Unless you live where true north and magnetic north are nearly identical, magnetic declination keeps a compass from displaying solar (true) south. Magnetic declination describes the difference between true north and magnetic north for a particular site. At a location with a magnetic declination of 10°E, the magnetic north pole is 10° east of the actual north pole. For accuracy, solar assessment tools must account for this difference.
In the United States, sites east of the Mississippi River Valley have a western declination; sites west of the Mississippi River Valley have an eastern declination. For example, the magnetic declination in Baltimore, Maryland, is approximately 11°W. If you were trying to determine solar south in Baltimore with a compass, you would find magnetic south, then rotate 11° to the west.
Magnetic declination changes over time. The variation is slight for locations in the central United States—for instance, the declination in New Orleans will change by 1° in eight years. But in Fairbanks, Alaska, the declination is currently changing by 1° every three years. The National Geophysical Data Center’s declination calculator provides current data on a site’s magnetic declination (bit.ly/DeclCalc). Aerial maps are aligned with true north and true south.