Magnetic Declination

There are three types of north: True north, Grid north, and Magnetic north. According to the American practical navigator Nathaniel Bowditch, the magnetic declination is the angle between the magnetic and geographical meridians at any place which is expressed in degrees and minutes east or west to indicate the direction of magnetic north from true north.

What Is Magnetic Declination?

Magnetic declination is defined as the angle between magnetic north and true north on the horizontal plane, which is not constant and keeps changing depending upon the position on the earth’s surface and time. The Greek letter δ is used as the symbol for magnetic declination and is also known as magnetic variation.

When the magnetic north is east of true north, the declination is positive and when the magnetic north is west of true north, the declination is negative. The other terms used are isogonic lines (when the lines along the declination are constant) and agonic lines (when the lines along the declination are zero)

Interested to learn more concepts related to the magnetic field, below are the links:

What Is True North?

True north is defined as the direction along the earth’s surface towards the true north pole or geographical north pole. It is also known as geodetic north and is different from the magnetic north which is the direction pointed by the compass and from the grid north which is in the direction along the grid lines towards the north.

What Is Grid North?

Grid north is defined as the direction which is in northwards along the grid lines on a map projection. This term is used for navigation and the deviation of grid north from the true north is very less.

What Is Magnetic North?

Magnetic north is defined as the direction which is pointed by the compass needle in response to the earth’s magnetic field. The deviation between the true north and the magnetic north varies from place to place as the earth’s magnetic poles are not fixed with respect to its axis.

Difference Between Magnetic North And True North

Following is the table explaining magnetic north vs true north:

Magnetic north True north
It is the north direction pointed by the compass needle which is along the earth’s magnetic field. It is the geographical north pointing towards the north pole.

What Is Magnetic Dip?

The magnetic dip is defined as the angle made with the horizontal by the earth’s magnetic field lines. It is also known as dip angle or magnetic inclination and was discovered by Georg Hartmann in the year 1544. When the inclination is positive it indicates that the earth’s magnetic lines are pointing downward in the northern hemisphere and when the inclination is negative it indicates that the earth’s magnetic lines are pointing upward in the southern hemisphere.

In the year 1581 Robert Norman discovered a dip circle which is a method used to measure the dip angle. The other terms used are isoclinic lines (when the contour lines are equal at the earth’s surface) and aclinic lines (when the locus of the points are having zero dip).

How To Calculate Magnetic Declination?

Following are the different ways used to calculate the magnetic declination:

  • From the declination calculator: The declination calculator is an easy way to calculate the declination of any location on the earth. By providing the year, latitude and longitude of a given location, the calculator gives the declination on the basis of magnetic reference field models.
  • From a magnetic declination chart: A magnetic declination chart is a map with the earth’s magnetic fields available on it.
  • From a compass: There are three types of bearing, they are true, magnetic and compass bearing. A compass can be used to calculate the declination as it is one of the errors of the compass and the other is magnetic variation. These three are related by:
T = M + V
M = C + D
T = C + V + D (which is a general equation relating compass and true bearings)

Where,

  • C is the compass bearing
  • M is the magnetic bearing
  • T is the true bearing
  • V is the variation
  • D is the compass deviation
  • V < 0, D < 0 for westerly variation and deviation
  • V > 0, D > 0 for easterly variation and deviation

Following is the way to calculate compass bearing from true bearing:

  • True bearing – variation = magnetic bearing
  • Magnetic bearing – deviation = compass bearing

Following is the way to calculate true bearing from compass bearing:

  • Compass bearing + deviation = magnetic bearing
  • Magnetic bearing + variation = true bearing

Now it becomes very clear that when we check for north direction using a compass, the needle is actually pointing towards the earth’s magnetic north and not the true north. To know more about other Physics concepts, stay tuned with BYJU’S.

Leave a Comment

Your email address will not be published. Required fields are marked *