Compass needle

No, this is not a dumb question.

Always North.

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"The needle of a compass is a small magnet, one that is allowed to pivot in the horizontal plane. The needle experiences a torque from the ambient magnetic field of the Earth. The reaction to this torque is the needle's preferred alignment with the horizontal component of the geomagnetic field. Having said this, the preferred directionality of a compass can be affected by local perturbations in the magnetic field, like those set up by (say) a near-by electrical system; a compass can also be affected by local magnetization of the Earth's crust, particularly near large igneous or volcanic rock deposits."

http://interactive2.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=475

The "North" end of a compass needle points to the magnetic North pole in either hemisphere kinda-sorta. Actually, a compass needle more or less orients itself to an imaginary line connectin the planet's 2 magnetic poles, North and South, which happen, BTW, not to be the geographic poles of the planet. Both poles wander some, so the exact lay of the line varies a bit. Tables offering adjustments, or declination offsets, to correct Magnetic Compass North to True North are readily available. Lotsa geomagnetic info - including a nifty interactive declination calculator,HERE

A trivia bit: electromagnetically, the Magnetic North pole actually is a "South" pole, and vice-versa. Don't let that bother you, though - acompass works like a compass just about wherever you are on the planet, with the exception of locations known as magnetic anomalies.

There are only a few differences between the northern hemisphere and the southern, the weather, seasonal differences, and of course the angle of the sun.

Our longest day, June 21st, the summer solstice, is their shortest day, the sun travels through the northern sky as apposed to our southern sun. Tornados in OZ spin clockwise versus their Kansas counterparts that spin counter clockwise...which is due to the Coriolis effect, which does not...no matter what that episode of "The Simpsons" said...does not, effect the rotation of toilet or drain water.

Some more trivia (having been a navigation officer and studied history :wink: ):
During the 15th century it became apparent that the compass needle did not point true north from all locations but made an angle with the local meridian. This phenomenon was originally called by seamen the northeasting of the needle but is now called the variation or - more common - declination.

The difference between compasses designed to work in the northern and southern hemispheres is simply the location of the "balance", a weight placed on the needle to ensure it remains in a horizontal plane and hence free to rotate.
- In the northern hemisphere, the magnetic field dips down into the Earth so the compass needle has a weight on the south end of the needle to keep the needle in the horizontal plane.
- In the southern hemisphere, the weight needs to be on the north end of the needle. If it wouldn't be adjusted, the needle would not rotate freely, and hence would not work properly.

Kick up a rumpus
But don't lose the compass
And get me to the church on time

Compass knowlege
I want to thank everyone (2Packs, timber, Walt [and even Mc Tag]) for the responses and the links which are becoming fascinating to me.

Beause of all the pirate stories lately, as well as the History Channel which about a year ago did a fascinating piece on "Navigation; longitude and latitude," I am becoming curious about learning to navigate the old fashioned way with a sextant, charts, etc. Because of GPS it seems to be a lost art. It would seem that it would be a good skill to know if your GPS goes on the blink. And also a hobby.

I know enough trig to do it (I think) but I know I'd have to buy a sextant (very expensive) and get the necessary tables and charts. But I think it would be fun and a challenge even though I'd never use it. Can you use a sextant to find your position on land? I don't have a boat, but I think it would be interesting (and educational for the grandkids) to drive about and stop and practice with the sextant. Could be risky though. I'd probably be arrested for unusual and strange behavior, especially at night.

Thanks again to all.

Re: Compass knowlege


Well, not really - there are always some (actually, many and all professionals) who don't believe electricity runs 24/24, 7/7, 12/12 on a ship ...

Provided you have an unrestricted view of the natural horizon, a sextant can be used on land. Of course, that horizon view provision makes it problematic at best.

On holiday this year, on a sailing ship in the North Sea, I met an American who had lived in Texas (now based in Germany) who had taught himself the sextant while living there....Tx. And had read widely.

He even gave impromptu seminars in celestial navigation to those interested who were off-watch. What that guy didn't know about navigation, just isn't worth knowing. So it's certainly possible to tutor yourself.

By the way, if you have an interest in this as a general subject, and like a good read, you could dip into "The Way of a Ship" by Derek Lundy, ISBN 0-676-97371-X

This deals, as a kind of a novel, with sailing ships, navigation, and maritime history. I'm currently ploughing my way through it at at rate of a few pages a night.

Even McTag

there is also s type of star-base navigation called an astro-inertial navigation system. we use it to take pictures of the sky and determine the location of the jet based on the shift of light and whatnot. it's kinda neat.

Well, if I want to do such



I rely more on old-fashioned stuff like this :wink:

I also found in this month's yachting magazine (I buy one every summer!)

"The Barefoot Navigator by Jack Lagan, published by Adlard Coles Nautical
This 148-page paperback is the antidote to increasing reliance on electronic navigation....starts by looking at how ancient mariners found their way across the seas..."

I may be too late with this information, perhaps our man has already gone to purchase a sextant and is even now studying the instruction manual, and scanning the heavens....

A minor correction: the paperback will be published (out in September) by Sheridan House (with 160 pages) ... which I've ordered :wink: