The Moon's sphere is the first heaven in Dante's Paradiso. Dante and Beatrice begin their ascent through the celestial spheres there, before moving outward through Mercury, Venus, the Sun, Mars, Jupiter, Saturn, the Fixed Stars, and the Primum Mobile.
In Dante's medieval cosmology, the Moon is the lowest of the nine heavens surrounding Earth. Its changing appearance becomes an allegory for inconstancy. The souls Dante encounters there, including Piccarda Donati and Constance of Sicily, are associated with vows that were not fully kept because external force disrupted their religious lives.
The Moon's sphere is also the setting for Beatrice's discussion of lunar markings in Canto II. She challenges Dante's initial explanation and describes an experiment involving reflected light, making the passage one of the poem's notable encounters between theology, cosmology, and observation.
Mercury is the second sphere, not the first, while Venus is third and Mars is fifth. A common confusion comes from the phrase “first heaven”: in Dante's poem it means the first planetary sphere reached during the ascent, namely the Moon, not the outermost heaven or the Empyrean.