Editing Almagest (section)

=== Ptolemy's planetary model ===
[[File:Ptolemaicsystem-small.png|thumb|16th-century representation of Ptolemy's [[geocentric model]] in [[Peter Apian]]'s ''Cosmographia'', 1524]]

In Almagest, Ptolemy assigned the following order to the [[planetary spheres]], beginning with the innermost:<ref>See Book IX, Chapter I, "On the order o f the spheres o f sun, moon and the 5 planets" in the Toomer's translation.</ref>
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# Moon
# Mercury or Venus (undecided)<ref>For example, in Albert Van Helden , Measuring the Universe Cosmic Dimensions from Aristarchus to Halley, Chicago University Press, 1985, page 20, it is stated that "He left the order of Venus and Mercury with respect to each other undecided in the Almagest."</ref>
# Mercury or Venus (undecided)
# Sun
# Mars
# Jupiter
# Saturn
# Sphere of fixed stars
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Later, in his "Planetary Hypothesis", he concludes that Mercury is the second closest planet. Other classical writers suggested different sequences. [[Plato]] ({{Circa|427|347 BC}}) placed the Sun second in order after the Moon. [[Martianus Capella]] (5th century AD) put Mercury and Venus in motion around the Sun. Ptolemy's authority was preferred by most [[Astronomy in medieval Islam|medieval Islamic]] and late medieval European astronomers. <!-- Comparison with Aristotle's model should be added -->

Ptolemy inherited from his Greek predecessors a geometrical toolbox and a partial set of models for predicting where the planets would appear in the sky. [[Apollonius of Perga]] ({{Circa|262|190 BC}}) had introduced the [[deferent and epicycle]] and the eccentric deferent to astronomy. Hipparchus (2nd century BC) had crafted mathematical models of the motion of the Sun and Moon. Hipparchus had some knowledge of [[History of Astronomy#Mesopotamia|Mesopotamian astronomy]], and he felt that Greek models should match those of the Babylonians in accuracy. He was unable to create accurate models for the remaining five planets.

[[File:HipparchusConstruction.svg|thumb|upright=1.35 |Geometric construction used by Hipparchus in his determination of the distances to the Sun and Moon, which was later incorporated into Ptolemy's work]]

The ''Syntaxis'' adopted Hipparchus' solar model, which consisted of a simple eccentric deferent. For the Moon, Ptolemy began with Hipparchus' epicycle-on-deferent, then added a device that historians of astronomy refer to as a "crank mechanism":{{sfn|Hoskin|1999|p=44}} he succeeded in creating models for the other planets, where Hipparchus had failed, by introducing a third device called the [[equant]].

Ptolemy wrote the ''Syntaxis'' as a textbook of mathematical astronomy. It explained geometrical models of the planets based on combinations of circles, which could be used to predict the motions of celestial objects. In a later book, the ''Planetary Hypotheses'', Ptolemy explained how to transform his geometrical models into [[three-dimensional space|three-dimensional spheres]] or partial spheres. In contrast to the mathematical ''Syntaxis'', the ''Planetary Hypotheses'' is sometimes described as a book of [[cosmology]].