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Diagrams and instruments

Escorial

This diagram, from a text known as the Theorica planetarum, is a recurrent element of university manuscripts dealing with mathematical astronomy. In this particular instance, the outer scale of this diagram, representing the zodiac, looks like it has been extracted from an astrolabe ream (?). The inner diagram is more abstract and mathematical. It represents the theory of the Sun. The Sun itself is represented six times as a small circle enhanced with red ink: once on the limit of the four quadrants; two more times in symmetric intermediary position. The circle on which the Sun revolves is centered on h. It is thus eccentric with respect to the Earth which is also the center of the zodiacal outer ring (point e). This eccentricity is the geometrical device used to account for the varying speed of the Sun over the course of the year (faster when it is near the Earth at the bottom of the diagram, slower when it is far from the Earth at the top of the image). Apart from the top and bottom position, we see that for the four others, three lines are drawn from e and h toward the Sun. Each time two of them are parallel, and one oblique. These lines allow us to define a mean and a true position of the Sun. These two quantities are the basis on which a complementary tabular expression of the solar theory allows computations of the Sun position in the zodiac as seen from the Earth.