Having survived the depths of Hell (described in the Inferno), Dante and Virgil ascend to the Mountain of Purgatory on the far side of the world. The mountain is an island, the only land in the Southern Hemisphere. Dante describes Hell as existing underneath Jerusalem, created by the impact of Satan's fall. Mount Purgatory, on exactly the opposite side of the world, was created by a displacement of rock, caused by the same event. Dante announces his intention to describe Purgatory by invoking the mythical Muses, as he did in Canto II of the Inferno:
"And of that second kingdom will I sing Wherein the human spirit doth purge itself, And to ascend to heaven becometh worthy. But let dead Poesy here rise again, O holy Muses, since that I am yours,"
Allegorically, the Purgatorio represents the penitent Christian life. In a contrast to Charon's ferry across the Acheron in the Inferno, Christian souls here arrive escorted by an angel, singing In exitu Israel de Aegypto (Canto II). In his Letter to Cangrande, Dante explains that this reference to Israel leaving Egypt refers both to the redemption of Christ and to "the conversion of the soul from the sorrow and misery of sin to the state of grace." Appropriately, therefore, it is Easter Sunday when Dante and Virgil arrive.
The Purgatorio demonstrates the medieval knowledge of a spherical Earth. During the poem, Dante discusses the different stars visible in the southern hemisphere, the altered position of the sun, and the various timezones of the Earth. At this stage it is, Dante says, sunset at Jerusalem, midnight on the River Ganges (with the constellation Libra overhead there), and dawn in Purgatory:
"By now the sun was crossing the horizon of the meridian whose highest point covers Jerusalem; and from the Ganges, night, circling opposite the sun, was moving together with the Scales that, when the length of dark defeats the day, desert night's hands; so that, above the shore that I had reached, the fair Aurora's white and scarlet cheeks were, as Aurora aged, becoming orange."