You can enter your location on the US Historic Declination calculator by either entering your zip code or entering the longitude and latitude manually. Only locations in the conterminous (lower 48) United States can be entered.

A degree is a way to measure position on a circle. There are 360 degrees in a circle. Units of degrees are used both for the entry of the location (latitude and longitude) and for the declination results. Latitude is measured north from the equator (0°) to 90° North at the North Pole and south to 90° South at the South Pole. Longitude is measured east (through Europe) and west (over the Atlantic) from Greenwich, England (0°) with 180° E the same as 180° W in the Pacific. Therefore, 45°N 90°E is in China while 45°N 90°W is in Wisconsin. This calculator accepts values in the range of 25° to 52° North latitude and 130° to 65° West longitude. Because only west longitudes are allowed, the longitude is entered as a positive value. Magnetic declination is measured positive east and negative west from 0° at the magnetic north. Declination values run from 0°-180° east and 0°-180° west. For more information on magnetic declination, see the geomagnetic information FAQ page.

Minutes are a subdivision of a degree. There are 60 minutes in one degree.

This calculator accepts date values between 1750 and 2010. Because the declination is estimated from past observations, results for the 1700's are typically only available in the eastern United States where observations exist. Declination values are not available for all years in all areas.

Declination is the angle of difference between true North and magnetic North. For instance, if the declination at a certain point were 10° W, then a compass at that location pointing north (magnetic) would actually align 10° W of true North. True North would be 10° E relative to the magnetic North direction given by the compass. Declination varies with location and slowly changes in time. See the Geomagnetic FAQ page for more information.