Castor and Pollux

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In the passage of the Purgatorio, the periphrasis «Se Castore e Poluce» is used to indicate the constellation of the Twins, named after the two mythological brothers, sons of Jupiter and Leda. The exegetical note clarifies that the phrase has an astronomical value: if the sun were in the Twins, that is, near the summer solstice, it would appear even closer to the north than it does when it is in Aries. This observation serves to explain why, in the southern hemisphere represented by the Purgatorio, the sun is seen rising toward the left, in the opposite direction to that of the northern hemisphere. The reference to the Dioscuri thus allows the introduction of the problem of the different perception of the sun's path in the two hemispheres, which will be developed in the subsequent verses with the image of the «specchio che di qua sé menando, di là sù conduce».