In the study of geomagnetism, declination is the angle between true and magnetic north, while astronomers employ this term in the coordinate system they use to describe fixed positions of stars in the celestial sphere. The intended meaning is usually clear from the context where the term is used. In both cases, it allows for very precise location measurements, a critical necessity when a small error can translate into a significant distance on land or in space.
The Earth's magnetic field is complex. It can vary from point to point, and over time, magnetic north and south tend to wobble around the true north and south poles. Depending on where and when someone takes a compass bearing, the accuracy of the compass in terms of locating true north can be quite variable. In some parts of the world, for example, the declination may be as much as 30 degrees. This means that when the compass points north, true north is actually 30 degrees away.
Declination charts for the surface of the Earth are available to help people orient themselves and use compasses accurately. If magnetic north is to the west of true north, this is expressed in a negative, like -15°. When it lies to the east, the declination is positive. It can be written in two different ways, with a directional symbol or a +/- symbol, as in 30E or +30. To correct for the variance, it is necessary to add or subtract to get the right measurement.
Electronic direction finding equipment can automatically correct for declination and generate an accurate true bearing. This is useful for sailors and hikers who rely on such equipment. It is important to periodically calibrate and test it to make sure it functions properly, as errors could be catastrophic. Otherwise, it is necessary to read a recent chart to determine how to correct the compass to get the right bearing. Many compasses are adjustable to allow people to correct for declination before they take a directional sighting.
Astronomers use declination to fix items on the celestial sphere relative to the celestial equator. If something is north of the equator it has a positive reading, while items south are negative. The projected south pole, for example, is located at -90° on the celestial sphere. This coordinate system allows astronomers to precisely observe and report stars and other phenomena. This acts much like latitude on Earth to determine where items are in relation to the imaginary line drawn at the equator.