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Mercury is so special

At the bottom of this folio, in the right margin it is found a little diagram in black and red ink. This diagram is meant to show the effect of the ‘small circle’ of Mercury. Mercury complex motion have always been a concern for astronomers. In the Ptolemaic traditions, Mercury model is specific because, in addition to the usual epicycle and deferent circle, it has a supplementary ‘small circle’ in order to adjust the planet distance to the earth. This peculiar device is usually not reflected in the structure of Mercury tables but can be perceived in some of their values. Peurbach and then Regiomontanus were particularly interested in this small circle and its effect. In this diagram and marginalia we can see one attempt of Johannes Schöner to reflect on the ‘small circle’ of Mercury.

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