Magnetic declination

In many places on Earth, there is a difference between true or polar north—the North Pole—and magnetic north, or the direction a compass points to as north. This is caused by variations in the magnetic field of Earth's outer core and often requires compasses to be adjusted to point to true north. The angle formed between the lines of true north and magnetic north is known as magnetic declination. Magnetic declination changes by location and over time and is calculated by various government agencies, including the National Geophysical Data Center and the U.S. Geological Survey's National Geomagnetism Program.

The concept of magnetic declination is important to navigators of air, land, and sea and is known by different names. The modern term, magnetic declination or simply declination, is most often used by those navigating on land. Because the term declination is also used to refer to the angle of a planet or star above the celestial equator, contemporary pilots and mariners often use the term variation instead of magnetic declination.

Magnetic Fields Explained

Scientists believe the center of Earth is a large solid core made up mostly of iron (Fe) surrounded by an outer core that also includes iron but also other metals, including nickel. Due to the lower pressure here, the outer core is molten. This outer core has currents due to differences in composition, temperature, and pressure from Earth’s surface and its rotation.

Physicists believe the way this liquid core flows around the more solid iron core creates electric currents. Where currents run in the same direction, they attract each other. Where they run in the opposite direction, they repel each other. This attraction and repulsion creates magnetic fields. Earth is surrounded by a magnetic field that spreads out in large circles and comes together at magnetic north and south poles.

Magnetism and Navigation

Centuries ago the Greeks and the Chinese both discovered that certain rocks had the power to attract iron. They noticed that when a metal needle was stroked with one of these magnets the needle also developed attractive properties. The Chinese tried suspending such a needle so that it could spin and found that it assumed a north-south alignment. This was the earliest compass, and variations soon became standard tools for navigators in Asia and eventually in Europe.

Navigators soon discovered slight differences between the direction a compass indicated as north and what the stars told them. They determined that they need to compensate for magnetic declination. It is believed the earliest calculations of the declination angle were made by the Chinese in the early eighth century, hundreds of years after they made the first compass. The Chinese are also credited with realizing that the angle of magnetic declination changed over time. On his first voyage to North America in 1492, Christopher Columbus observed his compass varied from pointing east of north while he was closer to Europe and west of north when he was closer to the New World. This observation made him the first to record differences in magnetic declination based on location. The earliest attempts at measuring and recording magnetic declination for different locations were made in the 1500s. It would be nearly two centuries before Edmond Halley (1656 – 1742), the English astronomer and mathematician who would also calculate the recurring orbit of the comet that now bears his name, established a way of marking magnetic declination with contoured lines on a map. He created the first chart of the Atlantic Ocean that indicated magnetic declination.

Zero Declination

On his first voyage to the New World, Columbus also noted that in an area of the Atlantic Ocean near the Azores the compass was pointing directly north. This is the first recorded instance of a place of zero declination, where true north and magnetic north line up. A number of places on Earth also display the characteristics Columbus saw. Just as Columbus noted, a compass will point east of true north in locations on the west side of this line and west of polar north on the east side.

Adjusting for Magnetic Declination

Magnetic north is moving northward about 30 km a year, making accurate calculations difficult yet necessary. Modern travelers have easier ways to set a course based on true north than the ancient mariners did. Many maps have information about magnetic declination printed near the scale bar, but because the angle can change by location and is affected over time by solar storms and other atmospheric disturbances, magnetic declination is available online through various government agencies that create updated calculators and other resources. One example is the Mobile Declination Calculator from the National Oceanic and Atmospheric Administration, available at http://www.ngdc.noaa.gov/geomag-web/. Most compasses have a mechanism that allows them to be adjusted using these magnetic declination calculations to allow for easy navigation.

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