• ‘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 the Stagirite’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 the Stagirite’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).
