Commentary Par IX 118-119

The point at which ends the shadow of the earth cast by the Sun, a cone stretching nearly 900,000 miles above the earth according to Alfraganus, reaches only as far as the sphere of Venus (and thus marks the planet only when it is on the lower half of its epicyclical rotation).  Most early commentators, if they cite any astronomical authority, refer to Ptolemy (for the relevance of his chapter on eclipses, first mentioned by Jacopo della Lana).  Beginning in the late nineteenth century, Alfraganus becomes more widely used as Dante scholars began to understand the extent of the poet's debt to the Latin translation of the ninth-century Arabian astronomer's work, in fact the probable source for whatever Ptolemy he knew.  (See Alfergano and Tolommeo.)

In the Old Testament, Rahab has a major role in the second chapter of Joshua (2:1-21), where she aids and abets two spies from Joshua's army; then she is rescued during the destruction of the city by a grateful Joshua (Joshua 6:22-25).  Bosco/Reggio (DDP Bosco.Par.IX.118-126) point out that her salvation is not original with Dante, but is a matter of biblical record, with such witness as offered by St. Paul (Hebrews 11:31) and St. James (James 2:25), the first of whom insists on her faith in God, while the second extols her good works.  And see Matthew 1:5, where Rahab is listed as the mother of Boaz, and thus a distant ancestress of Jesus.  For a figural understanding of Rahab, see Auerbach (Auer.1946.1), pp. 482-84.