25
Ptolemy describes the
design of his stone plinth in
the Almagest. It was a quadrant,
an instrument that measures
angles between 0° and 90°.
See also: The geocentric model 20 ■ Shifting stars 22 ■ The Copernican model
32–39 ■ The Tychonic model 44–47 ■ Elliptical orbits 50–55
FROM MYTH TO SCIENCE
Ptolemy produced a catalog
of 1,022 star positions and listed
48 constellations in the part of
the celestial sphere known to the
Greeks—everything that could be
seen from a northern latitude of
about 32o. Ptolemy’s constellations
are still used today. Many of their
names can be traced even further
back to the ancient Babylonians,
including Gemini (twins), Cancer
(crab), Leo (lion), Scorpio (scorpion),
and Taurus (bull). The Babylonian
constellations are named on a
cuneiform tablet called the Mul
Apin, which dates back to the
7th century bce, however, they are
thought to have been compiled
about 300 years earlier.
Early quadrant
To improve his measurements,
Ptolemy built a plinth. One of the
earliest examples of a quadrant,
his plinth was a huge rectangular
block of stone, one of whose vertical
sides accurately aligned in the
north–south plane. A horizontal
bar protruded from the top of the
stone, and its shadow gave a
precise indication of the height
of the sun at noon. Ptolemy took
daily measurements to obtain
accurate estimates of the time of
the solstices and equinoxes, which
confirmed previous measurements
showing that the seasons were
different lengths. He believed that
the orbit of the sun around Earth
was circular, but his calculations
led him to the conclusion that
Earth could not be at the exact
center of that orbit.Ptolemy the astrologist
Like most thinkers of his day,
Ptolemy believed that the
movements of the heavenly bodies
profoundly affected events on
Earth. His book on astrology,
Tetrabiblos, rivaled the Almagest
in popularity over the following
1,000 years. Ptolemy had not only
provided a means to calculate
planetary positions, but he had
also produced a comprehensive
interpretation of the ways those
movements affected humans. ■Claudius Ptolemy
Ptolemy was a polymath and
produced works on a wide
range of topics, including
astronomy, astrology,
geography, music, optics,
and mathematics.
Very little is known about
him, but he probably spent
all his life in Alexandria, the
Egyptian seaport with a
reputation for scholarship
and a great library, where he
was taught by the renowned
mathematician Theon of
Smyrna. Many of his prolific
writings have survived. They
were translated into Arabic
and Latin, disseminating his
ideas across the medieval
world. Geography listed the
locations of most of the places
in the known world, and
was carried by Christopher
Columbus on his voyages of
discovery in the 15th century.
The Almagest remained in
continual use in academia
until about 1643, a century
after Ptolemy’s model of the
universe had been challenged
by Copernicus.Key worksc.150 ce Geography
c.150 ce Almagest
c.150 ce Tetrabiblos0o90oSun’s heightHorizontal barSun’s
shadowStone plinthSun