the equator. Summer in this region lasts for about 2 to 3 months, and there is almost 24-hour sunlight during summer. The sun's rays are always slanting, so provide less heat per horizontal surface area. The North Frigid Zone includes the United States (only the state of Alaska), the northern regions of Canada (the Northwest Territories, Nunavut, and Yukon), Greenland (Denmark), Norway, Finland, Sweden, and Russia. The South Frigid Zone includes only Antarctica. History The concept of a geographical zone was first hypothesized by the ancient Greek scholar Parmenides and lastingly modified by Aristotle. Both philosophers theorized the Earth divided into three types of climatic zones based on their distance from the equator. Like Parmeneides, thinking that the area near the equator was too hot for habitation, Aristotle dubbed the region around the equator (from 23.5° N to 23.5° S) the "Torrid Zone." Both philosophers reasoned the region from the Arctic Circle to the pole to be permanently frozen. This region thought uninhabitable, was called the "Frigid Zone." The only area believed to be habitable was the northern "Temperate Zone" (the southern one not having been discovered), lying between the "Frigid Zones" and the "Torrid Zone". However, humans have inhabited almost all climates on Earth, including inside the Arctic Circle. As knowledge of the Earth's geography improved, a second "Temperate Zone" was discovered south of the equator, and a second "Frigid Zone" was discovered around the Antarctic. Although Aristotle's map was oversimplified, the general idea was correct. Today, the most commonly used climate map is the Köppen climate classification, developed by Russian climatologist of German descent and amateur botanist Wladimir Köppen (1846–1940), which divides the world into five major climate regions, based on average annual precipitation, average monthly precipitation, and average monthly temperature. See also Circle of latitude Climate classification Clime Hardiness zone Polar regions of Earth Subtropics Temperate climate Tropics References and footnotes Geographical zones