation
between the areas of influence of Spain and Portugal, 100
leagues W of the Azores and Cape Verde Islands. After some
portuguese protest this was shifted, in 1494, to 370 leagues
W of the Cape Verde Islands. The line was used as the prime
meridian of some maps of the period.
|Cape Verde Islands||
The Cape Verde islands were used by Ortelius, Jannson,
Blaeu, et al, 17th century:-
Isla del Fuego, ie Fogo, centre = 24d 25m W approximately
While nations chose what they would for their prime
meridian, navigators on ships tended to write their logs
giving longitude from their last landfall, for example W of
The Lizard , which had a currency as a prime meridian about
the 17th-18th century.
The Lizard = 5d 12.3m W
John Seller, mapping Hertfordshire, 1676, was first to
use London for a prime meridian on an english county
|London, St Paul's The one inch map of Surrey by Andrews and Dury, 1777, uses a prime meridian through St Paul's Cathedral, London. The bottom longitude scale states:-||
... Long. West from St. Pauls.
The map shows the greenwich observatory.
Measuring off the longitude scales - notice that the meridians slant across the map, and are at 2 minute intervals - Greenwich is 0d 5m 53s E; thus:-
St Paul's Cathedral = 0d 5m 5s W of Greenwich
St Paul's was used to define the prime meridian by General Roy, 1790, just before the Ordnance Survey was founded, 1791.
The Greenwich Meridian was established by John Flamsteed,
born Denby, Derbyshire 1646, died 1719, 1st Astronomer
Royal, 1675; he held the position to 1719. The Royal
Observatory, Greenwich was founded by Charles II, 1675.
A slightly different meridian was established at
Greenwich by Edmund Halley, born London 1656, died 1742, 2nd
Astronomer Royal, 1725; holding the position 1720-42.
The precise position of the meridian is determined by the exact position of a transit telescope. New instrument; new meridian.
The Bradley Greenwich meridian was established by James
Bradley, born Sherborne, Gloucestershire 1693, died 1762,
Astronomer Royal, 1742-62.
When publication of the Nautical Almamac began, 1767, it made the use of this meridian international. The first series of charts to use the Greenwich meridian systematically was the Atlantic Neptune, of the east coast of America, by J F W DesBarres, published 1784. This meridian is still used for OS map making in Britain. It is 5.79 metres W of the international Prime Meridian. On everyday maps this is less than a pencil line difference
Sir George Airy, born Alnwick, Northumberland 1801, died
1892, Astronomer Royal 1851-811, set up a new transit
telescope 5.79 metres East of Bradley's. The position of
this instrument, since 1884, defines the international Prime
The Greenwich Meridian (Airy) was chosen for international use at the International Meridian Conference, Washington, United States, 22 October 1884. 25 nations were present; Greenwich won the honour by 22 votes to 1; San Domingo against; 2 abstaining, France and Brazil. Resolved:
I. That it is the opinion of this Congress that it is desirable to adopt a single prime meridian for all nations, in place of the multiplicity of initial meridians which now exist.
II. That the Conference proposes to the Governments here represented the adoption of the meridian passing through the center of the transit instrument at the Observatory of Greenwich as the initial meridian for longitude.
III. That from this meridian longitude shall be counted in two directions up to 180 degrees, east longitude being plus and west longitude being minus.
By this date the USA had already decided on Greenwich for its time zone system; and british Admiralty Charts, using Greenwich, were in predominant use in the world's commerce. Countries not at the conference have since adopted the convention.
The prime meridian also sets midnight/zero for international time, GMT. Algeria, a french possession, did not like the phrase Greenwich Mean Time , and wanted it called Paris Mean Time diminished by 9 minutes 21 seconds .
: 1880 (11 December):: Illustrated London News:: about Airy's transit instrument.
Andrewes, William J H: 1996: Quest for Longitude: Collection of Historical Scientific Instruments:: ISBN 0 9644329 0 0
Brown, Lloyd A: 1977 (new edn) & 1949: Story of Maps: Dover Publications (New York, New York, United States):: ISBN 0 486 23873 3 (pbk)
Ferguson, James: 1843: Lectures on Select Subjects in Mechanics, Hydrostatics, Pneumatics, Optics, and Astronomy: Tegg, Thomas (London)
Howse, Derek: 1980: Greenwich Time, and the discovery of longitude: Oxford University Press
Keith, Thomas: 1842: On the Use of the Globes, A New Treatise: Tegg, Thomas (London)
Ptolemy, Claudius (90-160AD) & Stevenson, Edward L (trans and ed): 1991: Claudius Ptolemy, the Geography: Dover Publications (New York, New York, United States):: ISBN 0 486 26896 9 (pbk)
Sobel, Dava & Andrewes, William J H: 1998: Illustrated Longitude, The: Fourth Estate:: ISBN 1 85702 714 0