Home   Sun, Moon & Space   Magnetic Declination

What Is Magnetic Declination?

If you have ever used a compass, you know it points to the north. But did you know it usually doesn't point to the geographic North Pole or to the magnetic North Pole?

Illustration image

North on a compass can deviate from true north.


Three Versions of North

There are two main versions of north: true north and magnetic north. There is also a third version, grid north, but this is not related to magnetic declination, so it is only briefly described at the bottom of this article.

The difference between true north and magnetic north is called magnetic declination or magnetic variation. Some sources also erroneously refer to it as magnetic deviation. It's essential to take this phenomenon into account when navigating with a compass.

Magnetic North vs. True North

The magnetized needle of a compass usually does not point towards the geographical North Pole, or true north. Rather, it aligns itself with the direction of the Earth's magnetic field, showing magnetic north. In most locations on Earth, magnetic north differs from true north, so magnetic declination has to be taken into account.

Magnetic declination is expressed in degrees east or west. A western declination is usually stated as a negative value while an eastern declination is represented by a positive value. For example, in Miami, Florida, magnetic north currently lies around 7 degrees west of true north, so the city has a magnetic declination of -7°.

Trying to figure out a compass heading for the true-north directions shown in our Sun Calculator or Moon Calculator? Please see instructions below.

Large Local Variations

The difference between true and magnetic north varies by location and over time. In most populated parts of the world, it currently ranges from nearly 30 degrees west (south-eastern tip of Africa) to about 26 degrees east (southern tip of New Zealand). However, most locations on Earth have a considerably smaller declination.

On the west coast of the United States, the compass direction lies between 10 and 16 degrees east of true north. In locations on the east coast, the magnetic declination is about 7–16 degrees west. Cities like New Orleans, Louisiana and Minneapolis, Minnesota are situated very close to an agonic line, meaning that the magnetic declination is close to 0, so a compass actually shows true north there.

The same is true for London, UK and Madrid, Spain. In general, western Europe has a comparatively small declination, which ranges from -1 to 5 degrees.

The Magnetic North Pole

The magnetic North Pole is the location in the Northern Hemisphere where the planet's magnetic field lines point straight downwards, penetrating the surface of the Earth. Its location changes at a rate of about 56 kilometers (35 miles) per year. Currently, it is located in the Arctic Ocean north of Russia, hundreds of kilometers away from the geographic North Pole.

In the Arctic, the magnetic declination is largely a result of that distance. For example, a compass positioned between the magnetic and the geographical North Pole will point due south, towards the magnetic pole and away from the geographical pole, amounting to a magnetic declination of around 180°.

Compass Rarely Points to Magnetic North Pole

In other parts of the world, away from the Arctic, a compass needle usually does not point towards the magnetic North Pole. As one moves away from the magnetic North Pole, its distance to the geographical pole becomes increasingly irrelevant for magnetic declination.

Currents in the Earth's core as well as deposits of iron and other chemical elements in the Earth's crust deflect the magnetic lines on the surface. On a global scale, this causes vast, geographically irregular, and ever-shifting variations in the magnetic declination.

On a world map, these variations are usually indicated by isogonic lines. These are lines showing areas sharing the same magnetic declination.

Without these variations, the magnetic declination on the meridian south of the magnetic North Pole would be 0 because both the magnetic and the geographical poles are due north. However, the actual declination at that longitude is between 10 and 20 degrees on the equator. In practice, following a compass to the magnetic North Pole will get you there—but not via the shortest route.

Converting True North Heading to Compass Heading

All horizontal directions, or azimuths, displayed on timeanddate.com refer to true north. They are displayed in degrees, with 360 degrees in a full circle, counted in a clockwise direction from the north. An azimuth value of 0 degrees signifies true north, pointing directly towards the geographical North Pole. Similarly, 180 degrees is the direction from the selected location to the geographic South Pole.

To convert the true north values displayed in our Sun Calculator or Moon Calculator to compass headings—for example, if you wish to take pictures of the sunrise or moonrise—start by finding out the magnetic declination of your location.

Then simply subtract the declination angle from the true heading to find out the compass heading.

Example for locations with positive (eastern) declination: Moscow has a magnetic declination of about 12°. Each year on July 1, the Sun rises at an angle of 44° in the Russian capital. To convert this true north heading to a compass heading, you have to subtract 12° from 44°, so the sunrise occurs at 32° on your compass.

Example for locations with negative (western) declination: In New York, the sunrise on July 1 is at 58° each year. New York has a declination of roughly -13°. Since subtracting a negative value is equivalent to adding a positive value, the compass heading is 58° + 13° = 71°.

Grid North

Grid north is a third type of north that applies to map navigation. It refers to the north direction as shown on some maps. For example, the maps produced by the United States Geological Survey and the UK Ordnance Survey use straight vertical and horizontal lines to divide the map into a grid. Here, the vertical lines do not necessarily point to true north but to grid north.

Since the Earth is a globe and its surface is curved, lines running along the north-south axis, pointing to true north, would have to be slightly curved when displayed on a plane surface, such as a map. Grid north ignores this directional distortion caused by map projection, as expressed by the straight vertical lines.

The deviation between grid north and true north varies from one portion of the map to another. The vertical line in the middle of the map actually points to true north, whereas the lines right and left (east and west) of the center line do not. The difference is largest at the edges of the map.

Another factor affecting the difference between grid north and true north is the scale of the map. The smaller the scale, the larger the deviation. However, most topographic maps used for navigation have a fairly large scale, meaning that the difference is usually negligible for most purposes.

Topics: Sun, Moon, Astronomy, Earth, Geography