of a great many subjects of the utmost importance, thus widening its range and increasing its variety." Dante called the poem "Comedy" (the adjective "Divine" was added later, in the 16th century) because poems in the ancient world were classified as High ("Tragedy") or Low ("Comedy"). Low poems had happy endings and were written in everyday language, whereas High poems treated more serious matters and were written in an elevated style. Dante was one of the first in the Middle Ages to write of a serious subject, the Redemption of humanity, in the low and "vulgar" Italian language and not the Latin one might expect for such a serious topic. Boccaccio's account that an early version of the poem was begun by Dante in Latin is still controversial. Scientific themes Although the Divine Comedy is primarily a religious poem, discussing sin, virtue, and theology, Dante also discusses several elements of the science of his day (this mixture of science with poetry has received both praise and criticism over the centuries). The Purgatorio repeatedly refers to the implications of a spherical Earth, such as the different stars visible in the southern hemisphere, the altered position of the sun, and the various time zones of the Earth. For example, at sunset in Purgatory it is midnight at the Ebro, dawn in Jerusalem, and noon on the River Ganges: Dante travels through the centre of the Earth in the Inferno, and comments on the resulting change in the direction of gravity in Canto XXXIV (lines 76–120). A little earlier (XXXIII, 102–105), he queries the existence of wind in the frozen inner circle of hell, since it has no temperature differentials. Inevitably, given its setting, the Paradiso discusses astronomy extensively, but in the Ptolemaic sense. The Paradiso also discusses the importance of the experimental method in science, with a detailed example in lines 94–105 of Canto II: A briefer example occurs in Canto XV of the Purgatorio (lines 16–21), where Dante points out that both theory and experiment confirm that the angle of incidence is equal to the angle of reflection. Other references to science in the Paradiso include descriptions of clockwork in Canto XXIV (lines 13–18), and Thales' theorem about triangles in Canto XIII (lines 101–102). Galileo Galilei is known to have lectured on the Inferno, and it has been suggested that the poem may have influenced some of Galileo's own ideas regarding