Declination is an angular measurement defining a position relative to a primary reference direction or plane. The term describes different concepts depending on whether one is navigating on Earth or charting the heavens. In a terrestrial context, it involves the difference between True North and Magnetic North, a practical concern for anyone using a compass. In astronomy, declination is a coordinate used to map the precise location of celestial bodies in the sky.
Declination in Earth Navigation
Magnetic declination is the measured angle between True North and Magnetic North at any given location on the planet’s surface. True North refers to the fixed geographic North Pole, which is the northern point of the Earth’s axis of rotation. Magnetic North is the point toward which a compass needle aligns, following the planet’s magnetic field lines.
This difference is important for wilderness hikers, surveyors, and others who rely on a map and compass for accurate travel. Since a compass points to Magnetic North, a navigator must correct the compass reading by adding or subtracting the local magnetic declination to find the true bearing to a destination. The declination value is positive, or easterly, if Magnetic North is to the east of True North, and negative, or westerly, if it is to the west.
Topographical maps typically include a declination diagram showing this angle. This diagram also often includes the date the measurement was taken and the rate of annual change. If this correction is not applied, a traveler could be steered off course by a significant distance. The angle of declination can range from zero degrees, along the agonic line, to more than 20 degrees in certain regions of the globe.
Declination in Astronomy
Astronomical declination is a coordinate used to specify the position of an object on the celestial sphere, an imaginary sphere surrounding the Earth. This measurement is the angular distance of a star, planet, or galaxy north or south of the Celestial Equator. The Celestial Equator is the projection of the Earth’s own equator out into space.
Declination is analogous to latitude on Earth, serving as the vertical coordinate in the equatorial coordinate system. It is measured in degrees, arcminutes, and arcseconds, with the Celestial Equator having a declination of zero degrees. Positions north of this line are positive, while those to the south are negative.
The maximum positive declination is +90 degrees, which corresponds to the North Celestial Pole, and the maximum negative value is -90 degrees, marking the South Celestial Pole. Using declination alongside right ascension, the horizontal coordinate, astronomers precisely pinpoint objects for telescopic observation.
Why Magnetic Declination Is Not Fixed
Magnetic declination changes due to processes within the Earth’s deep interior, specifically the outer core. This layer consists of molten iron and nickel, and its slow, convective movement generates the planet’s magnetic field, a process known as the geodynamo. The constant motion of this fluid material causes the magnetic field lines to shift and fluctuate over time.
This internal movement results in the phenomenon of polar drift, where the Magnetic North Pole constantly moves across the Arctic region. For example, the pole has been migrating rapidly toward Siberia in recent decades, causing declination values to change noticeably worldwide. Because of this movement, magnetic declination is location-dependent and time-dependent.
Map makers provide the specific epoch, or date, for which the magnetic reading was accurate, along with the annual rate of change. This allows navigators to perform a calculation to adjust the historical map data to the current year. Relying on old declination values can lead to navigational errors, as the rate of change is not uniform globally.