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: