Tropic of Cancer

The Tropic of Cancer is an important circle of latitude on Earth. It marks the northernmost point where the Sun can be seen directly overhead. This special event happens once a year, on the June solstice, when the Northern Hemisphere is tilted most toward the Sun.

The Tropic of Cancer is currently located about 23°26′ north of the Equator. It is one of two tropics on Earth, with the other being the Tropic of Capricorn in the Southern Hemisphere. These tropics, along with the Equator, Arctic and Antarctic circles, and the Poles, help us understand and map Earth's shape and movement.

The position of the Tropic of Cancer changes slowly over time because of the tilt of Earth's axis. In addition to its importance in geography and astronomy, the Tropic of Cancer also has meaning in geopolitics. For example, it marks the southern limit of NATO's mutual defence obligation, meaning member states are not required to defend areas south of this line.

Name

Long ago, people named this important line of latitude after the constellation Cancer, which looks like a crab. At that time, during the June solstice, the Sun appeared in the sky near this constellation. However, because the Earth’s tilt changes very slowly over thousands of years—a process called precession—the Sun is now in a different constellation called Taurus during the June solstice. The word “tropic” comes from an ancient Greek word meaning “turn,” because the Sun seems to turn back at this special time of year.

Drift

The Tropic of Cancer does not stay in the same place. It moves because the Earth's tilt changes over time. This tilt shifts every 41,000 years, and right now, the Tropic of Cancer is moving slowly toward the south. In 1917, it was at exactly 23°27′N, and by 2045, it will be at 23°26′N.

The distance between the Tropic of Cancer and the Antarctic Circle stays about the same as they move together. The exact spot of the equator also changes a little over time. For more information, see: equator, axial tilt and circles of latitude.

Geography

North of the Tropic of Cancer are the subtropics and the North Temperate Zone. The line of latitude south of the Equator is called the Tropic of Capricorn, and the area between them, centered on the Equator, is known as the tropics.

In the year 2000, more than half of the world's people lived north of the Tropic of Cancer. On this line, there are about 13 hours and 35 minutes of daylight during the summer solstice, and around 10 hours and 41 minutes of daylight during the winter solstice.

The Tropic of Cancer passes through 17 countries (including two disputed areas) and 8 water bodies, starting at the prime meridian and moving east.

Climate

The Tropic of Cancer usually has hot and dry weather. However, some places along it, like high mountains in China, coastal areas, and islands such as Hawaii, can be cooler or get a lot of rain—up to 4 metres (160 inches) each year! Most areas here have two clear seasons: a very hot summer, where temperatures can reach 45 °C (113 °F), and a warm winter, with highs around 22 °C (72 °F).

Many lands near the Tropic of Cancer are part of the huge Sahara Desert. Farther east, the weather pattern changes with heavy rains during a short wet season from June to September, and very little rain the rest of the year. The tallest mountain near the Tropic of Cancer is Yu Shan in Taiwan. During a cold period long ago, glaciers reached down to 2,800 metres (9,190 feet). Today, a few small glaciers still remain close by, such as in the Himalayas and on Pico de Orizaba in Mexico.

Circumnavigation

According to the rules of the Fédération Aéronautique Internationale, for a flight to try for a round-the-world speed record, it must travel a distance at least as long as the Tropic of Cancer, cross all meridians, and finish at the same place where it began.

The Tropic of Cancer is 36,788 kilometres (22,859 miles) long. For regular trips around the world, the rule is a bit easier, and the distance is set to at least 36,770 kilometres (22,850 miles).