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Last Updated: Thursday, 31 January 2008, 18:29 GMT
What is The Tropic of Capricorn?
A new four-part series for BBC Two, Tropic of Capricorn, charts a 20,000 mile-journey around the world. But what is Capricorn? Series Producer Sam Bagnall explains.

Prog 1: Namibia and Botswana Sunday 10 February, 2008
2000 GMT on BBC Two

From distant memories of geography lessons at school, most people are vaguely aware that the Tropic of Capricorn is one of two lines around the earth, quite near the Equator.

For the record, Capricorn is the southern one, whereas the Tropic of Cancer lies to the north.

Capricorn cuts through South America, Southern Africa and Australia, as well as thousands of miles of empty ocean. But what is it?

The answer to that question is not as straightforward as you might think.

When our team filmed their journey around the Equator in 2006, it was relatively simple - zero degrees latitude, the centre of the world, the border between the Northern Hemisphere and the Southern.

Shifting location

Capricorn is a little more tricky. The best and pithiest definition appears to be that the Tropic of Capricorn is the point furthest south where the sun can be seen overhead.

This occurs at noon on the Summer solstice in the Southern hemisphere.

But there is much more to it than that.

The earth is not a sphere, it is an ellipsoid, and because the position of the line changes so does its length
Mick Ashworth, editor-in-chief of The Times Comprehensive Atlas of the World, and his colleague Kenny Gibson dropped the first bombshell: the Tropic of Capricorn moves.

The angle of the earth's tilt changes over time - hence its angle to the sun, and hence the position of the Tropic.

Apparently it moves up to 15 metres a year over a 40,000 year cycle.

Prog 2: South Africa to Madagascar
Sunday, 17 February, 2008
2000 GMT on BBC Two

At the moment it is around 23 degrees 26 minutes or 23.4 degrees of latitude.

Variable length

Finding out the length of the line is a struggle too.

The earth is not a sphere, it is an ellipsoid, and because the position of the line changes so does its length.

On 19 November 2007, Kenny calculated that Capricorn was approximately 36,748,889.697 metres long - that is 22,835 miles.

By swiftly consulting something called Vincenty's formula, Kenny calculated the distance between Capricorn and its sister latitude Cancer to be 5,186,148.744 metres, just under 3,200 miles.

But that was on 19 November 2007 - it is different now.

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And just to get the statistics out of the way, 75.67% of Capricorn is over sea - mostly the Pacific Ocean.

Of the 24.33% that covers land, the country with by far the biggest section is Australia at 2350 miles.

Night Equinox

For our programme we wanted to demonstrate that on the Summer Solstice in the Southern Hemisphere at noon the sun is directly overhead - neatly encapsulating the whole concept of the Tropic of Capricorn.

Our schedule was always going to land us up at the end of the journey just before Christmas in Brazil - where the Summer Solstice fell on 22 December.

But a quick check on the US National Observatory website showed that the Solstice fell at 06.08 GMT - that is 04.08 in the morning in Brazil. Not ideal for solar observations.

It was time for the Geographers to step aside and for us to seek some astronomical guidance.

'Local noon'

The only place on the Tropic of Capricorn where the sun would be truly overhead was where the solstice fell precisely at local noon
According to a colleague, Russell Eberst of the Royal Observatory in Edinburgh was something of an anorak when it came to matters like this.

"If Russell tells you that you have to stand on one leg holding a bowl of custard to observe the Solstice on the Tropic - then I would believe him."

Prog 3: Australia
Sunday, 24 February, 2008
2000 GMT on BBC Two

So what about seeing the sun overhead at midday on the solstice in Ubatuba - the small town on the coast of Brazil that lies on the Tropic of Capricorn?

The first problem Russell highlighted was not only does the Tropic move, so does noon.

In fact what we consider to be 12 o'clock, midday, is really just an approximation.

Until Greenwich Mean Time was imposed in Britain in 1880, every town set its own time by taking a measurement of the sun in the sky. So Bristol was 16 minutes behind London. It played havoc with train timetables.

'Equation of time'

In reality each point along a line has its own "local noon" or "solar noon" where the sun is at its highest point. At Ubatuba that could be anything up to an hour different to local Brazilian time.

Prog 4: Chile to Brazil
Sunday, 2 March, 2008
2000 GMT on BBC Two

And what is more the only place on the Tropic of Capricorn where the sun would be truly overhead was where the solstice fell precisely at local noon.

Russell estimated that this would be around 88 degrees of longitude - somewhere in the middle of the Indian Ocean south of Calcutta.

To get it completely right we had to take account of another variable.

"Although the Earth's rate of spin is uniform, the speed of its orbit round the Sun changes. The correction is called 'the Equation of Time', and can easily be looked up," explained Dave Rothery of the Open University.

Apparently this could alter our observation time by up to 20 minutes.

Scientific consensus

It was getting complicated. But then it dawned that this was a current affairs documentary, not The Sky at Night.

The scientific consensus was that the degree to which the sun was not exactly overhead in Ubatuba at noon on Capricorn was 'insignificant' or 'negligible'.

"I'm honestly not sure that the sun will be directly overhead at noon on the solstice. But surely for the whole point of what the tropic means, it's probably good enough," said Dr Carolin Crawford of the Institute of Astronomy at Cambridge.

Miraculously the sun was out in the notoriously wet town on 22 December and we were able to experience that little moment when the sun is overhead for the one and only time in the year on this line.

So there you have it - the end of a long journey and the Tropic of Capricorn explained. Almost.



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