Begonia House cafe, Wellington Botanic Garden
20 January 2012
The day has turned out sunny and clear, after an unpromising start. Gillian and I meet in Kelburn near the Cable Car and decide to walk down into the gardens. I’ve known her for several years and we enjoy walking together. You can never walk too fast for Gillian. She is a world-class orienteer (I can feel her blushing beside me “you can’t write that…”) and keen mountain runner. She lectures in physics at Victoria and is great to talk to about sciencey things as she can explain ideas clearly, and she makes most things sound fascinating.
I want to know why the heroic-era explorers were so interested in Earth’s magnetism. I have read that magnetic research and geographical discovery carried equal weight as goals in Scott’s 1901 Discovery Expedition. It’s easy to understand the importance of the geographical bit, but less so the magnetic bit.
This is where Gillian comes in. She has written a wonderful book—North Pole, South Pole—subtitled “the epic quest to solve the great mystery of Earth’s magnetism”. It is the story of how physicists, mathematicians and explorers through history eventually came to understand Earth’s magnetic field. It’s a complex story, so this is the short version. We drink coffee and share chocolate brownie under the shade of the grape vine, on what locals refer to as a typical Wellington day.
She draws me into the topic with her obvious enthusiasm. “This part of the story starts with Karl Friedrich Gauss, a German mathematician, who was busy collecting measurements of Earth’s magnetic field in the 1830s. He needed real observations so that he could fit his mathematical model to them, and use the data to refine it.” Gillian thumbs through her book to find a picture of him. He was a fine looking chap.
“Gauss started off a movement of setting up observatories to keep regular records of the magnetic field over long periods, as it was known that the field varied not only from place to place, but also over time. Often astronomical observatories were built at the same places.”
By 1600, William Gilbert, gentleman scientist and physician to Queen Elizabeth I, had figured out that Earth’s magnetism came from inside the planet and that a compass needle points north because “the Earth is itself a magnet”. In 1701 Edmund Halley (of comet fame) had charted magnetic declination (or variation) across the Atlantic Ocean to aid navigation and cleverly invented the contour line as a way to show the results in a simple way.
“Edward Sabine, an Anglo-Irish scientist and naval officer, took on the baton from Gauss and convinced the British of the importance of taking magnetic measurements. This mission became known as the Magnetic Crusade. Naturally they had very little data from the southern hemisphere to go on. ”
In 1839, the ships Erebus and Terror under the command of James Clark Ross, left to set up magnetic observatories in the outer reaches of the empire and take measurements in the ‘antarctic seas’. He planted the British flag on Possession Island (in amongst the penguins) and named the area Victoria Land, for his queen. His dip needle was recording inclinations of 88° (you expect a dip of 90° at the south magnetic pole itself) as he sailed along the coast of the newly found continent, and so he eventually figured that the magnetic pole must be on the land beyond the Great Ice Barrier (what we now call the Ross Ice Shelf).
He had collected stunning data sets in Antarctica that advanced the Magnetic Crusade greatly and his voyage (and two later ones) also gave rise to many Antarctic place names.
Gillian continues, “I was surprised to learn that when Scott set out in Discovery it was in a purpose-built wooden ship, so that it could act as a magnetic observatory (without an interfering iron hull). It leaked really badly and even in Lyttelton, where they stopped on the way to stock up with sheep, there are reports of repairs going on.”
“Scott was determined to locate the south magnetic pole, and sent off the 21-strong Western Sledge Party to reach it. It was a horrendous journey by all accounts—blizzards, dead-ends, man-hauling sledges laden with instruments, scurvy and a near-fatal heart attack. It looks like they turned back once they got to the top of the Polar Plateau and were forced to give up on actually reaching the magnetic pole.”
Edgeworth David and his team, in Shackleton’s expedition, claimed what they estimated to be the pole, for England on Jan 16th 1909, and they were probably pretty close. Lengthy calculations and the instruments they had left at the previous night’s camp were needed for confirmation, so who could blame them for just making a good guess in those conditions? King Edward VII was evidently very pleased with their achievement and he sent them a congratulatory telegram when news reached him from New Zealand.
“By 1919 various pieces of evidence had led to the proposal that Earth’s magnetism was caused by a geodynamo—electric currents created by the liquid iron in the outer core of our planet swirling around. This would explain why the field changes over time and even how magnetic polarity reversals (when a compass needle would point south instead of north) are possible. Polarity reversals have occurred throughout history, two or three times per million years, and, statistically-speaking, we are long overdue for the next one.
“It took till 1995 to build a supercomputer hefty enough to run an actual simulation of the geodynamo (an important step towards proving the theory) because the mathematical equations are incredibly complex. After several thousand CPU hours (we’re talking months) Gary Glatzmaier and Paul Roberts produced a beautiful animation of Earth’s magnetic field changing over time. They were even lucky enough to see a polarity reversal appear during their simulation—something they couldn’t have expected, since they could only afford to simulate a few tens of thousands of years of geomagnetic field behaviour.
“So it’s really no surprise that the south magnetic pole is moving. It’s now in the ocean off Victoria Land. To be precise, it’s gone from around latitude 70° south in 1900 to 64° south in 2010. Scott could have sailed right up to it!”
Gillian’s research studies these changes of the magnetic field through time, based on the magnetism of rocks. Her latest project is set to fill in the record for the past 10,000 years in the south west Pacific – an extension of what the early explorers sought to do in Antarctica.
As we walk back up the hill to Victoria, Gillian fills me in on how interaction of the solar wind with Earth’s magnetic field shapes the Aurora australis. And she’s not even puffing as we reach the top.