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Written by Helen Bostock (marine geologist, NIWA)

Date: 15/02/2013
Location: 64.807561°S, 139.86694°E
Weather: Snowing and breezy – 20 knots
Sea state: 2 m swell

Today we passed over the South Pole.

Well, not quite, but very, very close by. How is this possible when we are on board a ship and haven’t set foot on Antarctica itself?

We have been close to the Magnetic South Pole. The Magnetic South Pole is the wandering point in the Earth’s Southern Hemisphere where the geomagnetic field lines are directed vertically upwards – the ‘magnetic inclination’.

The Magnetic South Pole currently sits at 64°24’S, 137°00’E, so the RV Tangaroa passed just west of this, within 100 km of the Pole. It is constantly shifting, however, due to changes in the Earth’s magnetic field, moving northwest at a rate of 10-15 km per year. Unfortunately it being the middle of the night meant that there wasn’t much to see, except that the compass kept changing.

The first calculation of the magnetic inclination to locate the Magnetic South Pole was made on January 23rd 1838, by French hydrographer Clément Adrien Vincendon-Dumoulin on the Dumont d’Urville expedition to Antarctica and Oceania in 1837-1840, on the ships L’Astrolabe and Zélée.

There were several early attempts to reach the Magnetic South Pole in the early 20th Century: during the Ernest Shackleton’s 1908-1909 Nimrod expedition when it was approximately 72°20’S, 155°12’E, on mainland Antarctica (Victoria Land) just west of the Ross Sea, and then during Douglas Mawson’s 1911-1913 Australasian Antarctic expedition (more on that in a later blog post).

So theWikiMiniAtlasSoSSo magnetic poles are different from the geographic poles. This is critical to know if you are using a compass for navigation as the compass needle points to magnetic north, not true north (or grid north). The difference between magnetic north and true north is called the ‘magnetic declination’.

Most map coordinate systems are based on true north, and magnetic declination is usually shown on the map legend so that the direction of true north can be calculated. In some areas the declination is only 5°, so you wouldn’t get too lost if you are only going short distances. In New Zealand, though, we need to correct for this difference, as the declination is greater than 20°.

Closer to the magnetic poles, the declination gets very large and compasses are not very useful. As a result, the officers on the RV Tangaroa will have to rely on their Global Positioning System (GPS), which uses multiple satellites to work out our exact position, rather than relying on the compass and earth’s magnetism.

Spotlight on bow of ship

Spotlight on the bow of the ship. [Jill Scott]