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chris l.
2002-Jan-07, 01:58 PM
Last night (Jan. 5) on the History Channel, Arthur Kent stated that there are only two place on earth at which a compass points to both the magnetic north pole and the geographic north pole. Aren't there actually an infinite number of locations? It seems to me that at any place along the longitude line that goes thru both pole, a compass would point to both poles. (the show was about the Bermuda Triangle.)

Mnemonia
2002-Jan-07, 03:16 PM
On 2002-01-07 08:58, tychobrahe wrote:
Last night (Jan. 5) on the History Channel, Arthur Kent stated that there are only two place on earth at which a compass points to both the magnetic north pole and the geographic north pole. Aren't there actually an infinite number of locations? It seems to me that at any place along the longitude line that goes thru both pole, a compass would point to both poles. (the show was about the Bermuda Triangle.)


Any place along the line not between the geographic pole and the magnetic pole. The compass can't point to both of them at the same time!

What I think the show meant was any spot on the longitude below the magnetic pole and above the magnetic equator (a longitude which I beleive passes through part of the Burmuda Triangle) as one spot and the same longitude directly opposite, i.e. 180 degrees around the globe, as the second.

<font size=-1>[ This Message was edited by: Mnemonia on 2002-01-07 10:17 ]</font>

GrapesOfWrath
2002-Jan-07, 04:17 PM
A compass doesn't actually point to the magnetic North Pole (actually a magnetic south pole), it just aligns itself with the local magnetic field. So the line of zero declination does not line up with a meridian line (See this illustration (http://www.geomag.nrcan.gc.ca/geomag/e_magdec.html)). But anywhere upon the line of zero declination, a magnetic compass will also point to geographic north.

And it changes with time. Here in Durham, NC, this site (http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl) shows that the declination has changed from 2 degrees west to almost 9 degrees west, just over the last hundred years. This calculator (http://www.ngdc.noaa.gov/seg/potfld/ushd.shtml) says that we were on the zero declination line sometime around 1855.

<font size=-1>[Fixed links]</font>

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-01-07 12:29 ]</font>

ToSeek
2002-Jan-07, 05:21 PM
Edited GoW's post to make links work:



On 2002-01-07 11:17, GrapesOfWrath wrote:
A compass doesn't actually point to the magnetic North Pole (actually a magnetic south pole), it just aligns itself with the local magnetic field. So the line of zero declination does not line up with a meridian line (See this illustration (http://www.geomag.nrcan.gc.ca/geomag/e_magdec.html")). But anywhere upon the line of zero declination, a magnetic compass will also point to geographic north.

And it changes with time. Here in Durham, NC, this site (http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl) shows that the declination has changed from 2 degrees west to almost 9 degrees west, just over the last hundred years. This calculator (http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/ushd.pl) says that we were on the zero declination line sometime around 1855.