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Tom,
interesting and intriguing questions. As far as it is in my knowledge,
astronomers at the Aragonese court in Naples determined the latitude of the
south tip of Calabria with an error of around 4 minutes, in the last quarter of
the XV century, and was the best value up to the end of XVIII century.
I remember that Ptolemy describes a sort of astronomical circle for determining
the inclination of the Ecliptic and I was sure he detected part of minutes. You
may see, for more details about Ptolemy's accuracy in that measure of latitude,
G. J. Toomer, Ptolemy's Almagest, Duckworth, London 1984, pp. 61-64 and foll.
Anyway, take into account that if Ptolemy and some more Arabian astronomer,
were able to take measure up to a minute, half a minute or more, it doesn't
mean the everybody was was able to do so, with any instrument, in any condition
and in any place.
The only other references I know on the matter (but are not updated) are
- J. Kirtland Wright, Notes on the knowledge of latitudes and longitudes in the
Middle Ages, ISIS, V (1923),pp. 75-98;
and
- a long essay by Luis de Albuquerque in History of Portuguese Cartography, by
Armando Cortesao, 1969-1971, vol. II, pp. 221-442 on practical astronomy,
navigations and determination of latitude in the Era of discoveries.
John Hessler might be of some help in more recent studies (if any).
The recurrence of the error, or better the difference, between old and
modern observations might depend on an inaccurate division either of the
circle, or of any other instruments used for taking rough measures of latitude.
. . . or from to tools you used for deriving the coordinates from the Ravenna
Cosmography!
vladimiro
Il giorno 05/ago/2011, alle ore 00.02, Tom Ikins ha scritto:
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Since the rotational axis of the earth has changed with time, but I have no
idea if there is any history of it in terms of the not-so-distant past, I ask:
Were I measuring latitude with a gnomon in Roman Britain along a line of 0
deg longitude during the period 50-150 AD, what would be the difference in my
calculated latitude versus current standard latitude?
A formula for conversion accounting for longitude?
The lines of latitude derived from the British section of the Ravenna
Cosmography are south of expected by about 1/6 degree.
How accurately can we expect a gnomon to measure latitude?
--
Tom Ikins
The Roman Map of Britain
http://www.romanmap.com
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