Tropic of Cancer

The Tropic of Cancer marks the northernmost latitude on Earth where the Sun can appear directly overhead, which happens during the June solstice when the Northern Hemisphere tilts most toward the Sun. At that same latitude, during the December solstice, the Sun sits 90 degrees below the horizon at solar midnight from every direction. It is the northern of the two tropics; its southern counterpart, where the Sun can be seen directly overhead at its most southerly point, is the Tropic of Capricorn. These two circles are among the seven major lines 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, and because that tilt shifts over time, the circles move as well. Using a regularly updated formula, the Tropic of Cancer currently sits at 23°26′09.0″ (or 23.43582°) north of the Equator. In geopolitics, it is notable as the southern limit for NATO’s mutual defense obligation: member states are not required to defend territory located south of this line.

The name dates back to the last centuries BC, when the Sun occupied the zodiacal constellation Cancer (Latin for Crab) at the June solstice (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,” meaning turn or inclination, referencing how the Sun appears to reverse direction at the solstices.

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

North of the Tropic of Cancer 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. As of the year 2000, more than half 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 of Cancer passes through 17 countries (including two disputed territories) and 8 bodies of water, starting at the prime meridian and heading eastward.

The climate along the Tropic of Cancer is generally hot and dry, except for cooler highland areas in China, marine environments like Hawaii, and easterly coastal zones where orographic rainfall can be very heavy—in some places reaching 4 meters (160 inches) annually. Most regions here have 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 of Cancer belongs to 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 of Cancer 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 closest surviving ones are the Minyong and Baishui in the Himalayas to the north and Pico de Orizaba in Mexico to the south.

According to the Fédération Aéronautique Internationale, for a flight to qualify for a round-the-world speed record, it 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 Tropic of Cancer’s length 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. For an ordinary circumnavigation, the rules are more relaxed, and the required distance is rounded to at least 36,770 kilometers (22,850 miles).