Claudius Ptolemy (ca. 90- ca. 168)
Claudius Ptolemy, was a mathematician, astronomer, geographer, astrologer, and poet.
He lived in Egypt under Roman rule.
It is unknown where he was born, but he probably spent his complete life in Alexandria, where he died around 168.
Ptolemy was the author of several scientific treatises, at least three of which were of continuing
importance to later Islamic and European science.
The first is the astronomical treatise now known as the Almagest ("Mathematical Treatise").
The second is the Geographia, which is a thorough discussion of the geographic knowledge of the Greco-Roman world.
The third is the astrological treatise known sometimes in Greek as the Apotelesmatika or as
the Tetrabiblos consisting of four books, in which he attempted to adapt horoscopic astrology to the
Aristotelian natural philosophy of his day.
Astronomy
The Almagest is the only surviving comprehensive ancient treatise on astronomy.
Babylonian astronomers had developed arithmetical techniques for calculating astronomical phenomena.
Later Greek astronomers such as Hipparchus had produced geometric models for calculating celestial motions.
Ptolemy, however, claimed to have derived his geometrical models from selected astronomical observations
by his predecessors spanning more than 800 years, though astronomers have for centuries suspected that his
models' parameters were adopted independently of observations.
Ptolemy presented his astronomical models in convenient tables, which could be used to compute the future
or past position of the planets.
The Almagest also contains a star catalogue, which is an appropriated version of a catalogue created by Hipparchus.
Its list of forty-eight constellations is ancestral to the modern system of constellations, but unlike the
modern system they did not cover the whole sky but only the sky Hipparchus could see.
Through the Middle Ages it was an authoritative text on astronomy.
The Almagest was preserved, like most of Classical Greek science, in Arabic manuscripts - hence its name.
Because of its reputation, it was widely sought and was translated twice into Latin in the 12th century, once in Sicily
and again in Spain.
Ptolemy's model, like those of his predecessors, was geocentric and was almost universally accepted until
the appearance of simpler heliocentric models during the scientific revolution.
His Planetary Hypotheses went beyond the mathematical model of the Almagest to present a physical realization
of the universe as a set of nested spheres, in which he used the epicycles of his planetary model to
compute the dimensions of the universe.
He estimated the Sun was at an average distance of 1210 Earth radii while the radius of the sphere of the
fixed stars was 20000 times the radius of the Earth.
Ptolemy presented a useful tool for astronomical calculations in his Handy Tables, which tabulated all the
data needed to compute the positions of the Sun, Moon and Planets, the rising and setting of the stars,
and eclipses of the Sun and Moon.
Ptolemy's Handy Tables provided the model for later astronomical tables or zijes.
In the Phaseis (Risings of the Fixed Stars) Ptolemy gave a parapegma, a star calendar or almanac based
on the hands and disappearances of stars over the course of the solar year.
Geography
Ptolemy's other main work is his Geographia.
This also is a compilation of what was known about the world's geography in the Roman Empire during his time.
He relied somewhat on the work of Marinos of Tyre, and earlier geographers of the Roman and ancient Persian Empire,
but most of his sources beyond the borders of the Empire were unreliable.
The first part of the Geographia is a discussion of the data and of the methods he used.
As with the model of the solar system in the Almagest, Ptolemy put all this information into a grand scheme.
Following Marinos, he assigned coordinates to all the places and geographic features he knew, in a grid that spanned the globe.
Latitude was measured from the equator towards the poles, as it is today, but Ptolemy preferred to express it as climata
defined by the length of the longest day at that latitude rather than degrees of arc.
The length of the midsummer day increases from 12h to 24h from the equator to the poles.
For example the city of Rhodos at 36°N was located in the 3rd climata with a length of 14.5 hours for the
midsummer day.
For the Longitude he used degrees and put the meridian of 0° longitude at the most western land he knew, the
"Blessed Islands", probably the Cape Verde islands (not the Canary Islands, as long accepted).
A 15th-century manuscript copy of the Ptolemy world map, reconstituted from Ptolemy's Geographia (circa 150),
indicating the countries of "Serica" and "Sinae" (China) at the extreme east, beyond the island of "Taprobane"
(Sri Lanka, oversized) and the "Aurea Chersonesus" (Malay Peninsula).
Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (oikoumenè)
and of the Roman provinces.
In the second part of the Geographia he provided the necessary topographic lists, and captions for the maps.
His oikoumenè spanned 180 degrees of longitude to the East from the Blessed Islands in the Atlantic Ocean to the middle
of China, and about 80 degrees of latitude to the West from Shetland to anti-Meroe (East coast of Africa).
Ptolemy was probably well aware that he knew about only a quarter of the globe, and an erroneous extension of China
southward suggests his sources did not reach all the way to the Pacific Ocean.
The maps in surviving manuscripts of Ptolemy's Geographia, however, date only from about 1300, after the text was
rediscovered by Maximus Planudes.
It seems likely that the topographical tables in books 2-7 are cumulative texts.
Texts which were altered and added to as new knowledge became available in the centuries after Ptolemy (Bagrow 1945).
This means that information contained in different parts of the Geographia is likely to be of different date.
Maps based on scientific principles had been made since the time of Eratosthenes (3rd century BC),
but Ptolemy improved projections.
It is known that a world map based on the Geographia was on display in Augustodunum, Gaul in late Roman times.
In the 15th century Ptolemy's Geographia began to be printed with engraved maps.
The earliest printed edition with engraved maps was produced in Bologna in 1477, followed quickly by a
Roman edition in 1478 (Campbell, 1987).
An edition printed at Ulm in 1482, including woodcut maps, was the first one printed north of the Alps.
The maps look distorted as compared to modern maps, because Ptolemy's data was inaccurate.
One reason is that Ptolemy estimated the size of the Earth as too small:
while Eratosthenes found 700 stadia for a great circle degree on the globe, in the Geographia Ptolemy uses 500 stadia.
It is highly probable that these were the same stadion since Ptolemy switched from the former scale to the
latter between the Syntaxis and the Geographia, and severely re-adjusted longitude degrees accordingly.
If they both used the Attic stadion of about 185 meters, then the older estimate is 16% too large,
and Ptolemy's value is 16% too small, a difference explained as due to ancient scientists' use of
simple methods of measuring the Earth, which were corrupted due to atmospheric refraction of horizontal light
rays by 16% of the Earth's curvature.
Because Ptolemy derived many of his key latitudes from crude "longest day values" (climata),
his latitudes are erroneous on average by roughly some degrees (2 degrees for Byzantium,
4 degrees for Carthage), though capable ancient astronomers knew their latitudes
to more like a minute (Ptolemy's own latitude was in error by 14').
He agreed that longitude was best determined by simultaneous
observation of lunar eclipses, yet he was so out of touch with the scientists of
his day that he knew of no such data more recent than 500 years before (Arbela eclipse).
When switching from 700 stadia per degree to 500, he (or Marinos) expanded longitude
differences between cities accordingly, resulting in serious over-stretching of the Earth's
East-West scale in degrees, though not distance.
Achieving highly precise longitude remained a problem in geography until the invention
of the marine chronometer at the end of the 18th century.
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