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This finely-made, silver instrument, by the famous London maker John Rowley, involves an interesting contradiction. The outer 'armillary' sphere is made up of rings (in Latin, armillae) that represent the prominent circles of the celestial sphere, including the equator, the tropics and the ecliptic (the apparent annual path of the sun through the stars). This spherical assembly can rotate on an axis supported within a vertical 'meridian ring', a rotation which follows the apparent motion of the sky, so this is a 'Ptolemaic' model of the cosmos, with the heavens rotating around a central, stationary earth

But within this celestial sphere we do not find a stationary earth. Here Rowley provides a model of the planetary system or 'orrery', which has the sun at the centre, with the earth as a planet in orbit around it. This is the 'Copernican' view of things. In Rowley's construction the 'orrery' includes gearwork which makes the earth rotate, implying that the movement of the stars is only apparent, not real. Should this instrument be seen as an incongruous mixture of two incompatible ways of considering the cosmos, or is it rather a subtle and clever didactic tool, which can introduce and compare the systems of Ptolemy and Copernicus in a single model?

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