Daily Dose · Maps / November 2, 2026 / Lines of Latitude
The Tropic Won’t Stay Put
The Tropic of Cancer is drawn on every globe as if it were fixed. It isn’t. Its latitude is set by Earth’s tilt, the tilt is slowly relaxing, and so the line is creeping toward the equator, leaving its monuments behind.
15 m
a year, southward. That’s how far the Tropic of Cancer slides as Earth’s axial tilt shrinks by about 0.47 arcseconds a year. It sat at 23°27′N in 1917 and will cross 23°26′N around 2045.
Schematic equirectangular map · 8°–38°N · vertically stretchedShaded band = where the line wanders over a 41,000-yr tilt cycle
On land, the line crosses → Mexico · Bahamas · Western Sahara · Mauritania · Mali · Algeria · Niger · Libya · Chad (grazes) · Egypt · Saudi Arabia · UAE · Oman · India · Bangladesh · Myanmar · China · Taiwan. In the Pacific it passes just south of Necker Island, Hawaiʻi.
01 · The rule
A line drawn by the Sun
The tropic marks the farthest-north spot where the noon Sun stands straight overhead at the June solstice. Its latitude equals Earth’s tilt, and that tilt isn’t constant.
Over roughly 41,000 years Earth’s axis nods between about 22.1° and 24.5°. Today it is 23.44° and falling. Even the name is out of date: the line was named in the last centuries BC, when the June Sun sat in the constellation Cancer. Thanks to precession, it now rises in Taurus.
02 · The drift
Two kilometres a century, give or take
At about 0.47″ a year, the tropic loses one full arcminute of latitude every ~128 years. That’s slow, but surveyors can measure it.
03 · Zoom: Shuishang, Chiayi
The monument the line walked away from
Taiwan first marked the tropic in 1908. By our calculation, the 2026 line now runs about 2 km south of the monument’s published coordinates.
Chiayi’s marker has been rebuilt and replaced several times: first raised in 1908, rebuilt in 1921, shifted about 50 m west in 1923, and replaced by today’s building in 1995. Hualien’s Ruisui marker (1933) sits at 23°27′56″N, also north of the current line.
In Mexico, on Federal Highway 83 near Ciudad Victoria, a row of yearly markers from 2005 to 2010 tracked the drift step by step. The signs have since been removed.
04 · Living on the line
Eleven of these cities sit inside the sweep zone
The tilt cycle moves the line across a band about 266 km wide. Many large cities fall inside it, so over millennia the tropic will pass right over them.
05 · Two tropics
The line shrinks; the climate belt grows
The astronomical tropics are narrowing by a few hundred metres a decade. The climate tropics, the belt of tropical weather, are widening far faster.
≈40%
of the world’s people lived in the tropics in the 2014 State of the Tropics report
≈50%
projected share of humanity living in the tropics by 2050
0.5°
of latitude per decade: observed widening of the tropical belt since 1979 (Staten et al., 2018)
Why it matters · what to notice on the map
A line on a map, set by the sky
Look at the shaded band. Over its 41,000-year cycle the tropic wanders across that whole stripe, and Havana, Dhaka, Muscat and Guangzhou all sit inside it.
Look at the arrow. The line is heading south right now at about 15 m a year. Every tropic monument built on the line ends up north of it.
Notice what the map can’t show. A century of drift (~1.5 km) is thinner than the printed line, which is why a zoom is needed to see it.
Don’t confuse the two tropics. The line is astronomy. The tropical climate belt is spreading poleward hundreds of times faster, toward places where a growing share of humanity lives.