aclinic

Aclinic refers to a line on the Earth's surface where the magnetic declination is zero. Magnetic declination, also known as magnetic variation, is the angle between magnetic north and true north at any given point. This angle varies geographically and over time. The aclinic line, therefore, represents locations where a compass needle, if free to move, would point directly towards true geographic north without any deviation.

Historically, the aclinic line was also referred to as the "agonic line." It is not a fixed line but rather shifts slowly due to changes in the Earth's magnetic field. Navigators have used the concept of the aclinic line to aid in accurate plotting of courses and positions. Before the widespread adoption of GPS and other electronic navigation systems, understanding and accounting for magnetic declination, and thus knowing one's position relative to the aclinic line, was crucial for maritime and aerial travel.

The aclinic line passes through specific points on the Earth's surface where the magnetic poles are aligned with the geographic poles. As the Earth's magnetic field is dynamic, the aclinic line is constantly moving. Maps that display magnetic declination often show lines of equal declination, known as isogonic lines, with the aclinic line being the isogonic line representing zero declination.