## Sep 2 The Many North Poles

Colloquially we talk of "The North Pole," but there are actually more than one way of defining it and all of these definitions place the Pole at different points on the Earth.

Let's start out with some of the more common meanings. Firstly the Terrestrial North Pole is the point on the Earth where the axis of rotation meets the surface of the Earth. This is the one we spin around, the one we define our longitude and latitude from and the one around which time zones are based. Officially it is not in a time zone and because there is no permanent base there (or at the South) we have never rectified this, so you can go there to be outside time.

At first this sounds neat enough to be "The Proper North Pole", but it isn't that easy. Euler predicted that the Earth's rotational axis would actually move relative to the Earth and this was confirmed by looking at stars and seeing them precess out of position. If you track where the Terrestrial North Pole is you will find it varying by a few meters. This isn't great when you have to fix a coordinate system by it, so the Terrestrial North Pole is defined at one particular point from longitude and latitude and the Instantaneous North Pole is the one we actually spin around.

The next one is the North Magnetic Pole which is where the Earth's magnetic field points directly downwards and touches the Earth. So if a compass was allowed to spin in any direction it would point straight downwards at this point.

Contrast this with the North Geomagnetic Pole which is what you get if you model the Earth as a simple bar magnet and extend the North end straight out to reach the surface of the Earth. These two points aren't actually in the same place, here's a map:

The First Three North Poles

The latter moves fairly slowly (about 0.1 degree West of latitude annually with a little fluctuation North and South as it goes) and is opposite the Geomagnetic South Pole. However the North Magnetic Pole moves much more erratically and isn't opposite the South Magnetic Pole. Here's its movement since 1590:

North Magnetic Pole Location

This very much matters if you are trying to navigate using a compass, but even more surprisingly the Magnetic (and Geomagnetic) North and the South Poles actually flip which side of the Earth they are on from time to time. The last reversal happened about 780,000 years ago and we look well due for another.

While the Rotational Pole and the Magnetic Pole usually don't differ by too much, Uranus has them an incredible 60 degrees out. Venus and Mars don't have Magnetic Fields. It is interesting to think about how you would define North on another planet or exoplanet. In space there is no up and while some solar systems are more or less planar, some newer ones are far from this ideal. Some people use a right hand rule linked to rotation to do this, which would make Venus have its Extraterrestrial South at the "top".

We have done the big three, but let's delve deeper. For explorers, the Northern Pole of Inaccessibility is majorly important. It is the point on the Northern Ice Sheet which is the Furthest from the Sea, although it was only in 2013 from satellite images that we realised we were 133 miles from the real one and the point that we had been trying to reach since 1927 had been miscalculated. So far no one has reached either of these Poles.

By drawing a line from the Pole Star (Polaris) to the surface of the Earth so that it makes a normal with the surface we define another North Pole; although this hasn't had a consistent name and hasn't been used since we erroneously thought that it was the magnetic pull of the star leading the compasses rather than the Earth. Which star is in Pole Position changes on a historical time scale.

Finally we have the best North Pole, which is where Father Christmas lives. Its exact location is left to your imagination.