The Tropic of Cancer is a line of latitude circling the Earth at approximately 23.5° north of the equator. It is the northernmost point on Earth where the sun's rays can appear directly overhead at local noon. It is also one of the five major degree measures or circles of latitude dividing the Earth (the others are the Tropic of Capricorn, the equator, the Arctic Circle and the Antarctic Circle).
The Tropic of Cancer is significant to Earth's geography because in addition to being the northernmost point where the sun's rays are directly overhead, it also marks the northern boundary of tropics, which is the region that extends from the equator north to the Tropic of Cancer and south to the Tropic of Capricorn.
Some of the Earth's largest countries and/or cities are at or near the Tropic of Cancer. For example, the line passes through United States' state of Hawaii, portions of Central America, northern Africa and the Sahara Desert and is near Kolkata, India. It should also be noted that because of the greater amount of land in the Northern Hemisphere the Tropic of Cancer passes through more cities than the equivalent Tropic of Capricorn in the Southern Hemisphere.
Naming of the Tropic of Cancer
At the June or summer solstice (around June 21) when the Tropic of Cancer was named, the sun was pointed in the direction of the constellation Cancer, thus giving the new line of latitude the name the Tropic of Cancer. However, because this name was assigned over 2,000 years ago, the sun is no longer in the constellation Cancer. It is instead located in the constellation Taurus today. For most references though, it is easiest to understand the Tropic of Cancer with its latitudinal location of 23.5°N.
Significance of the Tropic of Cancer
In addition to being used to divide the Earth into different parts for navigation and marking the northern boundary of the tropics, the Tropic of Cancer is also significant to the Earth's amount of solar insolation and the creation of seasons.
Solar insolation is the amount of incoming solar radiation on the Earth. It varies over the Earth's surface based on the amount of direct sunlight hitting the equator and tropics and spreads north or south from there. Solar insolation is most at the subsolar point (the point on Earth that is directly beneath the Sun and where the rays hit at 90 degrees to the surface) which migrates annually between the Tropics of Cancer and Capricorn because of the Earth's axial tilt. When the subsolar point is at the Tropic of Cancer, it is during the June solstice and this is when the northern hemisphere receives the most solar insolation.
During the June solstice because the amount of solar insolation is greatest at the Tropic of Cancer, the areas north of the tropic in the northern hemisphere also receive the most solar energy which keeps it warmest and creates summer. In addition, this is also when the areas at latitudes higher than the Arctic Circle receive 24 hours of daylight and no darkness. By contrast the Antarctic Circle receives 24 hours of darkness and lower latitudes have their winter season because of low solar insolation, less solar energy and lower temperatures.
Click here to see a simple map showing the location of the Tropic of Cancer.
Wikipedia. (13 June 2010). Tropic of Cancer - Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/Tropic_of_Cancer