Titanic global forces create a band of deserts along the Tropic of Capricorn at 23 degrees South. Warm air above equatorial oceans cools as it rises, creating torrential rains. Scrubbed of moisture, the air descends clear and brittle-dry along the tropics, creating a band of drylands. Other factors intensify - or offset - this global aridity: distance from the sea, rain-shadow effects of nearby mountains, and cold ocean currents that slow the supply of moisture to the atmosphere. All of the deserts in Extremes are linked by the Tropic of Capricorn.
23 degrees South
How do you know you're standing on the Tropic of Capricorn? One way is to head north from Alice Springs. When you pass the 'Tropic of Capricorn' monument on the side of the road you know you're there. If you're in Namibia on the west coast of Africa, head south of Walvis Bay and you will soon find another road marker (the signs are now in English, but they used to read 'steenbokskeerkring' or 'antelope circle'). Or, when in Chile, head inland from Antofagasta and by the highway you'll see a large rock inscribed 'Linea del tropico de capricornio'. All these spots are linked by an invisible line running around the earth at 23º 27' S: that's the Tropic of Capricorn.
There are other ways to figure it out. In fact the Tropic of Capricorn relates to the sun and is not, as commonly thought, the half-way line between the equator and the South Pole, although it is close. The tropics of Capricorn and Cancer are calculated by the movement of the sun in relation to the earth. The Tropic of Capricorn marks the farthest point south at which the sun can be seen directly overhead at noon. That day is the longest day of the year and is the summer solstice on our side of the equator.
So, if on 22 December at noon you find yourself in the Northern Territory, Namibia or northern Chile, and the sun is directly overhead, you will be standing at latitude 23º 27'. That means you will be standing on the Tropic of Capricorn and you will also be standing in one of the incredible southern deserts.
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