Magnetism and the compass

Magnetism and the compass

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Fig 4.1 – Magnetic lodestone mineral with magnetic abilities to attract nails.

The earliest form of magnetism is thought to have been found in the lodestone, a mineral containing iron oxide (Fig 4.1). Metals that have been magnetized by lodestone have the curious ability to align themselves in the north south direction. It is thought that properties of loadstone were first discovered and used in China. It was Zheng He (1371 – 1435) who is accredited to using it for navigational use and made several oceanic voyages between 1405 and 1433. It was introduced to medieval Europe 150 years later. The meaning of the word lodestone comes from middle english meaning ‘course stone’ or ‘leading stone’ indicating the navigational use of this discovered mineral. The greeks discovered the properties of lodestone by the 6th Century BCE from rocks on the Magnesia Peninsula and where we get the name Magnets from. Recent evidence suggests that the magnetic properties of the lodestone were first discovered by the Olmec civilisations around central america up to 1000BC a millennium before the Chinese incorporated it into their comparatively modern navigational compasses.

How a Compass Works

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Fig 4.2 – The earths can be thought of as a huge bar magnet.

The rotational forces and interactions between the earths core, mantle and crust are thought to be responsible for creating the magnetic field around the earth which can be thought to function like a huge bar magnet (Fig 4.2). This forms a magnetic field – the magnetosphere – around the earth. The compasses on the boat align with the magnetic poles.

The Three North Poles

Life would be much easier for a navigator if the magnetic north (magnetic north) pole and the true north pole (true north) were in the same place. Everything we do on a chart is in relation to the true north pole and need to come up with a system to compensate for a magnetic north pole which is continually on the move. We also have a quick look at another north pole (compass north) which is affected by the direction our vessel is heading. On a yacht we continually work between these north poles and need to have an understanding of the ‘errors’ we apply when converting bearings between true, magnetic and compass north. Variation can be though of as an error between the true and magnetic north pole and deviation as an error between the magnetic and compass north poles.

Geographic and Magnetic North Poles

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Fig 4.3 – The earth spins around a axis of rotation, the true north pole is where all meridians of longitude converge.

The earth spins around an axis of rotation, which is the ‘True’ geographic north and south poles. Our charts are orientated towards True north and with all meridians of longitude converging towards the geographic true north pole and parallels of latitude parallel with the equator. When we work on the chart, everything is calculated in relation to True north (Fig 4.3).

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Fig 4.4 – The magnetic pole is continually on the move and values of variation change over time. At the moment it is on its way to Siberia!

Unfortunately the Magnetic north pole is not in the same place as the geographic north pole. Our magnetic compasses point towards the Magnetic North pole which was in position 82.7°N 114.4°W in 2005 and was 438 nautical miles away from the Geographic True North pole. Whats more the Magnetic pole is on the move and in recent decades has speeded up and is heading towards Siberia at around 36 miles per year after a long slow moving period when it was moving at a much slower 9 miles per year (Fig 4.4).

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Fig 4.5 – The value of variation changes depending on your geographic location and can be east or west of the true geographic north pole and if the pole are in line there will be no variation.

The difference between the True geographic north pole and the Magnetic north pole is called magnetic ‘variation‘. You can think of it as being an ‘error’ in the earths magnetic field by not being in the same position as the true north pole. Whats more the value of this ‘error‘ is continually changing as the magnetic north pole wanders around, changes direction, speeds up and slows down.
Depending on where we are on the earths surface the magnetic variation error can be west or east of the true north pole or in line with it (Fig 4.5). Additionally as our perspective of the two north poles changes the value of this variation ‘error’ can increase or decrease. Thankfully, the nautical charts we use today gives us an estimation of the position and the rate at which the magnetic north pole is moving. We can use this information to covert our bearings from Magnetic North to True North. As we shall see below we can convert a ‘True’ bearing line drawn on the chart into a ‘Magnetic’ bearing for use with the handheld compass or we can take bearing of an object with a handheld compass and convert it so we can plot this bearing on the chart.

Being able to calculate variation and applying it to convert bearings is a fundamental part of marine navigation. Indeed values of variation may be small and seem insignificant but being able do deal with small magnetic ‘errors’ helps reduce cumulative errors, intrinsic in any calculation, and makes our navigation more accurate.