Wednesday, August 7, 2013

The Tropic of Capricorn Monument.

One of the five major circles of latitude that mark maps of the Earth, The Tropic of Capricorn (or Southern tropic) is the circle that contains the sub solar point on December 21st, the winter or southern solstice. It is the southernmost latitude where the Sun can be directly overhead. The northern hemisphere equivalent of the Tropic of Capricorn is the Tropic of Cancer. Extraordinarily, the position of the Tropic of Capricorn is not fixed, but varies over time. The reasons for this are pretty complex but as of 2013, its latitude is 23° 26' 14.908" south of the Equator. However, it is very gradually moving northward, currently at the rate of 0.47 arc seconds or 15 meters, a remarkable 48 feet per year. Most places along the Tropic of Capricorn have arid or semi-arid climates, though in Australia and Southern Africa this climate is exacerbated by the fact that tectonic activity and glaciations have been for the most part absent for about 300 million years. Here the aridity is compounded by extremely infertile soils, which explains all the famine in Southern Africa. In South America, the presence of the geologically young and evolving Andes means that regions in Brazil are on the western side of the subtropical anticyclones and receives warm and humid air from the Atlantic Ocean. As a result, these areas adjacent to the Tropic are extremely important agricultural regions, producing large quantities of crops including coffee. However, here on the west side of the Andes, the Humboldt Current makes conditions extremely dry and where no glaciers exist, helping to create the Atacama Desert, one of the driest deserts in the world. Vegetation as I have mentioned in previous blogs, is almost non-existent. Fortunately, unlike some of the southern nations of Africa, there is active trade among countries so even people who live along this section of the Tropic of Cancer in South America have access to food and water. Here in Antofagasta, The Tropic of Capricorn passes through the region to the north of the city, and is marked by a quite elaborate monument. The monument which was opened on December 21st, 2000 in celebration of this circle of latitude is clearly visible from the main road. Consisting of two parts, it is a huge steel structure with a copper surface that has developed a beautiful patina. Separated by about 18 inches (1/2 meter), the first structure is a large square frame and the second has an arched top. Depending on the time of year and how close to noon it is, the sun shines through it at various angles culminating at the winter solstice when the sun would be directly overhead. Since it was a hot day the sun cast plenty of shade around the structure for which Winston was eminently grateful, as we spent a long time reading the science behind the monument and taking photos. Given what we know, we could not work out the exact time of day at the solstice that the sun would divide the two parts or what affect the movement of the Tropic of Capricorn has on the monument but it was fascinating.

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