Tropic of Cancer

Tropic of Cancer is the northernmost circle of latitude on the Earth where the Sun can be seen directly overhead. This happens on the June solstice, when the Northern Hemisphere is tilted as far toward the Sun as it can go. On the December solstice, at solar midnight, the Sun is below the horizon in all directions at this latitude.

This circle is the northern of two tropics. Its Southern Hemisphere partner, the Tropic of Capricorn, marks the farthest south the Sun can be seen directly overhead. The two tropics are among seven major circles of latitude on maps of Earth. The others are the Arctic and Antarctic circles, the Equator, and the Poles, which are degenerate circles.

The position of these circles, measured from the Equator, depends on the tilt of Earth's axis of rotation compared with the plane of its orbit. Because this tilt changes, the locations of the two tropics also change. The Tropic of Cancer is currently about 23.4 degrees north of the Equator.

In geopolitics, the Tropic of Cancer is known as the southern limit of the mutual defence promise of NATO. Member countries of NATO are not required to defend territory south of this line.

For the novel by Henry Miller, see Tropic of Cancer (novel).

Name

This line of latitude was named over two thousand years ago. At that time, the Sun was in the zodiac sign Cancer (Latin: Crab) at the June solstice, at 90° ecliptic longitude. Because of the precession of the equinoxes, this is no longer true. Today the Sun is in the zodiac sign Taurus at the June solstice.

The word "tropic" comes from the Greek "tropē (τροπή)", which means turn or change. It describes how the Sun seems to "turn back" at the solstices.

Drift

The Tropic of Cancer does not stay in one place. It moves over time because of axial precession. This is a slow wobble in how the Earth is tilted compared with the ecliptic, the flat plane in which the Earth travels around the Sun. The Earth's axial tilt changes over a 41,000-year cycle, from about 22.1 to 24.5 degrees. In 2000, it was about 23.4 degrees, and this value will stay roughly the same for about a millennium. Because of this wobble, the Tropic of Cancer is moving south by about 15 m (49 ft) each year. It was at exactly 23°27′N in 1917 and will be at 23°26′N in 2045.

The distance between the Antarctic Circle and the Tropic of Cancer is always very close to 90 degrees. Any small difference is due to the Earth being slightly flattened. For more details, see the equator, axial tilt and circles of latitude.

Geography

North of the tropic are the subtropics and the North Temperate Zone. The line of latitude south of the Equator is called the Tropic of Capricorn. The area between these two lines, centered on the Equator, is called the tropics.

In the year 2000, more than half of the world's people lived north of the Tropic of Cancer. On this line, the longest day of summer has about 13 hours and 35 minutes of daylight. The shortest day of winter has about 10 hours and 41 minutes. The line passes through 17 countries (including two disputed territories) and 8 bodies of water. It starts at the prime meridian and goes east.

Climate

The climate along the Tropic of Cancer is usually hot and dry. Cooler highland areas in China, ocean areas such as Hawaii, and eastern coasts can get very heavy orographic rainfall, reaching 4 metres (160 in) a year in some places. Most places on the Tropic have two clear seasons: a very hot summer, often reaching 45 °C (113 °F), and a warm winter, around 22 °C (72 °F). Much of the land on or near the Tropic is part of the Sahara Desert. To the east, the climate is monsoonal. It has a short wet season from June to September and very little rain for the rest of the year.

The highest mountain on or near the Tropic of Cancer is Yu Shan in Taiwan. Long ago, during the Last Glacial Maximum, its glaciers reached down as low as 2,800 metres (9,190 ft). Glaciers still exist around the Tropic today. The closest ones are the Minyong and Baishui in the Himalayas to the north, and on Pico de Orizaba in Mexico to the south.

Circumnavigation

According to the rules of the Fédération Aéronautique Internationale, a flight that wants to set a round-the-world speed record must travel at least as far as the Tropic of Cancer. It must cross all meridians and land at the same airfield where it started.

The Tropic of Cancer is about 36,788 km (22,859 mi) long. For an ordinary circumnavigation, the rules are a little more relaxed. The distance is set to at least 36,770 kilometres (22,850 mi).