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Tropic of Capricorn

Tropic of Capricorn.....................(3260.jpg)

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.

Tropic of Capricorn.....................(3259.jpg)

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.

Tropic of Capricorn.....................(3261.jpg)

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.

Tropic of Capricorn.....................(3258.jpg)

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


Images: (top, Namibia) Hans Hillewaert; (second from bottom, Brazil) Hermógenes Teixeira Pinto Filho; (bottom, solar observatory at Tropic of Capricorn, Chile) Marcos Escalier

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  • DAY

    August 30, 2016

    I have been noticing how the sun is shining on the northern side of my house, more and more for the last twenty years,,, Now, I am sure of it…

  • Bill

    July 12, 2013

    Is the 40,000 yr cycle influenced by the moon, sun, or other planetary alignments?

  • Ray

    February 7, 2013

    When it is summer in the Southern Hemisphere the average mean temperature of the South Pole is -28C while the North Pole is -40C. (ref Woods Hole Oceanographic Inst.)
    As for when the sun is actually over the Tropic of Capricorn on any particular year the actual temperatures at the poles could vary a lot depending on climatic factors.

  • Laura

    December 30, 2012

    What happens when the sun is over the tropic of capricorn? would the south pole be colder than the north pole?

  • Roshan

    July 2, 2008

    Good point – but that depends on your perspective, or more specifically where in the world you live. The sun reaches vertical position over the Tropic of Capricorn around Dec 22, the northern hemisphere’s winter solstice but the southern hemisphere’s summer solstice.

  • Dave

    July 2, 2008

    The sun is directly overhead at the Tropic of Capricorn on the Winter solstice, not the Summer solstice which is what have written in this article.

Any comments?