The Tropic of Capricorn

The Tropic of Capricorn is the southern boundary of the tropics. It is the southernmost latitude at which the Sun can stand directly overhead at any moment during the year, and the latitude where the Sun appears overhead on the December solstice. With the /learn/the-tropic-of-cancer support covering the northern counterpart, the two articles together describe the boundaries of the tropical belt the /learn/the-equator pillar bisects. This article covers the same physical geometry from the Southern-Hemisphere vantage: the definition, the December-solstice astronomy, the 10 countries the line crosses, the desert pattern it produces, and the precessional drift that has moved the December-solstice Sun out of its eponymous constellation.

Definition

The Tropic of Capricorn is the parallel at which the subsolar point — the point on Earth's surface where the Sun is directly overhead at any given moment — reaches its southernmost annual extent. The same axial obliquity ε that fixes the Tropic of Cancer at +ε fixes the Tropic of Capricorn at −ε. Per the NASA Earth Fact Sheet, ε is currently 23.4366°. The Tropic of Capricorn thus sits at about −23°26′22″ south, with the exact value evolving according to the IAU's obliquity formula:

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

where T is centuries since J2000.0. Because obliquity is currently decreasing at about 0.47 arcseconds per year, the Tropic of Capricorn is drifting north by about 14.5 metres per year. The Tropic of Cancer drifts south by the same amount: the two converge slowly toward the equator on the descending half of the 41,000-year Milankovitch obliquity cycle.

The two tropics are mirror images about the equator. Every property of the Tropic of Capricorn — its physical position, its astronomical significance, the climatic pattern that descends to its surface, even the naming convention — is the Tropic-of-Cancer story reflected through 0° latitude.

The December solstice

The Sun reaches the Tropic of Capricorn at solar noon on the December solstice, the moment Earth's southern axis is tilted most directly toward the Sun. Per USNO data, the solstice falls on December 21 or 22 in the Northern Hemisphere (occasionally December 20 in some time zones).

At that moment:

• Subsolar latitude = −ε ≈ 23.4366° S
• Day length at the Tropic of Capricorn reaches its annual maximum (about 13 hours 35 minutes for an unobstructed horizon)
• Day length is 24 hours everywhere south of the Antarctic Circle
• Day length is 0 hours everywhere north of the Arctic Circle

This is the height of Southern-Hemisphere summer. After the December solstice, the subsolar point migrates north again; on the March equinox it crosses the equator and, at the June solstice, reaches the Tropic of Cancer. The seasonal cycle in the tropics is the annual oscillation of the subsolar point between these two parallels.

Geography of the line

The Tropic of Capricorn crosses 10 sovereign states. Going eastward from the prime meridian:

| Region | Notable crossings | |---|---| | South Atlantic Ocean | The line begins in the open ocean off west Africa | | Namibia | Through the Namib desert; the line passes just south of Windhoek | | Botswana | Across the Kalahari Desert | | South Africa | Just clips the northern Limpopo province | | Mozambique | Across central Mozambique near Inhambane | | Indian Ocean | Through the Mozambique Channel | | Madagascar | Across the central island near Antsalova | | Indian Ocean | Across the open ocean | | Australia | Through Western Australia, the Northern Territory (just south of Alice Springs), and Queensland, crossing the coast near Rockhampton | | Pacific Ocean | Through French Polynesia (just south of Tahiti) | | Chile | Across the Atacama Desert; the city of Antofagasta is on the line | | Argentina | Across Salta, Catamarca, La Rioja, Córdoba, and Santa Fe provinces | | Paraguay | Through the central Gran Chaco | | Brazil | Through Mato Grosso do Sul, Paraná, and São Paulo — São Paulo city, at 23.55° S, is on the line | | South Atlantic Ocean | Back to the prime meridian |

The Australian crossing is particularly long; the continent spans about 3,800 km of longitude at the latitude of the Tropic, making Australia the country with the most Tropic-of-Capricorn frontage by far. Brazil's crossing passes within metres of central São Paulo — the most populous city on either tropic, with about 12 million residents in the city proper.

The southern subtropical highs

The descending branch of the Southern-Hemisphere Hadley cell sinks to the surface at the Tropic of Capricorn for the same reason it sinks at the Tropic of Cancer in the north: per the NOAA JetStream service, the Hadley cell lifts moist warm air at the equator (the ITCZ described in /learn/the-equator) and drops dry cool air at about 30° in each hemisphere. The descending dry air at the southern branch produces a Southern-Hemisphere subtropical-high belt.

The visible consequence is the southern desert chain:

• The Atacama Desert in northern Chile, the driest non-polar desert on Earth. Some weather stations in the Atacama have measured no rainfall in the entire historical record.
• The Namib Desert along the Atlantic coast of Namibia, one of the oldest deserts on the planet.
• The Kalahari Desert in Botswana, a semi-arid savannah rather than a sand desert but shaped by the same descending-air dynamics.
• The Australian Outback, including the Great Sandy Desert, the Tanami Desert, and the Simpson Desert.

The Southern Hemisphere subtropical highs are usually less continuous and less intense than their Northern counterparts, in part because the Southern Hemisphere has more ocean and less land at the subtropical-high latitudes. The Atacama, however, is more arid than the Sahara — a consequence of cold currents along the Pacific coast intensifying the descending Hadley flow.

Etymology and the precessional drift

The name “Tropic of Capricorn” comes from the constellation Capricornus, the sea-goat. Like the Tropic of Cancer, the name was established roughly 2,000 years ago when the December-solstice Sun was in Capricornus. Earth's precession has since shifted the December-solstice Sun westward through the zodiac at about 50.3 arcseconds per year. The December-solstice Sun is currently in Sagittarius, having departed Capricornus around AD 130, and will enter Scorpius around the year 2270.

The frozen-historical-state pattern is the same as on the Cancer side: the “tropic” part of the name continues to describe the astronomy faithfully (the Sun's southward turning), while the constellation-name part is purely commemorative.

Why the Capricorn climate differs from Cancer

Although the geometry is identical, the climate at the Tropic of Capricorn is meaningfully different from the climate at the Tropic of Cancer for one reason: the distribution of land and ocean.

At the latitude of the Tropic of Capricorn, only about a fifth of the surface is land — South America's narrow southern cone, the southern part of Africa, Madagascar, the Australian continent, and a scatter of islands. At the Tropic of Cancer, about two-fifths of the surface is land — the full Sahara, the Arabian peninsula, the Indian subcontinent, southern China, and Mexico. Land heats and cools faster than ocean, so the Northern subtropical-high belt drives more intense seasonal swings and more contiguous deserts than the Southern belt.

The Southern subtropical highs are also dominated by oceanic high-pressure systems — the South Pacific High and the South Atlantic High — that sit over open water rather than land. The pattern produces a more uniform climate around the Tropic of Capricorn but does not weaken its desert signature: the Atacama is the world's most arid non-polar desert, partly because the cold Humboldt Current along the Chilean coast magnifies the descending-air drying effect.

The astronomical infrastructure of the Southern Hemisphere is concentrated near the Tropic of Capricorn for the related reason that the southern subtropical highs produce exceptionally clear, dry skies. The European Southern Observatory's Paranal site (24°37′ S) and the ALMA array on the Chajnantor Plateau (23°01′ S) both lie within a degree of the Tropic, taking advantage of the same descending Hadley air that makes the Atacama a desert.

The two tropics together

The two tropics together define the tropical belt — the latitudes between which the subsolar point falls during the year. The full tropical belt spans about 47° of latitude (≈ 2ε). It contains roughly 36% of Earth's surface area, including most of South America, all of Africa between the Sahara and the Kalahari, the Indian subcontinent, Southeast Asia, much of Australia and Oceania, and the Caribbean.

Within the tropical belt, no point is ever far from a year in which the Sun is directly overhead at noon. North or south of the belt, the Sun is never directly overhead — its highest noon elevation is always less than 90°. This geometric fact is the foundation of every “why is it hotter near the equator” explanation: more direct sunlight per unit area, year-round. The qualifying caveats — descending air at the subtropical highs creates deserts, the equator itself is moderated by the ITCZ — were covered in this article and in /learn/the-tropic-of-cancer.

Markers and crossings

A few of the more visible Tropic-of-Capricorn monuments and crossings:

• Brazil — a monument outside São Paulo on highway BR-116 marks the line; another at Foz do Iguaçu in Paraná.
• Argentina — a monument near La Quiaca on Route 9.
• Chile — Antofagasta city itself straddles the line; informal markers along the Pan-American Highway.
• Australia — markers at Newman in Western Australia, on the Stuart Highway in the Northern Territory near Aileron, and at Rockhampton in Queensland (the “Tropic of Capricorn Marker” on the Capricorn Highway).
• Namibia — a roadside marker on the B1 highway between Windhoek and Mariental.

As with the Tropic of Cancer monuments, none of these is repositioned to track the northward drift; the actual line as of 2026 sits a few metres north of each marker, with the discrepancy growing slowly as obliquity decreases. The Rockhampton marker in Queensland is the most photographed of the southern monuments and the only one positioned to permit a tourist to stand simultaneously in tropical and subtropical Australia.

Sources

• NASA, Earth Fact Sheet — obliquity to orbit.
• IAU WGCCRE Cartographic Coordinates and Rotational Elements (2015) — obliquity formula and secular variation.
• USNO Earth's Seasons — solstice dates.
• NOAA JetStream — Atmospheric Circulation — Hadley cell meteorology.
• USGS GNIS — geographic names.

For the polar-circle counterparts and the full set of named parallels, see /learn/the-arctic-circle, /learn/the-antarctic-circle, and /learn/parallels-of-latitude.