• ‘Theoretical model of the universe describing the positions and apparent motions of the Sun, Moon, and
planets, formulated by the Alexandrian astronomer and mathematician Ptolemy
about AD 140 and recorded by him in
his Almagest.
Aristotle’s model of the universe had trouble explaining some planetary phenomena. The most striking of
these was retrograde motion. In retrograde motion each planet seems to slow down at times, and move in reverse or retrograde
before resuming its course.
Planets also grow brighter or dimmer as they move through the sky. Aristotle’s model could account for
neither phenomenon very well. The most important solution to this problem was proposed by Claudius Ptolemy in the third century
AD.
He argued that planets move on two sets of circles: a deferent and an epicycle. This explained retrograde motion while keeping the planets in their circular orbits around
the earth.
Where this did not fit Ptolemy proposed an eccentric. An eccentric orbit had a centre
different from the earth and accounted well for changes in a planet’s brightness.
Ptolemy’s last device was the equant. In an equant a
planet speeded up and slowed down, but when seen from an off-centre point actually appeared to be moving with uniform speed. From
earth, however, a planet’s motion was quite irregular.
The Ptolemaic System held ground for centuries, until too many discrepancies cried for new solutions’. (©Encyclopaedia Britannica).
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[1]‘Ptolemy: flourished AD 127–145, Alexandria (Latin in full Claudius Ptolemaeus): ancient astronomer,
geographer, and mathematician who considered the Earth the centre of the universe (the ‘Ptolemaic system’). Virtually nothing
is known about his life’. (©Encyclopaedia Britannica).
[2]‘a deferent: in the Ptolemaic system: the circular orbit of the centre of the epicycle in which a planet
was thought to move’. (Oxford Dictionary).
[3]‘an epicycle: a circle in the Ptolemaic system, in which the planets were regarded as moving in circles
whose centres moved round larger circles (deferents) centred on the earth. (Oxford Dictionary).
[4]‘an eccentric: a circle or orbit not having the earth precisely in its centre’. (Oxford Dictionary).
[5]‘the equant: an imaginary circle introduced with the purpose of reconciling the planetary movements with
the Ptolemaic hypothesis of uniform circular motion’. (Oxford Dictionary).
