A representation used to predict or explain the apparent arrangements and motion of celestial objects. Agreement with a limited observation set need not identify a unique physical arrangement.
An epicycle is a circle whose centre moves along another path, traditionally a larger circle called the deferent. Combining these motions can reproduce loops and changes of apparent speed in a planet's position on the celestial sphere. In Ptolemy's geocentric model, such constructions supplied quantitative planetary predictions.[ref]
A simple planar example isThe second term is the epicycle relative to its moving centre. Suitable radii and angular rates can produce locally backward motion, illustrating apparent retrograde motion. The construction is a geometric representation; fitting an observed track does not by itself establish what force governs the actual orbit.
A model in which the Earth is stationary at the centre of the relevant celestial system. Distinguish this broad class from a particular ancient planetary arrangement.
A geoheliocentric arrangement with the Sun circling a stationary Earth and other planets circling the Sun. It can reproduce the phases of Venus without an orbiting Earth.
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