The Tropic of Cancer is where June’s sun stands overhead
Earth’s northernmost latitude with a vertical noon sun sits at the Tropic of Cancer on the June solstice, when the Northern Hemisphere tips toward the Sun. Capricorn mirrors it in the south. NATO even treats the line as a southern edge of mutual-defence duty.
The line took its name in the last few centuries BC, when the June solstice sun sat in the zodiac sign of the Crab. The slow precession of the equinoxes has since shifted that alignment, so today the solstice sun lies in Taurus instead. The word tropic comes from Greek tropē, a turning, because the sun seems to reverse course at each solstice. Six months after its overhead moment, at solar midnight on the December solstice, the sun sits a full 90 degrees below the horizon for anyone standing on the line.
Its latitude is not fixed. Earth's axial tilt swings between roughly 22.1 and 24.5 degrees over a 41,000-year cycle, and the line now creeps toward the Equator by close to half an arcsecond, about 15 meters, each year. It stood at exactly 23°27′N in 1917, currently sits near 23.436 degrees, and should reach 23°26′N in 2045. The gap between it and the Antarctic Circle always stays very close to 90 degrees.
Along the circle the longest day brings about 13 hours 35 minutes of daylight and the shortest about 10 hours 41 minutes. It crosses 17 countries, counting two disputed territories, plus eight bodies of water. The climate there is mostly hot and dry, with summer highs often near 45 °C and winter maxima around 22 °C; much of the land is Sahara, while wetter eastern coasts can receive up to four meters of rain a year. Taiwan's Yu Shan is the tallest peak on or beside the line, and in 2000 more than half of humanity lived to its north.
The circle is about 36,788 kilometers long, and that figure matters to aviators: the Fédération Aéronautique Internationale requires a round-the-world speed record attempt to fly at least that distance, cross every meridian, and land where it took off.
Source: Tropic of Cancer
