The Tropic of Cancer and the Tropic of Capricorn are two important lines of latitude that mark the northernmost and southernmost places on Earth where the Sun can appear directly overhead at local solar noon during the year. The Tropic of Cancer is located at approximately 23.44 degrees north latitude, while the Tropic of Capricorn is located at approximately 23.44 degrees south latitude. They are imaginary circles that run around Earth roughly parallel to the Equator.
Their positions are determined by the tilt of Earths rotational axis relative to the plane of its orbit around the Sun. Because Earths axis is tilted by approximately 23.44 degrees, the apparent position of the overhead Sun moves seasonally between these two latitudes. The tropics are therefore defined geographically and astronomically by the relationship between Earths axial tilt and its revolution around the Sun.
The name Tropic of Cancer is derived from the historical association of the northern tropic with the constellation Cancer, while Tropic of Capricorn similarly refers to the constellation Capricornus. The names originated in ancient astronomical geography, when the apparent position of the Sun at the northern and southern solstices occurred against those constellations. Because of the gradual apparent shifting of the background stars caused by axial precession, the solstitial Sun is no longer located in those constellations.
The traditional names have nevertheless remained in use. The word tropic comes from a Greek term associated with turning, referring to the apparent seasonal turning point of the Sun. Cancer and Capricorn are therefore historical astronomical names rather than descriptions of physical characteristics of the land through which the lines pass.
The Tropic of Cancer corresponds approximately to the latitude reached by the Sun at the northern summer solstice, while the Tropic of Capricorn corresponds approximately to the latitude reached by the Sun at the southern summer solstice. At the June solstice, the Sun can be directly overhead at the Tropic of Cancer at local solar noon. At the December solstice, it can be directly overhead at the Tropic of Capricorn.
The exact latitude of the tropics changes gradually because Earths axial tilt is not perfectly constant. Modern geographical maps commonly use approximately 23.5 degrees north and 23.5 degrees south for simplicity, whereas more precise astronomical calculations use the current value of the obliquity of the ecliptic. This distinction matters when precise geographical or astronomical measurements are required.
The two tropics divide the Earth into several broad astronomical and climatic zones. The region between the Tropic of Cancer and Tropic of Capricorn is called the tropics or tropical zone and extends across the Equator. Within this region, every location experiences the Sun at or north or south of the zenith at some point during the year, although locations very close to the tropics experience this phenomenon only around one solstice.
Outside the tropics, the Sun is never directly overhead at local solar noon. The tropics are therefore different from climate classifications, although they have major climatic significance. Tropical climates are influenced strongly by high solar elevation, seasonal movement of the Intertropical Convergence Zone, atmospheric circulation, ocean temperatures and regional geography, so the tropics are not uniformly hot or wet.
The Tropic of Cancer passes through portions of several countries and territories in the Northern Hemisphere, including Mexico, the Bahamas, Western Sahara, Mauritania, Mali, Algeria, Niger, Libya, Egypt, Saudi Arabia, the United Arab Emirates, Oman, India, Bangladesh, Myanmar, China and Taiwan, depending on the precise treatment of political boundaries and the modern latitude of the line. The Tropic of Capricorn passes through areas including Namibia, Botswana, South Africa, Mozambique, Madagascar, Australia, French Polynesia, Chile, Argentina, Paraguay and Brazil. These lines are not physical boundaries visible on the ground. In some locations they are marked by monuments, signs or tourist attractions, but most of the tropics have no physical indication that the latitude has been crossed.
The tropics have important effects on solar geometry, seasons and the distribution of solar energy. Near the Equator, day and night remain relatively similar in length throughout the year, whereas seasonal changes in daylight become more pronounced as latitude increases. At the tropics, the maximum solar elevation is extremely high, and around the relevant solstice the Sun reaches the zenith. The tropics also form an important reference framework for understanding the global circulation of the atmosphere. Strong solar heating in low latitudes contributes to rising warm air, large scale circulation cells and the broad distribution of tropical rainfall, although actual rainfall varies greatly according to atmospheric circulation, mountains, oceans and local conditions.
A common misconception is that the Tropic of Cancer and Tropic of Capricorn are equivalent to the boundaries of hot weather or that temperatures necessarily become colder immediately outside them. They are astronomical latitude lines, not precise climatic boundaries. Another misconception is that the Sun is always directly overhead somewhere on each tropic. Direct overhead position occurs at the appropriate solstice and specific local solar noon, not continuously. The tropics also should not be confused with the Equator. The Equator is at zero degrees latitude and divides the Northern and Southern Hemispheres, whereas the tropics are approximately 23.44 degrees from the Equator and identify the extreme latitudes of the annual apparent northward and southward movement of the overhead Sun.
The Tropic of Cancer and Tropic of Capricorn are therefore useful reference lines connecting astronomy, geography, cartography and climatology. They provide a simple way of expressing the consequences of Earths axial tilt and orbital motion and help explain why the Sun changes position through the seasons. Their importance extends beyond map reading because latitude affects solar angle, day length, atmospheric circulation, ecosystems and climate.
The tropics contain a large proportion of Earths biological diversity and include environments ranging from equatorial rainforest to tropical savanna, desert and high mountain environments. Understanding the two tropics therefore requires distinguishing their precise astronomical definition from the broader geographical and climatic meanings associated with the tropical region.
