| Old Hampshire Mapped
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| Prime Meridian
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longitude 0
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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.
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on the ground
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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.
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Ptolemy 2nd century
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... 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,
which is Winchester, at longitude:-
18° 40' E
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Laplace 1800
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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.
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Keith 1842
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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.
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Ferguson 1843
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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.
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Lewis Carol
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'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.
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past and present
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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.
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Rhodes
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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.
Rhodes on the Island of Rhodes is about
28°2' E
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Canary Islands
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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.
The Canaries are centred:-
16° 0.0' W
The western edge of Ferro, Ile de Fer or Hierro, is about:-
18° 10' W
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 is about:-
16° 45' W
The 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 to be 20° 02' E and London,
St Paul's 17° 37.5' E, from this meridian.
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Azores
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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 Portugal.
The Azores centred about:-
28° W
their western edge is about:-
31° W
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W of the Azores
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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.
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Cape Verde Islands
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The Cape Verde islands were used by Ortelius, Jannson, Blaeu, et
al, 17th century; Isla del Fuego, ie Fogo, centred at:-
24° 25' W
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The Lizard
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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 is about:-
5° 12.3' W
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London
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John Seller, mapping Hertfordshire, 1676, was first to use
London for a prime meridian on an english county map.
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London, St Paul's
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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; ie St Paul's is about:-
0° 5' 5" 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.
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Greenwich Flamsteed
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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.
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Greenwich Halley
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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.
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Greenwich Bradley
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This Greenwich meridian was established by James Bradley,
born Sherborne, Gloucestershire 1693, died 1762, Astronomer
Royal, 1742-62.
When 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
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Greenwich Airy
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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 Meridian.
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1884 onwards
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Greenwich Meridian
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.
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References
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: 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
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