Midwinter in Britain means midsummer in the southern hemisphere. With the sun moving into Capricorn, now is the moment to take off for the summer, along a line that girdles the Earth and marks the boundary between the Tropics and the rest

Tropic of Capricorn? Where's that?

Tropic of Capricorn? Where's that?

Parallel with the Equator, but 26? 30" south of it. It is the circle that marks points where the sun is directly overhead at noon on midwinter's day. The name was chosen because the constellation of Capricorn rises above it on the summer solstice. The line passes through Chile, Argentina, Paraguay, Brazil, Namibia, Botswana, South Africa, Mozambique, Madagascar, Australia and French Polynesia, clipping New Caledonia, Fiji, Tonga and the Cook Islands before landfall at Pitcairn.

So it's hot then?

In general, yes. The tropics experience little difference between seasons so, between the Tropic of Capricorn and the Tropic of Cancer (which is an equal distance north of the Equator), life is generally warm and sunny. However, there's a lot more variety here than you might think. The Atacama and Kalahari deserts, the steamy urban frenzy of Rio de Janeiro and the chilly peaks of the Andes mountains all lie beneath the Tropic of Capricorn. And the vast majority of the world's coffee is grown in this region.

Where does it start?

Strictly speaking, it should be at that other infamous imaginary marker, 0? longitude, but that's somewhere out in the Atlantic Ocean. The first landfall is the desert coast of Namibia, at the Sandwich Harbour. These days it's no longer a harbour. But a fleet of dedicated bird-watchers regularly steer their way here from the capital, Windhoek. This is more than can be said for Martin Luther, an old steam train that lies in ruins stranded in the desert a few miles east of Swakopmund. Not unlike its zealous 16th-century namesake, this train sits passively before God. And like its locomotive counterpart in Conrad's Heart of Darkness, the train stands as testament to the incompatibility of Africa with the technology that made Europe "great". No water? No wood? No steam.

Further on, among the massive dunes of the Namib Desert, the Tropic passes close to Rehoboth, the centre of the Baster community, and to towns constructed in Bavarian style when Namibia was German Southwest Africa. Today they rise from the landscape as absurdly as the succulent plants that somehow thrive in the surrounding sand.

What comes next?

Cross the border to Botswana and enter the Central Kalahari Game Reserve. Two decades of conservation has ensured the survival of many native animal species and, in so doing, has guaranteed plenty of revenue from tourists. The reserve's largely featureless terrain makes this a particularly good location for spotting big cats.

On into South Africa, the Tropic crosses the Limpopo River, the great watercourse which rises in the Transvaal and flows through South Africa, Botswana and Mozambique to the Indian Ocean.

The Tropic of Capricorn touches only the tip of South Africa's northernmost province, but that's enough to include the Kruger National Park. The park is the local tourist industry's equivalent of the crown jewels and a huge variety of game animals are easily seen on conventional tours. However, the boundaries of the park are pretty arbitrary on the ground. Thanks to years of strenuous conservation, elephants are now abundant here and, beyond the park boundaries they've become a bit of a problem, consuming crops, destroying fences and crashing their way through vegetation and trees. In response to this, limited culls have been introduced.

Time to move on...

Further down the Limpopo, in Mozambique, nature has reimposed itself on the landscape. The ancient kingdom of Gaza was struck by devastating floods in March and environmentalists believe that, ironically, it was the deliberate removal of "fever trees" that brought floods and cholera to the region. Nine out of ten of the people displaced by the deluge have returned to try and reconstruct their lives, but this particular region may not be back on the tourist trail for some time.

Where are people going now?

A lot of them are heading to Madagascar, which the Tropic of Capricorn bisects. South of it, the country is arid and dry. This is the home of the baobab, the extraordinary tree that retains water and soil nutrients in its bloated trunk - hence the appearance of having been uprooted and replanted upside-down. Legend has it that a forest of baobabs grew from where the spindly twigs of one giant tree touched the ground at the heart of the forest. Deep in the root structure lurks a giant serpent, which devours the unwary.

North of the line the atmosphere grows more turbid and the mangroves deepen. Madagascar is scheduled to be the most cosmic place on Earth on 21 June next year, when the Solstice coincides with a total solar eclipse.

What about those tropical paradises?

Yes indeed. Just above the line, about 500 miles east of Madagascar, is Réunion, every bit your classical tropical idyll, with spectacular beaches, lush forests, an active volcano and swarms of European tourists. Administered and heavily influenced by France, Réunion offers a wide range of beach holiday resorts in which to relax.

I sense you're holding something back...

Well, the Tamil people of Réunion celebrate Temeedee in December, which involves walking on burning embers and piercing themselves in extreme ways. It's an extraordinary show of devotion from a branch of the oldest religion in the world, Hinduism. We owe some of our understanding of the astronomy that defines our Tropic to ancient Hindu Vedic scholars of the Indian subcontinent, the distant ancestors of the Tamils of Réunion. It was they who observed in 3200BC that the paths of stars changed in predictable increments.

That's enough education. Let's get moving...

Passing over the massive potential gas and oil fields of the Indian Ocean, the Tropic of Capricorn makes landfall again in Australia. It breaches the Western Coast between Carnarvon and Exmouth and makes passage through the dry and unremarkable Little Sandy Desert. Into the Northern Territories, it passes over Aboriginal land and runs over the West MacDonnell Ranges close to Alice Springs. The most isolated town in Australia, Alice is the jumping-off point for visiting Uluru (Ayers Rock), where sun-hatted and fly-netted people squint at the dimple in which the wombat fought the snake spirit way back in the Dreamtime.

The Capricorn Highway runs through the interior, uncannily following the precise line of the Tropic, to the coast at Rockhampton. Above a road that is baked and washed regularly by the tropical climate, birds of prey hover at regular intervals, swooping close to passing cars in search of roadkill and the thermals that radiate from the tarmac. In Rockhampton itself, flies crawl up the white tiles of the local hotel-bars while backpackers depart to island hop in the Whitsunday Islands, pick fruit in Bundaberg and visit Great Keppel Island.

The South Pacific equals exotic, right?

It does in French Polynesia. In 1913 the poet Rupert Brooke observed "In the South Seas, the creator seems to have laid himself out to show what he can do." The cherry on the cocktail here is the island of Bora Bora, with its silver sanded beaches protected by a coral reef lagoon.

Sounds great, when do we get there?

We don't - not on this itinerary. Bora Bora is at least 600km north of the Tropic. Instead it passes through the sparsely populated Austral islands. To the north is the Tubai and its reefed lagoon, a tiny outcrop that surely does belong to Tangaora, the god of the sea. The local Kahuna priests drink Kava to stay in touch with the spirits. Kava's relaxing, psychotropic properties have made it a popular drink among locals and visitors alike. If you want to avoid that morning-after sinking feeling bear in mind that the Mangaian folklore of the Austral Islands says that spirits can only ascend to heaven on the solstice, along the Milky Way, the path of transmigrating souls.

Wait a minute, this sounds familiar...

You're probably thinking of Graham Hancock, whose many books have included Fingerprints of the Gods and Heaven's Mirror, in which he sets out to shift the way we perceive history.

The similarities between the Austral island myths, the Inca Milky Way Spirit Road and the activities of the Vedic scholars in the Indus River Valley, among other examples, are the tiniest echoes (says Hancock) of an ancient global culture. One thing we do know is that humanity arrived in the vicinity of the Austral islands in around 1500BC, from South East Asia and from Indonesia. Their next step, almost two millennia later, was a move to Easter Island, which, funnily enough, is the next destination on the Capricorn trail.

But Easter Island is 300km off the Tropic. Can't I go back to Bora Bora?

No chance. Easter Island is the aerial junction between Australasia and South America. Take off from here to the Chilean capital, Santiago, and head north to the Tropic. In the area east of Antofagasta, space agencies test lunar landing craft, and enormous telescopes peer up into the sky. As you move towards the Andes, you reach San Pedro de Atacama, a picturesque oasis in the middle of the Atacama desert. This is the place to stay while visiting the Reserva Nacional Los Flamencos, where three colonies of flamingos breed on a huge salt lake. The town's fascinating museum includes some of the world's best-preserved mummies. Nearby, the El Tatio geysers pump out boiling, mineral-rich water, forming weird crystalline structures as the water evaporates.

Hang on. I want tropical beaches, cocktails and 24-hour partying. You only seem interested in giving me a geography lesson...

Calm yourself. Skip the arid Gran Chaco of Paraguay and plunge on past Rio de Janeiro, straight into the arms of Brazil's most popular religion, Candomble. At celebrations in late December, bisexual priestesses strut among the faithful on Copacobana, Ipanema and Loblon beaches, smoking cigars and drinking Caipirinhas. This powerful cocktail is mainly Cachaca, a fire water distilled from Brazilian sugar-cane juice. Knock it back it with lime and soda and it'll melt your brain. The hedonistic congregation here worships Iemanja, the goddess of the sea and carnal pleasure. In Rio you might understand why Henry Miller named his twin erotic books Tropic Of Cancer and Tropic Of Capricorn.

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