Tropic of Cancer

The Tropic of Cancer marks the northernmost latitude on Earth where the Sun can appear directly overhead, an event that happens during the June solstice when the Northern Hemisphere tilts maximally toward the Sun. At the same latitude, during the December solstice’s solar midnight, the Sun sits 90 degrees below the horizon in every direction.

It is the northern of the two tropics; its southern counterpart, the Tropic of Capricorn, marks the southernmost point where the Sun can be directly overhead. Together, these two lines are among the seven major circles of latitude used on maps—alongside the Arctic and Antarctic circles, the Equator, and the Poles (which are considered degenerate circles). Their positions relative to the Equator depend on the tilt of Earth’s rotational axis compared to its orbital plane. Because that tilt shifts over time, the circles move as well. Using a continuously updated formula, the Tropic of Cancer currently sits at 23°26′09.0″ (or 23.43582°) north of the Equator.

Geopolitically, this line is notable as the southern limit for NATO’s mutual defense obligation: member states are not required to defend territory south of the Tropic of Cancer.

**Name** When this latitude was named in the last centuries BC, the Sun occupied the zodiac sign Cancer (Latin for Crab) during the June solstice (at 90° ecliptic longitude). Due to the precession of the equinoxes, that is no longer true; today, the Sun is in Taurus during the June solstice. The word “tropic” comes from the Greek *tropē* (τροπή), meaning turn or inclination, referencing how the Sun appears to “turn back” at the solstices.

**Drift** The Tropic of Cancer’s position is not fixed. It shifts because of axial precession—Earth’s longitudinal alignment relative to the ecliptic (the plane of its orbit) changes over time. Earth’s axial tilt varies over a 41,000-year cycle from about 22.1 to 24.5 degrees; as of 2000, it was roughly 23.4 degrees, a value that will hold for about a millennium. This wobble causes the Tropic of Cancer to drift southward by nearly half an arcsecond (0.468″) of latitude—about 15 meters (49 feet)—per year. The circle was exactly at 23°27′N in 1917 and will reach 23°26′N in 2045. The latitudinal distance between the Antarctic Circle and the Tropic of Cancer is always very close to 90°, with any slight difference due to Earth’s oblateness.

**Geography** North of the Tropic lie the subtropics and the North Temperate Zone. The equivalent line south of the Equator is the Tropic of Capricorn, and the region between them, centered on the Equator, is the tropics. In the year 2000, more than half of the world’s population lived north of the Tropic of Cancer. During the summer solstice, locations on this line get about 13 hours, 35 minutes of daylight; during the winter solstice, about 10 hours, 41 minutes. Using 23°26′N as the reference, the Tropic passes through 17 countries (including two disputed territories) and 8 water bodies, starting at the prime meridian and heading eastward.

**Climate** The climate along the Tropic of Cancer is generally hot and dry, except in cooler highland regions in China, marine environments like Hawaii, and easterly coastal areas where orographic rainfall can be very heavy—in some places reaching 4 meters (160 inches) annually. Most regions here experience two distinct seasons: an extremely hot summer with temperatures often hitting 45 °C (113 °F) and a warm winter with highs 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 torrid monsoonal, with a short wet season from June to September and very little rain the rest of the year. The highest mountain on or adjacent to the Tropic is Yu Shan in Taiwan, which had glaciers descending as low as 2,800 meters (9,190 feet) during the Last Glacial Maximum. Glaciers still exist near the Tropic today; the nearest surviving ones are the Minyong and Baishui in the Himalayas to the north and Pico de Orizaba in Mexico to the south.

**Circumnavigation** According to Fédération Aéronautique Internationale rules, a flight competing for a round-the-world speed record must cover a distance no less than the length of the Tropic of Cancer, cross all meridians, and end at the same airfield where it started. The length of the Tropic of Cancer is 36,788 km (22,859 mi), calculated using the formula: *l = 2π cos(φ) 6378137 (1 − 0.00669438 (sin(φ))²)⁻⁰·⁵* where φ is the latitude of the Tropic. For an ordinary circumnavigation, the rules are somewhat relaxed, and the distance is set to a rounded value of at least 36,770 km (22,850 mi).