17:00

Magnetic North

The Earth's magnetic field has always been a source of mystery. It was originally thought that it was to do with the Lodestar - the North Star or Polaris - around which the heavens pivot and to which the Lodestone (which was Magnetite) pointed. William Gilbert in the 16th century in his treatise,  De Magneta, postulated rightly that it was something in the Earth's core that caused the Lodestone to react in this way.

What we now know is that the core of the earth is a solid iron sphere about the size of the moon and that that is surrounded by heated liquid iron up to the earth's crust and this liquid iron is moving, partly to the same Coreolis Effect which is caused by the spinning of the earth; the same force that makes your bath water go round in different directions in different hemispheres. This creates a gigantic Dynamo that forms a magnetic field currently centred over northern Canada where it is moving at about sixty kilometres a year. This movement means that magnetic north is always changing whereas True North is always at the North Pole.

One practical  manifestation can be seen on airport runways which are numbered according to their direction; an east/west runway, which is 90 degrees and 270 degrees respectively,
is numbered 09/27 and these numbers have to be periodically repainted as the Magnetic Pole moves. Heathrow was repainted in the 1980s.

In the days before satellite navigation, navigation in the polar regions was a hazardous affair. With no sight of the stars due to bad weather and a useless compass, sailors and explorers were dependant on dead reckoning which was dangerous in the extreme in areas of few charts and little knowledge of tides and currents. 

When you realise the limits of their navigational tools and their dependancy on wind and furs, you cannot but admire them.
SHARE:

No comments

Post a comment

Blogger Template Created by pipdig