Magnetic Poles, Declination and Inclination
Geography › Interior of the Earth and Magnetic Field · Topic 2.6
The magnetic poles are the points where the Earth's magnetic field lines are vertical, and they do not coincide with the geographic poles. Magnetic declination is the angle between true north and magnetic north at a place, magnetic inclination or dip is the angle between the field and the horizontal, and the magnetic equator is the line where the dip is zero.
5 min read · UPSC Prelims · Geography · Interior of the Earth and Magnetic Field
At a Glance
- Declination Angle between true north and magnetic north
- Inclination (dip) Angle between field and horizontal
- Dip at magnetic equator 0 degrees
- Dip at magnetic poles 90 degrees
- Pole position Not fixed; drifts every year
- Exam link UPSC Prelims, Geography: Interior of the Earth and Magnetic Field
Where Does It Fit?
This is the sixth topic of the category on the interior of the Earth and its magnetic field. The previous topic explained how the field originates, and this one describes how it is measured on the surface. The next topic covers geomagnetic reversal and geomagnetic poles. Previous topic: Dynamo Theory and the Magnetic Field of the Earth. Next topic: Geomagnetic Reversal and Geomagnetic Poles.
What Are the Magnetic Poles?
In short: the magnetic poles are where a compass needle would point straight down or straight up.
The magnetic poles are the two points on the surface of the Earth where the magnetic field lines are vertical. At the North Magnetic Pole a freely suspended magnet points straight down, and at the South Magnetic Pole it points straight up. By convention the north-seeking end of a compass needle is attracted to the magnetic pole in the northern hemisphere, which is physically the south pole of the Earth’s magnet. The magnetic poles do not coincide with the geographic poles, which are the ends of the axis of rotation, and they are not exactly opposite each other. The North Magnetic Pole lies in the Arctic and has been drifting from the Canadian Arctic towards Siberia, at a rate of tens of km each year, while the South Magnetic Pole lies off the coast of Antarctica.
What Is Magnetic Declination?
A compass points to magnetic north, but a map is based on true or geographic north. The horizontal angle between the two directions at a place is the magnetic declination or variation. Declination is positive or east when magnetic north lies east of true north, and negative or west when it lies to the west. It is zero along a line called the agonic line, where a compass points exactly to true north. Lines of equal declination are called isogonic lines. Declination changes with place and with time, so surveyors, pilots and navigators use updated charts. In India the declination is small, generally less than about 2 to 3 degrees, and it varies slowly across the country.
What Is Magnetic Inclination or Dip?
The magnetic inclination, or dip, is the angle that the magnetic field makes with the horizontal plane at a place. A compass needle that is free to move up and down would tilt downward in the northern hemisphere and upward in the southern hemisphere. The magnetic dip changes with latitude.
| Location | Dip angle | Field direction |
|---|---|---|
| Magnetic equator | 0 degrees | Horizontal |
| Mid-latitudes | Between 0 and 90 degrees | Inclined |
| Magnetic poles | 90 degrees | Vertical |
The line joining places with zero dip is the magnetic or dip equator. Lines of equal dip are called isoclinic lines. The magnetic equator does not coincide with the geographic equator, and it moves slowly.
Why Is the Magnetic Equator Near Thiruvananthapuram Important?
The magnetic equator passed close to Thumba near Thiruvananthapuram in Kerala when India chose it for its first rocket launching station in 1963, since a launch site on the magnetic equator is ideal for studying the equatorial electrojet, a strong electric current in the upper atmosphere, and for upper atmosphere research. The Thumba Equatorial Rocket Launching Station was the base from which the first Indian sounding rocket was launched, and it later grew into the Vikram Sarabhai Space Centre. As an analytical point, the magnetic equator keeps drifting, so the claim that it passes through a particular town should be read as true for a given date, and the dip equator today lies a little away from where it was in the 1960s.
How Are Poles Different: Geographic, Magnetic and Geomagnetic?
| Pole | What it is | Behaviour |
|---|---|---|
| Geographic pole | End of the axis of rotation | Almost fixed |
| Magnetic pole | Place where the field is vertical | Wanders and is not antipodal |
| Geomagnetic pole | End of the best-fit dipole axis through the centre | Antipodal and changes slowly |
The idea of the geomagnetic pole is in Geomagnetic Reversal and Geomagnetic Poles. The source of the whole field is the core, as described in Dynamo Theory and the Magnetic Field of the Earth.
Why Is This Topic Important?
- Navigation Compass use
- India link Thumba and the magnetic equator
- Concepts Declination versus dip
What Came Next?
The next topic describes geomagnetic reversals and the geomagnetic poles. Next topic: Geomagnetic Reversal and Geomagnetic Poles.
Key People and Terms
- Declination Angle from true north
- Inclination Angle of dip from the horizontal
- Isogonic line Line of equal declination
- Agonic line Line of zero declination
- Isoclinic line Line of equal dip
- Magnetic equator Line of zero dip
Exam Corner
Points to Remember
- Dip is 0 degrees at the magnetic equator and 90 degrees at the poles.
- Declination is measured from true north.
- Isogonic lines join equal declination.
- Magnetic poles drift each year.
- Thumba was chosen as it lay on the magnetic equator.
Do Not Confuse With
- Declination and inclination Horizontal angle versus vertical angle
- Isogonic and isoclinic Equal declination versus equal dip
- Geographic and magnetic equator Different lines that do not coincide
Memory Hook
Declination deviates from true north; inclination dips down; zero dip at the equator.
Mains Angle
Questions ask for definitions and use. These points add depth.
- Navigation Correct compass readings
- Drift Poles and equator move
- Research Equatorial electrojet
A Question You May Face
An original practice question, not a past paper question.
“Distinguish between magnetic declination and magnetic inclination and explain their importance for navigation.”
How to Answer
- Define Declination and dip.
- Compare Horizontal and vertical.
- Lines Isogonic and isoclinic.
- Use Navigation and research.
GS Relevance
Frequently Asked Questions
What is magnetic declination?
Magnetic declination is the horizontal angle between true north and the direction in which a compass needle points, magnetic north. It varies from place to place and changes slowly with time, so navigators and surveyors correct their compass readings for it.
What is magnetic inclination or dip?
Magnetic inclination, or dip, is the angle that the Earth's magnetic field makes with the horizontal at a place. It is zero at the magnetic equator, where the field is horizontal, and 90 degrees at the magnetic poles, where the field is vertical.
What is the magnetic equator?
The magnetic equator is the line on the Earth joining places where the magnetic dip is zero, that is, where the field is horizontal. It does not coincide with the geographic equator, and it passed close to Thumba near Thiruvananthapuram when the rocket station was set up in 1963.
Do the magnetic poles coincide with the geographic poles?
No. Geographic poles are the ends of the axis of rotation and are almost fixed, while the magnetic poles are where the field lines are vertical. The magnetic poles wander each year, and they are not exactly opposite each other on the globe.
What are isogonic lines?
Isogonic lines are lines on a map that join places with the same magnetic declination. The isogonic line of zero declination is called the agonic line, along which a compass points exactly to true north.
PYQ Practice — Statement Analysis
1 Magnetic dip is zero at the magnetic equator.
The field is horizontal there.
2 Declination is measured from the horizontal plane.
Declination is measured from true north.
3 The magnetic poles are fixed in position.
They drift over the years.
4 Isoclinic lines join places with equal magnetic dip.
Isogonic lines join equal declination.
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