Tropic of Capricorn

Lore & Background

The Tropic of Capricorn is the southernmost latitude where the Sun can appear directly overhead, an event that occurs at the December solstice. At the June solstice, the Sun at solar midnight is 90 degrees below the horizon in all directions from this circle. Its current latitude is approximately 23.4 degrees south of the Equator, but this position is not fixed.

Due to a wobble in Earth’s axial tilt, which varies over a 41,000-year cycle between about 22.1 and 24.5 degrees, the Tropic is drifting northward at a rate of nearly half an arcsecond (0.468″) of latitude, or about 15 meters, per year. In 1917 it was exactly at 23°27′S, and it will reach 23°26′S in 2045. The distance between the Arctic Circle and the Tropic of Capricorn remains essentially constant. The length of this circle at 23°26′11.7″S is roughly 36,788 kilometers.

During the June solstice (Southern Hemisphere winter), daylight lasts about 10 hours and 41 minutes; during the December solstice (Southern Hemisphere summer), it lasts about 13 hours and 35 minutes. The Tropic divides the Southern Temperate Zone to the south from the Tropics to the north. Its name originates from when the Sun was in the zodiacal constellation Capricorn at the northern solstice; due to precession of the equinoxes, the Sun now appears in Sagittarius at that solstice.

A Slowly Migrating Boundary: The Shifting Axial Tilt

Perhaps the most fascinating aspect of the Tropic of Capricorn is that it is not, in any strict sense, a fixed line. Its latitude depends entirely on the obliquity of the ecliptic—the tilt of Earth's axis relative to the plane of its orbit around the Sun—and that tilt is in constant, slow flux. These changes, though imperceptible in a human lifetime, mean that the Tropic of Capricorn is a slowly migrating boundary whose position will shift noticeably over tens of thousands of years.

Reader's Guide

The Tropic of Capricorn is the dividing line between the Southern Temperate Zone to the south and the Tropics to the north. Its position is not fixed, but constantly changes because of a slight wobble in the Earth's longitudinal alignment relative to its orbit around the Sun. Earth's axial tilt varies over a 41,000 year period from about 22.1 to 24.5 degrees and currently resides at about 23.4 degrees. This wobble means that the Tropic of Capricorn is currently drifting northward at a rate of almost half an arcsecond (0.468″) of latitude, or 15 metres, per year (it was at exactly 23°27′S in 1917 and will be at 23°26'S in 2045).

Therefore, the distance between Arctic Circle and the Tropic of Capricorn is essentially constant moving in tandem. There are approximately 10 hours and 41 minutes of daylight during the June solstice (Southern Hemisphere winter). During the December solstice (Southern Hemisphere summer), there are 13 hours and 35 minutes of daylight. The length of the Tropic of Capricorn at 23°26′11.7″S is 36,788 km (22,859 mi).