Hemispherical compass trivia

[SynCitizen]

After reading in another thread about equipment, which led to compass's, then a comment about northern and southern hemisphere compass's, which is something I've never heard about before got me searching the internet about an answer (you gotta love the net!) Interesting stuff.

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Global Magnetic Regions:
Because of the Earth’s curvature and variances in the Earth’s magnetic field, a compass designed for the northern hemisphere will ‘stick’ in the southern hemisphere, as it will basically be trying to point towards the ground.


The image to the left shows the 5 magnetic zones the compass industry has defined. Depending where in the world you are any standard compass you purchase generally has its needle balanced only for that region. e.g: most compasses sold in New Zealand will only be most effective in New Zealand and Australia.



Original link

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Where does a compass really point? A compass in the Northern Hemisphere truely does point in a northerly direction, but not to the North Pole. Instead, the compass points to the North Magnetic Pole, which, as Sir James Clark Ross discovered in 1831, is located at the northernmost point of the Artic coast of North America. Similarly, a compass in the Southern hemisphere always points to the South Magnetic Pole, which is firmly planted south of Australia, in Antarctica.
The different directions their compasses pointed, when traversing the high-seas of the Northern Hemisphere, baffled ancient mariners. Their modern counterparts understand, and compensate for, the differences in the North Pole and the Northern Magnetic Pole, and chart their courses accordingly. The differences in the poles proves minor, in comparison to the tricks the Northern Magnetic Pole pulls from its home of Boothia. The bane of boyscouts, as they attempt to navigate with, or without, the benefit of their trusty compasses, is the fact that this Pole chooses to roam about in a 20-mile circle, and to shift its course between day and night.
This 20-mile variance, however, is not one of global proportions. Modern sea-farers compensate for the Northern Magnetic Pole's perpetual motion, by using charts, and tools other than the compass. All things considered, 20 miles is a minor measure for distant travelers to take into account in adjusting their travel agenda.
Thankfully, the Southern Magnetic Pole spares sailors the navigational nightmare its Northern nemesis does. In the south, compass needles actually do point true South, to the South Magnetic field.


Original link

[SynCitizen]

Ok this leads me to another question since the sport of scuba diving often leads us to travel in oceans, lakes, and water all over the world. Should you not rely on your home compass when you travel? Do the newer electronic DC compass's take the magnetic deviations into account?

[ScubaToys Larry]

I think the thing you need to realize here is we're scuba diving - and going a few hundred yards - not trying to fly around the globe. And when you use a compass, you are normally setting a heading, then doing a return heading. So if you are following a heading of 10 degrees, and come back at 190 - does it really matter if you actually were swimming 15 and came back 195??

And people are going to give you directions based on standard compass readings: "Hop in at our dock, and take a heading of 120 for about 200 feet and you'll see this anchor." That 120 will be based on standard compass reading - if you correct - up to 15 or 20 degrees depending where you are - you might miss the thing you're looking for.

So while the science and thought of this is good - in a practical application for our use as divers - it's immaterial.

[SynCitizen]

Good points on practicality. Not really note worthy when doing navigation on the scale of doing a dive. I just had never thought or heard of the impact that could be had on compass's other than the differences in true vs mag north and thought it was an interesting phenomena.

[fisheater]

Next, we'll start having debates about Longitude & Latitude coordinates versus using the Universal Transverse Mercatur coordinates! :-)

Navigation can get very complicated, but only if you let it.

[OTGav]

I think you have to have a diffent compass if you live in Southern hempisphere compared to Northern, but I could be confusing that with some other bit of trivia.

Just remember a discussion where the answer was - buy a local compass.

Not sure that it would have made a huge difference unless you were doing serious Nav - and passing on your readings to someone else though.

[CompuDude]

Here's another thought, though.

Who cares if your compass is pointing slightly down? It's not DIRECTLY down, after all. Underwater is a 3D environment. Tilt your wrist a bit and you should be good to go!

I used my California-purchased Suunto SK7 compass in Thailand, the other side of the world, just fine. Perhaps it would have been different in Australia, but the reality is, most of the time I'm not trying to follow precise directions with precise headings underwater. Most of the time, I just need to note that "Land is to the North, according to this compass on my wrist" and "From the anchor line, I'm going to head West... according to this compass... so heading back East will bring me back to the anchor line." As long as the Earth's magnetic pole doesn't shift dramatically DURING my dive, things will be fine with seat-of-the-pants navigation like that.