The Earth as a magnetic body
A compass responds to the Earth's magnetic field, not directly to the geographic grid. The first task is to separate geographic poles, magnetic poles and magnetic polarity.
The field around the Earth
A magnet produces a field in the space around it. A freely suspended magnet aligns with that field. The Earth has a magnetic field produced mainly by electrical currents in its conducting outer core. For elementary navigation the field is represented by a powerful bar magnet near the Earth's centre. This dipole model is useful, but the real field is irregular and changes with place and time.
Magnetic materials such as iron, steel, nickel and cobalt can be attracted or magnetised. Like poles repel and unlike poles attract. In the elementary bar-magnet model, the force between two poles weakens rapidly as their separation increases, approximately with the square of the distance.
A field line shows the direction in which the north-seeking end of a small test magnet would point. Near the surface, the local field normally has both a horizontal part and a vertical part. A compass uses the horizontal part to establish magnetic north.
Geographic poles and magnetic poles
The geographic poles are the ends of the Earth's axis of rotation. True north is the direction along a meridian towards the Geographic North Pole. The magnetic poles are locations where the Earth's field is vertical. They do not coincide with the geographic poles, they are not exactly opposite each other, and they move slowly.
| Reference | What defines it | Navigation use |
|---|---|---|
| Geographic North Pole | Earth's rotation axis | True north and true direction |
| Northern magnetic pole | Field is vertical in the northern region | Nearby horizontal directive force becomes very small |
| Magnetic north at an observer | Horizontal direction of the local Earth field | Reference for magnetic direction |
The blue pole and red pole convention
In the convention used in the source books, the north-seeking end of a magnet is the red pole and the south-seeking end is the blue pole. Unlike magnetic poles attract. The red end of a compass therefore points towards a blue magnetic polarity in the northern region. The place is still called the North Magnetic Pole because of its geographic location, even though its physical polarity attracts the red north-seeking end.
A model, not a perfect bar magnet
A central bar magnet explains the broad pattern of field lines, magnetic dip and the separation between true and magnetic north. It does not explain every local feature. The real field is not symmetrical, the poles wander, and local magnetic minerals can produce anomalies. Current charts and a current magnetic model are therefore essential.
Bali's source-era description places the northern magnetic pole about 750 NM from the Geographic North Pole and gives an approximate 960-year circuit around it. Treat those figures as an illustration of secular movement, not as a present-day pole position. Oxford's successive source dates likewise show the pole moving between surveys.
Magnetic dip and magnetic latitude
The field does not usually lie flat against the Earth's surface. Its angle below or above the local horizontal is magnetic dip.
Angle of dip
Magnetic dip, also called magnetic inclination, is the angle in the vertical plane between the local horizontal and the total Earth magnetic field. In the Northern Hemisphere the field is normally inclined downwards towards the north. In the Southern Hemisphere it is inclined upwards towards the north, or equivalently downwards towards the south.
A free magnet that can rotate about both a vertical and a horizontal axis follows this inclined field. A heading compass is designed to remain nearly horizontal, so the vertical tendency is a source of compass limitations developed in Chapter 4.
Dip through the field
| Magnetic region | Dip | Field direction |
|---|---|---|
| Magnetic equator | 0 degrees | Field is horizontal |
| Intermediate magnetic latitude | Between 0 and 90 degrees | Field has horizontal and vertical components |
| Magnetic pole | 90 degrees | Field is vertical |
The magnitude of dip generally increases with magnetic latitude. A simple dipole model produces a smooth increase, as shown in the figures. The operational field is more irregular, so actual dip is taken from field data rather than assumed from geographic latitude.
Isoclinals and the aclinic line
An isoclinal is a line joining places of equal magnetic dip. The aclinic line joins places where dip is zero and is therefore the magnetic equator. It is not the same as the geographic Equator, because the magnetic field is tilted and irregular.
Magnetic latitude is not geographic latitude
Magnetic latitude describes position within the magnetic field. Geographic latitude is measured from the geographic Equator. Because the magnetic and geographic axes are separated, two places on the same geographic parallel need not have the same dip.
Total force and its components
The total magnetic force T can be resolved into horizontal force H and vertical force Z. This triangle explains both dip and the geographical limit of a magnetic compass.
The force triangle
The total field strength is represented by T. Its horizontal component is H, and its vertical component is Z. These three quantities form a right triangle. The angle between H and T is the angle of dip.
Worked example from components
Suppose H is 30 microteslas and Z is 40 microteslas. T equals the square root of 30² plus 40², so T is 50 microteslas. Tan dip is 40 divided by 30, giving a dip of about 53.1 degrees.
Worked example from total force
If T is 50 microteslas and dip is 60 degrees, H is 50 cos 60 degrees, or 25 microteslas. Z is 50 sin 60 degrees, or approximately 43.3 microteslas. These values also satisfy T² = H² + Z².
Directive force
The horizontal component H is called the directive force because it turns the compass magnet into alignment with the local magnetic meridian. At the magnetic equator H is greatest relative to T and Z is zero. Towards a magnetic pole, Z increases while H weakens. At the pole H is zero and the field is vertical.
A direct-reading magnetic compass cannot indicate a dependable horizontal direction when H is too small. Oxford uses a notional minimum field of about 6 microteslas for a compass detector. Whatever the equipment threshold, the principle is the same: no useful H means no useful magnetic heading.
Bali gives about 70 degrees magnetic north or south as a teaching limit beyond which a heading compass becomes unreliable because of dip. It is a rule of thumb, not a substitute for the limitations of the installed system.
Magnetic north and variation
True north comes from the Earth's axis. Magnetic north comes from the local horizontal field. The angle between those two directions is variation.
Magnetic meridian
At any position where H is usable, the vertical plane through the local horizontal field is the magnetic meridian. The horizontal direction towards magnetic north is the direction indicated by a freely suspended magnet affected only by the Earth's field.
Magnetic variation
Variation, also called magnetic declination, is the horizontal angle at a place between true north and magnetic north. It is named east when magnetic north lies east of true north, and west when magnetic north lies west of true north. Variation belongs to the place and date, not to the aircraft.
| Local arrangement | Name | Meaning |
|---|---|---|
| Magnetic north east of true north | East variation | Magnetic meridian lies clockwise from true meridian |
| Magnetic north west of true north | West variation | Magnetic meridian lies anticlockwise from true meridian |
| Magnetic north coincides with true north | Zero variation | The place lies on an agonic line |
Why variation changes with position
The true meridian at each place points towards the geographic pole, while the magnetic meridian follows the local field. Moving the observer changes the angle between these directions. Even the ideal tilted dipole can therefore produce east variation, west variation or zero variation at different places.
The maximum possible variation is 180 degrees. This exceptional geometry can occur in the region between a geographic pole and the corresponding magnetic pole, where true north and magnetic north may be opposite directions. It is not a normal mid-latitude chart value.
Situation near the poles
At a geographic pole every outward direction is south, so the ordinary true-north reference becomes singular. At a magnetic pole H is zero, so a magnetic direction is not defined by a horizontal compass. Isogonals appear to converge in the regions of both geographic and magnetic poles because the reference geometry becomes singular there.
Isogonals, agonic lines and chart use
A navigation chart turns thousands of individual variation values into a field of labelled lines. The line pattern must be read with its date.
Isogonals
An isogonal joins places having equal magnetic variation at the stated epoch. A line marked 4 degrees east is therefore a locus of points where the predicted variation is 4 degrees east for the chart's reference date. Between lines, variation is normally interpolated.
The agonic line
An agonic line is an isogonal of zero variation. Along it, true north and magnetic north coincide at that date. It is irregular and can form more than one segment on a world map. It is not a meridian and must not be confused with the aclinic line.
Isogonal compared with isoclinal
| Line | Joins equal values of | Zero-value name |
|---|---|---|
| Isogonal | Variation | Agonic line |
| Isoclinal | Magnetic dip | Aclinic line, or magnetic equator |
Read the chart annotation
A chart may print the isogonal value, its direction east or west, the epoch to which it applies, and an annual change. Apply the annual change for the elapsed time from the epoch. Do not assume that an old printed value remains current.
Variation changes with time
The Earth's field is dynamic. Slow drift matters for chart updating, while shorter changes explain why a magnetic value is never absolutely fixed.
Secular change
Secular change is the slow long-term change in the strength and direction of the main magnetic field. The magnetic poles wander and the isogonal pattern moves. A chart therefore states an epoch and often gives annual change so that variation can be brought forward to the date of flight.
Magnetic models predict the field for a defined epoch and a limited period. Oxford treats roughly ten years as the outer limit for a useful long-range forecast. Models are updated because the future motion of the field cannot be known indefinitely. A current chart or database is the operational source, not a memorised variation from an old edition.
Annual and diurnal changes
An annual variation is associated with the yearly cycle. Diurnal variation is the small daily change linked to solar heating and ionospheric current systems. Oxford notes that normal diurnal variation may be up to about 0.1 degree. These small cycles sit on top of secular change.
Magnetic storms
Disturbances from solar activity can cause rapid, irregular magnetic changes called magnetic storms. Solar activity follows an approximate 11-year cycle. Oxford records historical short-term variation changes of up to about 7 degrees during severe events. Such a disturbed value is not handled by treating the normal chart annual change as exact.
Local anomalies and altitude
Magnetic rocks and mineral deposits can distort the field locally. Charts may warn of significant anomalies. Variation can also alter slightly with altitude because the aircraft is sampling the field at a different point in space, although ordinary chart use normally employs the published surface model.
| Change | Typical timescale | Operational response |
|---|---|---|
| Secular | Years and decades | Use epoch and annual change, then use updated charts |
| Annual | One year | Included in the predicted field model |
| Diurnal | One day | Usually small, about 0.1 degree in normal conditions |
| Magnetic storm | Hours to days | Expect irregular disturbance during solar activity |
| Local anomaly | Fixed to location | Observe charted warnings |
India, field models and the complete picture
For Indian navigation, the method is the same as anywhere else: take variation and annual change from current approved chart data, with the field model epoch clearly identified.
WMM and IGRF
The World Magnetic Model, abbreviated WMM, and the International Geomagnetic Reference Field, abbreviated IGRF, describe the large-scale field mathematically. Each published version has an epoch. The model supplies predicted variation, dip and field strength for position, altitude and date within its valid range.
Keep the four north references separate
This chapter needs only true north and magnetic north. Compass north is introduced in Chapter 4 because it includes aircraft deviation. Grid north belongs with grid navigation. Keeping the reference words explicit prevents an Earth-field question from turning into a compass-correction question.
| Exam cue | Correct idea |
|---|---|
| Earth represented as a magnet | Approximate dipole model, not a perfect central bar |
| Northern magnetic polarity | Blue pole attracts the red north-seeking compass end |
| Dip | Angle of T to the horizontal in the vertical plane |
| Directive force | Horizontal component H |
| Variation | Horizontal angle between true north and magnetic north |
| Equal variation | Isogonal |
| Zero variation | Agonic line |
| Equal dip | Isoclinal |
| Zero dip | Aclinic line, the magnetic equator |
| Long-term drift | Secular change, handled by epoch and annual change |
What carries forward
The Earth provides a total field T. Its horizontal component H supplies heading information and its vertical component Z produces dip. Variation relates the local magnetic meridian to the true meridian. Chapter 4 adds the aircraft's own magnetic field, compass deviation and the practical compass systems.