The book looks at ways of world-building in prose fictions of cosmic voyage in the seventeenth century. With the rise of the New Astronomy, there equally was a resurgence of the cosmic voyage in fiction. Various models of the universe were reimagined in prose form. Most of these voyages explore imagined versions of a world in the moon, such as the cosmic voyages by Johannes Kepler, Francis Godwin and Savinien Cyrano de Bergerac. In Margaret Cavendish's The Blazing World, an eponymous imaginary planet is introduced. The book analyses the world-building of cosmic voyages by combining theories of world-building with contemporary concepts from early modern literature. It shows how imaginary worlds were created in early modern prose literature.