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The magnetic declination and inclination are essential properties of the earth’s magnetic field. The predominant source of the magnetic field is the dynamo effect inside the planet. The dynamo effect is a naturally occurring phenomenon in which heat from the earth’s core produces a series of electric currents, which creates a magnetic field. A three-dimensional vector represents the magnetic field of the earth at any location. An ordinary compass is sufficient to measure the direction of the magnetic field.
Magnetic inclination
Magnetic inclination, or magnetic dip, is the angle formed between the earth’s surface and the planet’s magnetic lines. The magnetic inclination can be observed when a magnet is trying to align itself with the earth’s magnetic lines.
Since the earth is not flat, the magnetic field lines are not parallel. Hence, the compass needle’s north end will be either upward (southern hemisphere) or downward (northern hemisphere). The degree of inclination varies with the location on the earth.
The inclination value is:
- Positive if the inclination is downwards.
- Negative if the inclination is upwards.
The location with a 00 dip is called the aclinic line. This is present at the equator.
The dip angle formed by a compass, when held vertically, can be measured with an instrument known as the dip circle. George Hartmann found the dip angle in 1544, and Robert Norman elucidated the method to measure the dip circle with the dip angle in 1581 in England.
The magnetic inclination is represented by the letter I.
The formula for this is
I = tan-1 (ZH)
Z is the intensity associated with the magnetic field vector in the vertical direction and H is the intensity associated with the magnetic field vector in the horizontal direction.
Magnetic declination
Magnetic declination of the earth, or magnetic variation, is the angle formed between the magnetic north of the compass and the true geographical north. The value of the declination changes with location and time. It is represented by the letter D or the Greek alphabet δ.
The value of declination is:
- Positive if the magnetic north is along the east side of the true north.
- Negative if the magnetic north is along the west side of the true north.
Isogonic lines join points on the earth’s surface which have a common declination value that is also constant. Agonic lines are lines along with the value of declination is zero.
It is represented by the equation
D = Tan-1 (XY)
where X and Y are components of the magnetic field of the earth in the north and the south, respectively.
How does magnetic declination vary?
The value of the magnetic declination depends on the place and time. This is because some locations may have more iron ore deposits which can strongly influence the earth’s declination. It also depends on how much time has passed. For example, the UK had a one-degree declination towards the west in 2014, which reduced to zero degrees in 2020. It is said that the magnetic declination of the earth changes every hundred years by 20 to 2.50.
Methods to determine the magnetic declination of the earth
- By direct measurement
This method measures the declination at a particular location by directly referring to the celestial poles. The celestial poles are points in the sky around which stars revolve. These show the true north and south directions. The North Star or Pole Star or Polaris indicates the approximate position of the north celestial pole. A declinometer is used to find the value of the magnetic declination of the earth.
- With a map
A map is a way to get a rough estimate of local declination. The declination can be measured using an isogonic chart of the particular continent or the world. Isogonic lines will be illustrated in aeronautical and nautical charts. With the help of an illustration, local declination can be found on large-scale local maps.
- By a compass
A compass embodies three varieties of bearings: true bearing, magnetic bearing, and compass bearing.
These three values are related by the equations
T = M + V
M = C + D
T = C + V + D (general relation for all the bearings)
Where C is the compass bearing
M is the magnetic bearing
V is the magnetic variation
T is the true bearing
D is the compass deviation
The variation and deviation can be determined by
- If V < 0 and D < 0 then it is westerly variation and deviation
- If V > 0 and D > 0 then it is easterly variation and deviation
Conclusion
Magnetic declination and inclination are important features of the earth’s magnetic field. A three-dimensional vector represents the magnetic field at any location on earth. The direction of the magnetic field is measured using an ordinary compass. The magnetic inclination is the angle between the earth’s surface and the magnetic field lines. Magnetic declination is the angle between the magnetic north of the compass and the true north. The declination can be measured using a compass, the direct measurement method, and a map.
