Tropic of Capricorn
So you think you know where the Tropic of Capricorn is? The Equator, well, that's easy – it's the middle of the earth, zero degrees latitude, where north meets south. The Tropic of Capricorn would seem to be similarly located, sort of halfway towards the South Pole, right? More precisely, 23.5° south of the equator? Well, yes and no. Apparenty it's not that simple. Apparently the Tropic of Capricorn moves.
Technically it's the furthest point south at which the sun can be seen directly overhead. This happens at noon on the summer solstice. However, a recent BBC expedition following the Tropic of Capricorn exposed what a confusing little line it is. The planet isn't round, and the speed at which it orbits the Sun changes. As if that didn't confuse things enough, the angle of the earth's tilt changes, so the Tropic of Capricorn also changes, moving about 15 meters a year towards the equator.
The positions of the Tropic of Cancer, the Tropic of Capricorn, the Arctic Circle and the Antarctic Circle all depend on the tilt of the Earth's axis relative to the plane of its orbit around the sun. As the axis tilts, so do all these lines. So, all the useful Tropic of Capricorn markers around the world (some of which are pictured here) aren't really all that accurate. They can't be.
The Tropic of Cancer line is therefore also unstable, meaning that every year the Tropics decrease in size by about 1100 sq km. The Arctic and Antarctic Circles are also moving towards the Poles, meaning the polar regions are technically shrinking too. This all takes place over a 40,000 year cycle, but what happens at the end of that period isn't clear. Maybe it moves back slowly. I certainly hope it doesn't bounce back suddenly. RM