What is the difference between true north and magnetic north called?

The difference between true north and magnetic north is known as variation, or magnetic declination. This measurement is critical for navigation, as true north refers to the geographic North Pole, while magnetic north is the direction that a magnetic compass points, which can vary depending on your location.

Variation is a key concept particularly for mariners and aviators because it helps them adjust their compass readings to ensure accurate navigation. Since compasses are influenced by the Earth's magnetic field, which can shift over time due to changes in the Earth's core or movements in the tectonic plates, understanding the local variation is essential for accurate navigation.

Other terms mentioned, such as deviation, drift, and set, refer to different navigational concepts. Deviation refers to the error introduced by local magnetic fields, drift pertains to the effect of currents or wind on a vessel's course over time, and set refers to the direction in which a current sets a vessel off its intended track. These terms are important in their own right, but they do not represent the difference between true north and magnetic north, which is specifically addressed by the term variation.