The Tropic of Cancer

The Tropic of Cancer is the northern boundary of the tropics — the belt within which the Sun can be directly overhead at some moment during the year. It is one of the five named parallels covered by the /learn/parallels-of-latitude support, and the southern counterpart in the /learn/the-tropic-of-capricorn support covers the same physics in the Southern Hemisphere. This article covers the astronomical definition that fixes the line at its latitude, the geography of the line across three continents, the climatic significance that turns much of it into desert, and why the “Cancer” in the name now refers to a constellation the Sun no longer occupies at the June solstice.

Definition

The Tropic of Cancer is the parallel of latitude at which the subsolar point — the point on Earth's surface where the Sun is directly overhead at any given moment — reaches its northernmost annual extent. That extent is set by the tilt of Earth's rotation axis relative to the orbital plane: the obliquity of the ecliptic, ε.

Per the IAU's working group on cartographic coordinates, the obliquity at J2000.0 was 23°26′21.448″, with secular variation given by

ε(t) = 23°26'21.448" − 46.8150"·T − 0.00059"·T² + 0.001813"·T³

where T is centuries since J2000.0. At the current epoch (2026) the value is about 23°26′22″, with the actual latitude of the Tropic of Cancer being exactly this number on Earth's surface. The NASA Earth Fact Sheet publishes the obliquity to four decimal places as 23.4366°, which agrees to within an arcsecond.

This means the Tropic of Cancer is not a fixed line — it migrates with the obliquity. Over a 41,000-year cycle the obliquity oscillates between about 22.1° and 24.5°, so the Tropic of Cancer wanders by about 2.4° of latitude (about 265 km of north–south range) on geologic time scales. Across a single year the line is fixed; across a single human lifetime it shifts by about a metre. The /learn/why-the-earth-is-not-a-sphere support covers the related precession and other Milankovitch cycles in more detail.

The June solstice

The Sun reaches the Tropic of Cancer at solar noon on the June solstice, the moment Earth's northern axis is tilted most directly toward the Sun. Per US Naval Observatory data, the June solstice falls on June 20 or 21 in the Northern Hemisphere (occasionally June 22 in some time zones), with the exact moment shifting about 6 hours each year and a 23-hour resync at every leap year.

At that moment:

• Subsolar latitude = +ε ≈ 23.4366° N
• Day length on the Tropic of Cancer is the year-long maximum at this latitude (about 13 hours 35 minutes for an unobstructed horizon)
• Day length is 24 hours everywhere north of the Arctic Circle (a fact covered in the /learn/the-arctic-circle support)
• Day length is 0 hours everywhere south of the Antarctic Circle

The day after the solstice, the subsolar point begins to migrate south again. On the September equinox, it crosses the equator and continues into the Southern Hemisphere. On the December solstice, it reaches the Tropic of Capricorn at −23.4366°, then turns back. This annual cycle of the subsolar point — bounded by the two tropics — is the geometric definition of seasons in the tropical belt.

Geography of the line

The Tropic of Cancer crosses 16 sovereign states across three continents and three oceans. Going eastward from the prime meridian, with the line in the open Atlantic:

| Region | Notable crossings | |---|---| | Western Sahara (disputed) | Just south of Dakhla | | Mauritania | Tiris Zemmour desert region | | Mali | Far north (uninhabited Saharan reaches) | | Algeria | Across the Tassili n'Ajjer | | Niger | Far north Ténéré desert | | Libya | Southern Fezzan | | Egypt | Near Aswan; just south of Lake Nasser | | Saudi Arabia | Across the Najd region | | United Arab Emirates | Grazes the southern Empty Quarter | | Oman | Western Empty Quarter | | India | Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura | | Bangladesh | Just south of Dhaka through central Bangladesh | | Myanmar | Through Magway and Sagaing regions | | China | Yunnan, Guangxi, Guangdong; the line passes near Guangzhou | | Taiwan | Across the southern part of the island, including Chiayi | | Pacific Ocean | Across the open ocean, including just south of Hawaii | | Mexico | Baja California Sur, Sinaloa, Durango, San Luis Potosí, Tamaulipas | | Bahamas | Through the central archipelago |

The mainland landmass crossed is dominated by the Sahara, the Arabian Peninsula, the Indian subcontinent, southern China, and the Mexican Plateau. The line passes through more named deserts than any other parallel.

Notable points on or near the line include the Karnak temple complex in Egypt (slightly north), the Pyramid of Khufu at Giza (well north), the city of Aswan (south of the line), Kolkata (just north), Guangzhou (north), and Mexico's state capital cities of Hermosillo and Culiacán (both south).

The Hadley cell and the subtropical highs

The Tropic of Cancer marks the latitude where the descending branch of the Hadley cell sinks to the surface. The Hadley cell is the planet-scale atmospheric circulation that lifts moist warm air at the equator (the /learn/the-equator pillar covers the ITCZ) and drops dry cool air at about 30° N and 30° S. The descending air at the cell's poleward branch reaches the surface near the tropics.

Per NOAA's JetStream educational service, sinking air warms and dries as it compresses; cloud formation is suppressed and rainfall is sparse. The result is the planet's subtropical-high belt — the band of permanent high-pressure cells that dominate weather at this latitude. The visible expression is the chain of subtropical deserts: the Sahara, the Arabian Desert, the Thar Desert in India, the Mexican Plateau deserts, and (in the Southern Hemisphere on the matching Capricorn parallel) the Kalahari, the Namib, the Atacama, and the Australian Outback.

Deserts are not the only signature. Mediterranean climates on the western sides of continents at this latitude (California, central Chile, the southern tip of Africa, Western Australia, the Mediterranean basin itself) are also driven by the seasonal migration of the subtropical high. The trade winds — the easterly winds between the subtropical high and the ITCZ — are the equatorward arm of the same Hadley circulation.

Etymology and the precessional drift

The name “Tropic of Cancer” reflects astronomy as it was roughly 2,000 years ago. Tropic derives from the Greek τροπή (tropē), “turning” — the parallel where the Sun's northward journey turns and begins moving south again. Cancer is the Latin name for the crab constellation: when the convention was established (the ancient Greek astronomers, refined by Ptolemy around AD 150) the June-solstice Sun was in the constellation Cancer.

It is not there any more. Earth's rotation axis itself precesses around the orbital pole on a 25,772-year cycle — a slow conical wobble that shifts the celestial coordinates of the equinoxes and solstices westward through the zodiac at about 50.3 arcseconds per year, or about one full constellation every 2,150 years. The June-solstice Sun is currently in Gemini, having departed Cancer around 1990 BC, and will enter Taurus around the year 2600.

The name “Tropic of Cancer” has not been updated. It is one of many cases where astronomical nomenclature was frozen at its historical state — the spring equinox is still called the “First Point of Aries” even though the equinox Sun has been in Pisces for centuries and will move into Aquarius in the late 26th century. The “tropic” part of the name still describes the physical phenomenon (the Sun's northward turning) faithfully; the “Cancer” part is purely historical.

Eratosthenes at Syene

The Tropic of Cancer figures in one of the earliest scientific measurements of Earth's shape. Around 240 BC the Greek polymath Eratosthenes, then chief librarian at the Library of Alexandria, learned that at solar noon on the June solstice the Sun cast no shadow in a deep well at Syene (modern Aswan). The well was “on the Tropic of Cancer” — Syene was within an arcminute of the line at the time, and the USNO records confirm the solstice geometry.

On the same day in Alexandria, about 800 km north, the Sun cast a clear shadow at noon. Eratosthenes measured the shadow angle as approximately 1/50 of a full circle — about 7.2°. He reasoned that if Alexandria and Syene were on the same meridian, and the Sun's rays were essentially parallel at Earth-distance, then the arc between them must subtend 7.2° at the centre of Earth. Multiplying by 50 gave the full circumference as 50 × 800 km ≈ 40,000 km — astonishingly close to the modern WGS 84 value of 40,075 km. Eratosthenes' measurement is the first quantitative estimate of Earth's size, and the Tropic of Cancer made it possible.

Markers on the ground

Several countries mark the line with monuments. The most prominent are in Mexico (a monument at Ciudad Mante on Mexican Federal Highway 85), in Taiwan (an obelisk in Chiayi County), and in China (a stone marker at Yongchuan, near the line's entry into Guangdong). The Indian state of Madhya Pradesh has a roadside marker near Rajgarh; Egypt has nothing visible at the line itself, although the city of Aswan a few kilometres south is the conventional reference point for “the Tropic” in Egyptian geography.

None of these monuments is rigorously placed — they were positioned with the surveying technology available at the time and most have not been moved to track the southward drift. The actual Tropic of Cancer as of 2026 lies a few metres south of each marker, and the discrepancy quietly grows by about 14.5 metres per century — a curiosity that is invisible at the scale of a tourist photo but accumulates over decades. The Taiwan monument at the village of Shuishang in Chiayi County is unusual in that the local government has rebuilt and repositioned it on multiple occasions over the past century, an effort that reflects the line's practical importance to Taiwanese geography rather than to navigation.

Sources

• NASA, Earth Fact Sheet — obliquity to orbit, 23.4366°.
• IAU WGCCRE Cartographic Coordinates and Rotational Elements (2015) — obliquity formula with secular variation.
• USNO Earth's Seasons — solstice and equinox dates.
• NOAA JetStream — Atmospheric Circulation — Hadley cell and subtropical-high meteorology.
• USGS GNIS — geographic-name authority for landmarks and crossings.

For the southern analogue, see /learn/the-tropic-of-capricorn; for the polar-circle counterparts, see /learn/the-arctic-circle and /learn/the-antarctic-circle.