Hello Judith: The Earth's magnetic field is in a constant state of change so there is no simple answer to your question. Every 5 years or so the World Magnetic Model is updated. This model will tell you what the magnetic field is for a given location and date anywhere on the Earth's surface (but not near anomalies like Iron mountains). For more see my sensors web page at: <http://www.pacificsites.com/~brooke/Sensors.shtml#Earth's Magnetic>
Brooke "Romano, Judith" wrote: > We all know that "True North" (Polaris) is not the same as "Magnetic North" > when orienting a sundial. My grandfather's Boy Scout pocket "Sunwatch" is a > true gem for our family -- loaded with information for any enthusiastic > sundial traveler (complete with a compass, dial, and table of corrections > for the major US cities). One of the corrections included was for "Degree > Variance" (East or West) of magnetic north in which to point the dial to > "True North" for an accurate reading. For example, Boston, MA is 14 degrees > West; Seattle, WA is 23 degrees East; and ironically, Cincinnati, OH has no > variance and is the same as magnetic north. > > Is anyone in the mailing list aware of the mathematical formula used to > compute this variance for True North? > > Judith Romano > mailto:[EMAIL PROTECTED] > > - -
