Tropic of Cancer

The Tropic of Cancer is a significant circle of latitude on Earth. It marks the northernmost point where the Sun can appear directly overhead. This phenomenon occurs during the June solstice. At this time, the Northern Hemisphere reaches its maximum tilt toward the Sun. This line is one of seven major circles of latitude used on maps. These include the Equator, the two tropics, and the Arctic and Antarctic circles. The Poles are also considered circles, though they are degenerate circles.

The position of this line is determined by Earth's axial tilt. This tilt is the angle of Earth's rotation axis relative to the plane of its orbit. This plane is known as the ecliptic. The tilt is currently about 23.4 degrees. This specific value will remain valid for roughly one thousand years. The relationship between this tilt and the latitude circles is clearly shown in diagrams.

This circle is not a fixed location. It changes because of axial precession. Precession is a slow wobble in Earth's orientation. The axial tilt also varies over a 41,000-year period. This cycle moves the tilt between 22.1 and 24.5 degrees. Because of this wobble, the Tropic of Cancer is drifting southward. It moves at a rate of almost 0.468 arcseconds of latitude per year. In 1917, the line was at 23°27'N. By 2045, it will reach 23°26'N.

The name of the line has a historical connection to the stars. In the last centuries BC, the Sun was in the constellation Cancer during the June solstice. Cancer is the Latin word for crab. Due to precession, the Sun is now in the constellation Taurus during the solstice. The term "tropic" comes from the Greek word "tropē." This means a turn or a change in direction. It refers to how the Sun appears to turn back at the solstices.

Many diverse regions lie along this latitude. The line passes through 17 countries and eight water bodies. It starts at the prime meridian and moves eastward. It touches the northernmost point of Western Sahara. It crosses the Red Sea, the Indian Ocean, and the Pacific Ocean. It also passes through many Mexican states, including Sinaloa and Tamaulipas.

In Mexico, a sign once marked the line on a highway. This was the only intersection where the drift could be seen between 2005 and 2010.

The climate along the Tropic of Cancer is often hot and dry. Much of the land near the line is part of the Sahara Desert. Many areas experience two distinct seasons. There is an extremely hot summer and a warm winter. In some eastern areas, the climate is torrid monsoonal. These areas have a short wet season from June to September. Other places, like Hawaii or highland China, are cooler. Some coastal areas experience very heavy orographic rainfall. This type of rain is caused by air rising over mountains.

Geography and human life are also connected to this line. In the year 2000, more than half of the world's population lived north of the Tropic of Cancer. The highest mountain near the line is Yu Shan in Taiwan. While much of the region is hot, glaciers still exist nearby. The closest surviving glaciers are in the Himalayas and on Pico de Orizaba in Mexico. Finally, the line has importance in aviation. To set a round-the-world speed record, a flight must cover at least the length of the Tropic of Cancer. This distance is approximately 40,075 kilometers.