Tropic

Geographic definition

Geographically, the tropics are bounded by the circles of latitude known as the Tropic of Cancer in the Northern Hemisphere and the Tropic of Capricorn in the Southern Hemisphere. Each tropic lies at approximately 23°26′22″ from the equator (about 23.44°), a position determined by the tilt of the Earth's axis. Between these two parallels the sun can be directly overhead at local solar noon on at least one day of the year.

Characteristics of the tropics

• Solar geometry: The sun reaches its zenith only within this band, cycling with the solstices and tied to the Earth's axial tilt.
• Climate: Many tropical locations have warm temperatures year-round; rainfall patterns vary widely, producing rainforests, monsoon climates and seasonally dry areas.
• Biological diversity: Tropical regions host a large share of the world's biodiversity and complex ecosystems.
• Human activity: Agriculture, settlement patterns and cultural practices have long adapted to tropical seasons and disease environments.

The name and concept also help define climatic and ecological classifications—often distinguished from temperate and polar zones—and are used in navigation, cartography and education. For a general article on the broader belt, see the entry on the tropics.

History of the term and other uses

The English term derives from the Greek tropikos, meaning "of a turn," a reference to the sun's apparent turning point at the solstices. Beyond its strict geographic sense, "tropic" appears in proper names and commercial uses: several towns and communities are named Tropic (for example, Tropic, Utah and a locality often cited as Tropic, Florida), and the word is used for products, place names and occasionally for vessels or routes in transportation. Such uses borrow the evocative idea of warmth, latitude or exoticism rather than the scientific definition.

Notable distinctions: the tropics as latitude lines are fixed by axial tilt and vary slightly over long timescales, whereas the tropical climate and ecosystems are defined by temperature and precipitation regimes that can shift with climate change and human activity.