Tropic of Cancer
Northernmost latitude where the Sun appears directly overhead.
The Tropic of Cancer is the northernmost latitude on Earth where the Sun can appear directly overhead, an event that occurs during the June solstice when the Northern Hemisphere is tilted most toward the Sun. At the corresponding moment of solar midnight on the December solstice, the Sun at this latitude sits 90 degrees below the horizon in every direction. This circle is the northern counterpart of the Tropic of Capricorn, and together they form two of the seven major circles of latitude, which also include the Equator, the Arctic and Antarctic Circles, and the Poles. The position of the Tropic of Cancer is not fixed; it shifts due to changes in Earth’s axial tilt, which varies over a 41,000-year cycle between roughly 22.1 and 24.5 degrees. Currently, the tilt is about 23.4 degrees, and the Tropic drifts southward by nearly half an arcsecond of latitude per year. In 1917, it was exactly at 23°27′N, and by 2045 it will reach 23°26′N. The line’s name originates from the last centuries BC, when the Sun at the June solstice stood in the zodiacal constellation Cancer (the Crab); precession of the equinoxes has since moved the solstice Sun into Taurus. The word “tropic” comes from the Greek “tropē,” meaning turn or inclination, referencing the Sun’s apparent reversal at the solstices. Geopolitically, the Tropic of Cancer marks the southern limit of NATO’s mutual defense obligation. North of this line lie the subtropics and the North Temperate Zone, while the region between it and the Tropic of Capricorn is the tropics. In the year 2000, over half the world’s population lived north of this circle.
- Daylight at summer solstice
- 13 hours, 35 minutes
- Daylight at winter solstice
- 10 hours, 41 minutes
- Countries crossed
- 17 (including two disputed territories)
- Highest mountain on or adjacent
- Yu Shan in Taiwan
Lore & Background
Geographically, the Tropic of Cancer passes through 17 countries and 8 water bodies. North of it lie the subtropics and the North Temperate Zone. The climate along the tropic is generally hot and dry, except for cooler highland regions and easterly coastal areas with heavy orographic rainfall. Much land on or near the Tropic is part of the Sahara Desert. In geopolitics, the Tropic of Cancer serves as the southern limitation for the mutual defence obligation of NATO; member states are not obligated to defend territory south of this line. For aviation, the Fédération Aéronautique Internationale requires a round-the-world speed record flight to cover a distance no less than the length of the Tropic of Cancer and cross all meridians.
Reader's Guide
The Tropic of Cancer is a fundamental reference line in geography, astronomy, and geopolitics. Astronomically, it marks the northernmost latitude where the Sun can be directly overhead, a phenomenon tied to Earth's axial tilt and the June solstice. Its drifting position—currently moving southward by about 15 meters per year—illustrates the dynamic nature of Earth's orientation in space, influenced by axial precession and long-term tilt variations. This drift has practical implications for mapping and navigation, as the line's exact location must be updated continuously. Geographically, the Tropic of Cancer defines the northern boundary of the tropics, a region that contains diverse climates from the Sahara Desert to monsoonal areas. The line also appears in international law, notably as the southern limit of NATO's mutual defence commitment, affecting strategic planning. Culturally and historically, the name 'Tropic of Cancer' reflects ancient astronomical observations, though precession has since shifted the Sun's position at the solstice from Cancer to Taurus. Overall, the Tropic of Cancer remains a key marker of Earth's geometry, climate zones, and human systems.
Did You Know?
- The highest mountain on or adjacent to the Tropic of Cancer is Yu Shan in Taiwan.
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