SAN FRANCISCO – This morning some of our Dear Readers will have come across the words "Agonic Line" in another sailing publication. Your Ed., for one, was not familiar with the term. To save others similarly situated here is an explanation.
Executive Summary, or "TL/DR" as our Reddit-reading millennial friends would say, for "Too Long / Didn't Read"
The Magnetic North Pole and Geographic (or True) North Pole are not in the same place, not even close really. Since its discovery in 1831, the MNP has been around (yes, it moves) Ellesmere Island in Northern Canada, about 500 miles (800 kilometers) from the GNP. However, there are points on the surface of the earth where the compass needle lines up with both MNP and, beyond it, GNP. A line connecting all such points is known as the Agonic Line.
In the words of the late, great Paul Harvey, "Now, the rest of the story"
The Geographic North Pole and the Magnetic North Pole are not in the same position on the earth. Lines of latitude and longitude on charts are neatly arranged and relate to the Geographic North Pole. Of course, the magnetic compass indicates the direction to the Magnetic North Pole, which is currently in northern Canada; yes, it moves with time.
From any position on the globe, a direction can be determined to either the GNP or to the MNP. These directions are expressed in degrees from 0-360, and also fractions of a degree. The difference between these two directions is known as magnetic declination (sometimes called magnetic variation.)
(While a compass is installed in a vehicle or vessel, anomalies local to the vessel can introduce error into the direction that a compass points. The difference between the direction to Magnetic North Pole and the direction that the compass is pointing is known as magnetic deviation.)
Magnetic declination (again, sometimes called magnetic variation) is different depending on the geographic position on the globe. The MNP is currently in Northern Canada and is moving generally South. A straight line can be drawn from the GNP, down to the MNP, and then drawn straight down to the equator. This line is known as the Agonic Line, and the line is also moving.
In the year 1900, the Agonic Line passed roughly through Detroit and then was East of Florida. It currently passes roughly west of Chicago and through New Orleans. If a navigator is located on the Agonic Line, then the declination (variation) is zero. Magnetic North Pole and the Geographic North Pole appear to be directly in line with each other. If a navigator is East of the Agonic Line, then the declination is westward. Magnetic North appears slightly west of the Geographic North Pole. If a navigator is West of the Agonic Line, then the declination is Eastward – the Magnetic North Pole appears to the East of the geographic North Pole. The further the navigator is from the Agonic Line, the greater the declination. The local magnetic declination is indicated on NOAA Nautical Charts at the center of the compass rose along with the year of that declination. The annual increase or decrease of the declination is also usually indicated. So that the declination for the current year can be calculated.
A GPS receiver natively relates to the Geographic North Pole, but can elegantly calculate Magnetic North based on its true position and data tables; the unit can then calculate the current location of, and hence direction, to the MNP.
Does the compass on an iPhone (and other smartphones) show Geographic North or Magnetic? The default on most phones is Magnetic North so that it reads like a handheld compass. But in Settings/Compass you can turn on "Use True North" and it will give you a GPS bearing, provided the phone's "Location Services" is also turned on.
Question of the Day: Does a Moral Compass use True North, Magnetic North or neither?