Wednesday, January 30, 2013

The wandering south magnetic pole


This post is a response to a question from the 8th grade class at Saint Columbkille Partnership School in Brighton, Massachusetts.  They wanted to know about the South Magnetic Pole and if the magnetic field down here was weird.  I knew a little about it, but I wanted to do some measurements of my own to determine where it was.  Here is what I found:

There are two types of Pole on Earth that each come with a North and a South; geographic poles and magnetic poles*.  The geographic poles are what we usually mean when we say “North Pole” (where Santa lives) and “South Pole” (where Amundsen-Scott Station is).  They are defined as the points around which the Earth rotates (if you are cooler than I am and can spin a basketball on your finger, the South Geographic Pole would be where your finger touches the ball).  These points of rotation stay fairly constant through time, so they are the poles we use when making maps.  Therefore, the North arrow on any map you pick up will point towards the North Geographic Pole.

The magnetic poles are basically where magnetic compasses point.  Their location is much more complicated and changes through time since the magnetic field is created by the churning of Earth’s Outer Core, a molten layer of nickel and iron deep inside.  This churning is complicated just like any fluid moving around (think of how crazily the smoke from a fire curls around), causing the magnetic poles to wander around, up to 100 miles a year.

Perhaps the most interesting thing about the magnetic and geographic poles is that their locations are similar but not identical.  For instance, you know that the South Geographic Pole is kind of in the middle of Antarctica, but did you know that the South Magnetic Pole is currently outside the boundaries of Antarctica?  It has not always been this way though; around 100 years ago, the first people to reach the South Magnetic Pole (one of whom also was named Alistair!) reached it on foot since it was still on land.  At any location, the difference between geographic North (what  you would find on a map) and magnetic North (where your compass points) is called the declination.  This is an important number if you try to navigate using a map and compass, so people who still sometimes use compasses (airplane pilots use them as a backup) need to know the declination of their location.

The difference between the location of the two types of pole can be confusing if you are trying to navigate by compass.  This is an image of me using a special geology compass called a Brunton Compass.  When you aim the pointy end of the compass at something, the white end of the needle points to a number that tells you the angle between that direction and magnetic North.  I am pointing it towards geographic North (determined from maps), so the number (reading 175o) tells the declination.
*There are other poles as well, such as the Pole of Inaccessibility, which is the point on a landmass farthest from any shoreline.  The Russians, in an attempt to one-up the US base at the South Geographic Pole, established their base (Vostok; you might have heard of it because of the drilling project at Lake Vostok, which is a lake buried deep under the ice that might harbor strange life) at the South Pole of Inaccessibility.

Science you can do at home

1.       If you know the magnetic declination (remember that this is the angle between the magnetic pole and the geographic pole) in two places, you can use a map to recreate that angle and find the current South Magnetic Pole.  I was curious to see if this was possible to do accurately, so I took one reading in Christchurch, New Zealand and second in the Antarctic Dry Valleys.  However, I do not have a map of the area around Antarctica with me, so I will post the data I took and rely on you to see if my readings were good enough to get a close approximation of the location of the magnetic pole.

In order to do this, first find the locations on a map.  Then, using a protractor, draw a line at the appropriate angle from a line of longitude (they run straight North-South).  The intersection of the two lines should be close to the actual location of the South Magnetic Pole.  You can check the actual location on the internet.

Christchurch, New Zealand – Compass says (magnetic) South is 23 degrees to the West of (geographic) South

Dry Valleys, Antarctica (Latitude: 77.87427, Longitude: 160.53845) – Compass says (magnetic) South is 175 degrees (yes, that is what I measured) to the East of (geographic) South

2. You can also measure magnetic declination at your house, if you have a compass and know which direction geographic North is (where the North Star is, or near where your shadow is at noon).  I have no idea what it is anywhere in the US, so if you feel so inclined, post your location (city, state, latitude/longitude; whatever you want) and the declination you get.

1 comment:

  1. Alistair - Great science and great writing. I didn't know the magnetic pole could move so fast! Best, Jack Cederquist

    ReplyDelete