Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Karen W. on 18/02/2008 18:39:46
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Also how do I rely on my compass..if that is true.... and north isn't really north... etc etc... Would I be unable to get home with just a compass???
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No, true north is not magnetic north (I believe they are about 8° away from each other); but magnetic north anyway wanders, so is not exactly the same year to year (and over longer periods of time, has even flipped over totally, so magnetic north pointed south).
You cannot get home just with a compass, which is why there was so much effort made to create accurate clocks that could keep their time while being knocked about on the ocean waves. With a good clock, a sextant, and a compass, you can get home.
Normally, a compass will help you navigate with a map, but that is only useful when you have a map, and over a featureless terrain (e.g. the high seas), it is not sufficient.
Where a compass does become pretty useless (not even merely insufficient, but useless) is when you are very close to the magnetic North or South poles, since the compass will point vertically rather than horizontally (since either the north of south pole will be under you feet, so that is where it will point - so if you want to get home, you'd better start digging).
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SOOOOO....I'd be lost any old way huh George??? LOL... Well I wondered what good those compasses were if I couldn't read them and since it would not make a difference since what would be the odds of me carrying a sextant and a good clock with me compass.. LOL Besides ...The odds would be even greater that I wouldn't know how to read a sextant or recognize one if it bit me on the tush!! LOL...
So... I wonder if they have classes for dummies like me to learn to read instruments like those ...??? LOL!
Oh and a map too... yikes... One best not leave home and roam the wilderness with out a load of directional items.... and a nice modern GPS and cell phone.. just in case you can get reception! LOL!
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It's here --> http://gsc.nrcan.gc.ca/geomag/nmp/daily_mvt_nmp_e.php (http://gsc.nrcan.gc.ca/geomag/nmp/daily_mvt_nmp_e.php)
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SOOOOO....I'd be lost any old way huh George??? LOL... Well I wondered what good those compasses were if I couldn't read them and since it would not make a difference since what would be the odds of me carrying a sextant and a good clock with me compass.. LOL Besides ...The odds would be even greater that I wouldn't know how to read a sextant or recognize one if it bit me on the tush!! LOL...
So... I wonder if they have classes for dummies like me to learn to read instruments like those ...??? LOL!
Oh and a map too... yikes... One best not leave home and roam the wilderness with out a load of directional items.... and a nice modern GPS and cell phone.. just in case you can get reception! LOL!
OK, I was probably a bit too dramatic.
Most of us have some kind of crude map of where we are in our heads much of the time. For instance, if we know there is a river running to the north of us (and there are no rivers running near the north pole, so I don't need to worry about how this would play out there), then the compass would tell us roughly the direction of the river, but it would not be enough to tell us where on the river we would reach. Thus, if we knew we lived in a house on the banks of the river, the compass could get us to the river, but it could not tell us whether to turn east or west once we reached the river. Still, at least having got to the river, you can then walk some distance east, if you don't find the house, then turn around and walk back west.
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Yep That is good advice .. I liked your original post also, and found humor in the fact that it would not do me any good to have a compass which I carry in my purse like a regular girl Scout! ( But I don't know how to use it.. although people have tried to show me..) LOL..
Thanks George that makes perfect sense!
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No, true north is not magnetic north (I believe they are about 8° away from each other)
It depends where you are on Earth. Magnetic compasses point towards the magnetic pole, so the so called deviation will differ, depending on where you are. The 'actual magnetic NORTH Pole' is in North America, at present. Apparently, it is drifting slowly in the direction of Siberia.
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No, true north is not magnetic north (I believe they are about 8° away from each other)
It depends where you are on Earth. Magnetic compasses point towards the magnetic pole, so the so called deviation will differ, depending on where you are. The 'actual magnetic NORTH Pole' is in North America, at present. Apparently, it is drifting slowly in the direction of Siberia.
Sorry, I was not clear - I did not mean its bearing was 8° off true (as you right say, the bearing depends on your own location), but that the location was about 8° off.
http://en.wikipedia.org/wiki/North_Magnetic_Pole
In 2001, the North Magnetic Pole was determined by the Geological Survey of Canada to lie near Ellesmere Island in northern Canada at 81.3° N 110.8° W. It was estimated to be at 82.7° N 114.4° W in 2005.
In other words, in 2001 it was 8.7° off the true North Pole, and in 2005, it was 7.2° south of the true North Pole.
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If you look at the plots of the Earth's actual magnetic field they look a real mess. It's a wonder they are any use at all for navigation!
http://www.geomag.bgs.ac.uk/earthmag.html#_Toc2075556 (http://www.geomag.bgs.ac.uk/earthmag.html#_Toc2075556)
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When doing naviagtion, there are calculations to convert the compass reading to a true reading for chart work. I think factor for the difference between mag and true was called 'declination'. There were other things in the calculation due to the amount of metal things near a ships compass.
If you look at a OS Landranger map, there is an explanation there.
The error will vary at different points on the earth, for instance, if you were about 110W, then the compass would point at true north, but at 20W the error will be at its greatest.
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It's called Magnetic Variation and, on a ship with much iron on it, it is also necessary to add in the Magnetic Deviation which is due to the structure and varies as the ship alters its heading. All good ships have a 'deviation Table' because the correction depends upon heading; you have to add both corrections before you are (more) sure where you're heading.
If you happen to be up there between true North and Mag North then you can be 180 degrees out.