# compasses **URL:** \ **Category:** Factual Questions\ **Created:** [June 9, 2003, 2:23pm UTC](https://boards.straightdope.com/t/compasses/180643 "2003-06-09T14:23:18Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Equal](https://avatars.discourse-cdn.com/v4/letter/e/e5b9ba/32.png) [@Equal](https://boards.straightdope.com/u/Equal)\ **Post date:** [June 9, 2003, 2:23pm UTC](https://boards.straightdope.com/t/compasses/180643/1 "2003-06-09T14:23:18Z")
Does a compass work because the earth is magnetic? Exactly how so? thank you. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [June 9, 2003, 2:29pm UTC](https://boards.straightdope.com/t/compasses/180643/2 "2003-06-09T14:29:16Z")
The Earth has a magnetic field that is generated in its spinning metallic core. The magnetized compass needle lines up with the Earth’s magnetic field lines, so it points North. Here’s a page with a schematic diagram and some more information: [http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html](http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html) ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [June 9, 2003, 2:30pm UTC](https://boards.straightdope.com/t/compasses/180643/3 "2003-06-09T14:30:50Z")
The needle is magnetic, so it has a north and south pole. Now the earth also has a magnetic north pole and magnetic south pole. The compass needle is balanced so that it may swing freely. Now the earth magnetic north attracts the south pole of the compass needle and the earth’s south pole attracts the compass’ north pole. The needle aligns itself to those poles. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [June 9, 2003, 3:12pm UTC](https://boards.straightdope.com/t/compasses/180643/4 "2003-06-09T15:12:12Z")
> [@](#): > > \*Originally posted by Podkayne \* > **The Earth has a magnetic field that is generated in its spinning metallic core.** There is a circulation in the core that is causing the magnetic field, but the core is not spinning as a unit. > [@](#): > > \*Originally posted by Grey \* > \*\*The needle is magnetic, so it has a north and south pole. Now the earth also has a magnetic north pole and magnetic south pole. > The compass needle is balanced so that it may swing freely. Now the earth magnetic north attracts the south pole of the compass needle and the earth’s south pole attracts the compass’ north pole. The needle aligns itself to those poles. \*\* And we have to be careful here, since the Earth North Magnetic Pole is actually a south magnetic pole–that’s why it attracts the north magnetic pole of the compass’s magnetic needle. ---
**Author:** ![Khadaji](https://avatars.discourse-cdn.com/v4/letter/k/9e8a1a/32.png) [@Khadaji](https://boards.straightdope.com/u/Khadaji)\ **Post date:** [June 9, 2003, 4:49pm UTC](https://boards.straightdope.com/t/compasses/180643/5 "2003-06-09T16:49:13Z")
[Here’s](http://science.howstuffworks.com/compass.htm) a good site on how stuff works. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [June 9, 2003, 7:19pm UTC](https://boards.straightdope.com/t/compasses/180643/6 "2003-06-09T19:19:57Z")
> [@](#): > > \*Originally posted by Grey \* > \*\*the earth magnetic north attracts the south pole of the compass needle and the earth’s south pole attracts the compass’ north pole. The needle aligns itself to those poles. \*\* This is not necessarily true. As pointed out by **Podkayne** , a compass does not “know” which way is north (or south, for that matter). The only thing it can do is align itself with the _ **local** _ magnetic field. We then make the _assumption_ that the local magnetic field is “pointing” toward magnetic north. This assumption may not always be correct. ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [June 9, 2003, 8:30pm UTC](https://boards.straightdope.com/t/compasses/180643/7 "2003-06-09T20:30:32Z")
Due probably to the iron ore, there is a lot of magnetic variations in Northern Wisconsin/UP Michigan. I don’t have a Green Bay sectional (aviation map) on me, but I know it says thinks like “Magnetic deviation as much as 15 degrees exits near \” Brian ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [June 10, 2003, 1:59am UTC](https://boards.straightdope.com/t/compasses/180643/8 "2003-06-10T01:59:44Z")
True. There’s also the fact that the poles move. Depending on where you are the difference between true north and magnetic north can be quite large. ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [June 10, 2003, 4:00am UTC](https://boards.straightdope.com/t/compasses/180643/9 "2003-06-10T04:00:42Z")
Actually is says “magnetic disturbances”, and it is 14 degrees near Escanaba and 18 degrees near the Lake Superior shore from Duluth to Grand Marais. but here is the kicker: Magnetic disturbances as much as 8 degrees at Grand Marais; other large disturbances exist at {list of 6 places} \*\* and as great as 50 degrees at Magnet and Pie Islands\*\* :eek: (emphasis and eek mine) I’m lucky in that geographic and magnetic north are very close where I live/fly. Brian