24
ALL MATTERS USEFUL
TO THE THEORY OF
HEAVENLY THINGS
CONSOLIDATING KNOWLEDGE
I
n his greatest known work, the
Almagest, the Graeco-Egyptian
astronomer Ptolemy produced
a summary of all the astronomical
knowledge of his time. Rather than
producing radical new ideas of his
own, Ptolemy mostly consolidated
and built upon previous knowledge,
particularly the works of the Greek
astronomer Hipparchus, whose
star catalog formed the basis
of most of the calculations in the
Almagest. Ptolemy also detailed
the mathematics required to
calculate the future positions of
the planets. His system would be
used by generations of astrologers.IN CONTEXT
KEY ASTRONOMER
Ptolemy (85 –165 ce)BEFORE
12th century bce The
Babylonians organize the
stars into constellations.350 bce Aristotle asserts
that the stars are fixed in
place and Earth is stationary.135 bce Hipparchus produces
a catalog of over 850 star
positions and brightnesses.AFTER
964 ce Persian astronomer
al-Sufi updates Ptolemy’s
star catalog.1252 The Alfonsine Tables
are published in Toledo, Spain.
These list the positions of the
sun, moon, and planets based
on Ptolemy’s theories.1543 Copernicus shows that
it is far easier to predict the
movement of the planets if the
sun is placed at the center of
the cosmos rather than Earth.Ptolemy’s model of the solar system
had a stationary Earth at its center,
with the heavens spinning daily
around it. His model required
complicated additions to make it
match the data and allow it to be
used to calculate the positions of the
planets; nonetheless, it was largely
unchallenged until Copernicus
placed the sun at the center of
the cosmos in the 16th century.The constellations devised by
Ptolemy are used in this 17th-century
star map. The number of stars per
constellation ranges from two (Canis
Minor) to 42 (Aquarius).