eduKate Learning Manual
Science | Earth, Water, Atmosphere & Celestial World
Understand → Teach → Learn → Memorize → Test → Go Deeper
The Compass
Why North Is Not Always True North
WAIT, WHAT? A Compass Does Not Point at the Geographic North Pole
Place a compass on a table and let the needle settle.
The marked end points roughly north.
It is tempting to imagine an invisible line connecting the needle directly to one magnetic pole somewhere in the Arctic.
That is not what the instrument is doing.
A freely pivoting compass aligns with the local horizontal direction of Earth’s magnetic field where the compass is sitting.
That local direction is usually not exactly the same as geographic true north.
The angular difference is called magnetic declination.
And because Earth’s magnetic field changes, the correction itself changes with location and time.
Modern Navigation Still Needs a Living Magnetic Map
NOAA and the British Geological Survey maintain the World Magnetic Model used in navigation systems. The current WMM2025 models Earth’s main magnetic field and its slow change through time.
The striking lesson is that an ancient-looking instrument still depends on a planet-sized field that must be measured and updated.
compass reading → local magnetic field direction → declination correction → true geographic direction.
Big Question: How does a tiny magnetized needle reveal Earth’s local magnetic field, and why can its “north” differ from true geographic north?
Quick Answer
A compass needle is a small magnet free to rotate horizontally.
Earth produces a magnetic field. At any location, that field has both horizontal and vertical components.
A normal compass is constrained to rotate mostly in the horizontal plane, so it aligns with the horizontal component of the local magnetic field.
Geographic true north is defined by Earth’s rotation axis. Magnetic north is defined from the magnetic field.
Those directions are not generally identical.
declination = angle between true north and magnetic north at a location.
Declination changes across Earth and changes slowly with time because the magnetic field generated in Earth’s fluid outer core evolves.
What You Will Learn
- Why a compass needle rotates.
- What Earth’s magnetic field is.
- Why a compass responds to the local field rather than a distant point.
- What horizontal magnetic-field component means.
- What true north means.
- What magnetic declination means.
- Why declination changes with place and time.
- Why compasses become unreliable near magnetic poles.
- What magnetic inclination or dip means.
- Why nearby metal and electric currents can disturb a compass.
- How modern navigation models correct magnetic headings.
- Why a phone compass needs calibration even though it has no swinging needle.
Part 1 — The Needle Is a Magnet
A compass needle has two magnetic poles.
When placed in an external magnetic field, the two ends experience forces that create a torque.
That torque rotates the needle until its magnetic moment is approximately aligned with the surrounding field.
The compass therefore measures direction by allowing a small magnet to turn toward a lower-energy orientation.
Part 2 — Earth Has a Magnetic Field
Earth is surrounded by a magnetic field extending far into space.
The large-scale field is generated mainly by electrically conducting liquid iron moving in Earth’s outer core.
Rotation, convection and magnetic induction maintain a geodynamo.
The field is broadly dipole-like, but real Earth is more complicated than one perfect bar magnet.
Part 3 — A Magnetic Field Has Direction
At one location, Earth’s magnetic field can be represented by a vector.
That vector can be split into:
- a horizontal component along the local ground plane;
- a vertical component pointing upward or downward.
A flat compass responds mainly to the horizontal component because the needle is designed to rotate around a vertical pivot.
Part 4 — The Compass Does Not Aim at One Magnetic Pole
NOAA explicitly warns against the common picture that the needle points directly toward a single magnetic pole.
Instead, it aligns with the local horizontal field direction.
Magnetic field lines curve through three dimensions. A compass at Singapore, London or Alaska samples the field locally, not by drawing a straight line to the Arctic magnetic pole.
Part 5 — What Is True North?
Geographic true north points along Earth’s surface toward the Geographic North Pole.
The Geographic North Pole is defined by Earth’s rotation axis at latitude 90° N.
Maps and geographic coordinate systems are built around this rotational geometry.
Part 6 — What Is Magnetic North?
For navigation, magnetic north means the direction of the local horizontal magnetic-field component.
That is the direction a well-behaved compass tends to indicate after local disturbances are removed.
It is a field direction, not merely the coordinate of the magnetic dip pole.
Part 7 — Magnetic Declination Is the Difference
Magnetic declination is the horizontal angle between magnetic north and true north.
NOAA defines east declination as positive and west declination as negative.
If magnetic north lies 5° east of true north, a compass heading must be corrected to obtain the true geographic bearing.
The correction depends on where and when the measurement is made.
Part 8 — Why Declination Changes From Place to Place
Earth’s magnetic field is not a perfectly centred dipole.
Different regions of the planet experience different combinations of northward, eastward and vertical field components.
Lines connecting locations with equal declination are called isogonic lines.
A world declination map therefore looks like a set of curved contours, not simple longitude lines.
Part 9 — Why Declination Changes With Time
The liquid outer core is moving.
As the flow of electrically conducting iron changes, the magnetic field changes too.
This slow variation is called secular variation.
A declination printed on an old map can therefore become inaccurate years later.
Part 10 — Why the World Magnetic Model Is Updated
The World Magnetic Model represents Earth’s large-scale main magnetic field and how it changes.
WMM2025 is the current standard model and is intended for the 2025–2029 epoch.
It is used by navigation systems in aircraft, ships, phones and other devices.
This is a striking connection between classroom magnetism and operational global navigation.
Part 11 — What Is Magnetic Inclination?
Earth’s magnetic field is usually tilted relative to the horizontal surface.
The angle between the field vector and the horizontal plane is called inclination or magnetic dip.
Near magnetic dip poles, the field becomes steeply vertical.
A simple flat compass becomes less useful there because the horizontal component becomes weak.
Part 12 — Why Compasses Become Unreliable Near Magnetic Poles
Near a magnetic dip pole, Earth’s field points strongly downward or upward.
The horizontal field component can become very small.
If the horizontal component is weak, tiny local disturbances can overwhelm the directional signal.
NOAA’s World Magnetic Model therefore defines caution and blackout zones around the magnetic poles where magnetic heading becomes degraded or unusable.
Part 13 — Why a Nearby Magnet Can Ruin the Reading
A compass responds to the total local magnetic field.
Bring a strong magnet close and its field can be much stronger than Earth’s field at the needle.
The compass then points toward the combined field instead of the undisturbed geomagnetic direction.
This is useful experimentally because it proves that the needle is a field detector, not a mystical north-seeking device.
Part 14 — Electric Currents Can Move a Compass Too
Electric current produces a magnetic field.
A wire carrying current near a compass can therefore rotate the needle.
Hans Christian Ørsted’s famous 1820 observation of a compass needle deflected by electric current helped establish the connection between electricity and magnetism.
The compass became evidence for a much larger physical theory.
Part 15 — Why Buildings and Vehicles Disturb Compasses
Steel structures, speakers, electric motors, cables and magnets can all distort the local field.
That is why a navigation compass should be used away from large ferromagnetic objects and strong electrical equipment where possible.
A ship or aircraft can require calibration for its own magnetic influence.
Part 16 — Why a Phone Compass Needs Calibration
A smartphone does not usually contain a traditional pivoting needle.
It uses miniature magnetic sensors to measure components of the local field.
Software combines those measurements with orientation sensors and a magnetic-field model.
Nearby magnets, phone cases, metal furniture and sensor offsets can bias the reading, which is why recalibration may be required.
Part 17 — Why GPS and Compass Are Different
GPS determines position from satellite signals.
A magnetic compass determines orientation from a magnetic field.
A device can know where it is without knowing which way it is facing, especially when stationary.
Modern navigation often fuses GPS, magnetometer, gyroscope and accelerometer information rather than relying on one sensor alone.
Follow One Compass Reading
- The compass lies flat away from large metal objects.
- Earth’s local magnetic field passes through the instrument.
- The horizontal component exerts torque on the magnetized needle.
- The needle rotates.
- Friction and oscillation gradually decrease.
- The needle settles along the local horizontal field direction.
- You read the magnetic bearing.
- A current magnetic model provides local declination.
- The declination correction converts magnetic bearing toward true bearing.
- The result can be compared with a geographic map or GPS-derived track.
A Text Diagram You Can Draw Anywhere
true north
↑
|\
| \ magnetic north
| ↗
| / declination D
|/
[ compass ]
Earth field vector B
= horizontal component H + vertical component Z
flat compass aligns mainly with HThink Like a Scientist — Disturb the Field, Not the Needle
Use a simple compass and a small classroom magnet.
- Place the compass away from phones, speakers and metal furniture.
- Mark the settled needle direction.
- Bring a small magnet toward the compass from 30 cm away.
- Move closer in measured steps without touching the compass.
- Record the needle angle at each distance.
- Remove the magnet and confirm the needle returns toward its original direction.
- Repeat from another side.
Keep strong magnets away from medical devices, magnetic storage, cards and equipment that can be damaged. The scientific question is how competing fields combine.
How Do We Know the Compass Follows the Local Field?
- nearby magnets deflect the needle predictably;
- electric currents deflect compasses;
- measured magnetic declination varies systematically across geography;
- declination changes over years as Earth’s field evolves;
- magnetometers measure the same local vector components used in field models;
- NOAA’s WMM predicts magnetic headings used operationally in navigation.
Observation vs Inference
- Observation: a compass needle settles in a repeatable direction.
- Observation: a nearby magnet changes that direction.
- Observation: magnetic headings differ from true geographic headings in many places.
- Observation: declination changes over time.
- Inference: the compass responds to the local magnetic field rather than directly sensing the geographic pole.
Common Misconceptions and How to Repair Them
| Misconception | Better model |
|---|---|
| The needle points straight at the magnetic pole. | It aligns with the local horizontal magnetic-field component. |
| Magnetic north and true north are the same. | Their angular difference is magnetic declination. |
| Declination is fixed forever. | It varies with location and changes as Earth’s magnetic field evolves. |
| Earth is exactly a giant bar magnet. | A dipole is a useful first model, but the real geomagnetic field is more complex. |
| A compass always works everywhere. | It becomes unreliable where the horizontal component is very weak or local interference is strong. |
| GPS and compass do the same job. | GPS gives position; a compass gives magnetic orientation. |
Checkpoint Questions
- Why does a compass needle rotate?
- What part of Earth produces most of the main magnetic field?
- What is the horizontal field component?
- Why does a flat compass respond mainly to it?
- What is true north?
- What is magnetic declination?
- Why does declination vary geographically?
- Why does it change with time?
- Why are compasses unreliable near magnetic dip poles?
- Why can a phone or steel table disturb a compass?
Apply It — Map North vs Compass North
A hiker’s map is aligned to true north. The local magnetic declination is 7° east.
The compass reads a magnetic bearing of 40°.
Using the NOAA convention that east declination is positive, estimate the corresponding true bearing.
Answer Key
Open after attempting the application
Add east declination to the magnetic bearing: 40° + 7° = 47° true. Navigation conventions must be applied carefully because maps and devices may present corrections differently, so operational users should follow the convention stated by their chart or instrument.
Can You Explain WHY?
- Why does a compass need no battery?
- Why does the needle align with a field rather than a distant point?
- Why is true north defined independently of magnetism?
- Why must declination be updated?
- Why does the compass become poor near a magnetic pole?
- Why can one small magnet overwhelm Earth’s field nearby?
Singapore Field Connection
Singapore is an excellent place to separate map north from magnetic north because a phone or handheld compass can be compared with a mapped street grid and satellite-derived position.
Do not memorise one Singapore declination number forever. Declination is modelled for a specific place and date and slowly changes.
Use NOAA’s current World Magnetic Model calculator when an accurate value matters.
Primary Science / PSLE Bridge
- magnets exert forces without touching;
- a compass contains a magnet;
- Earth itself produces a magnetic field;
- direction can be measured using repeatable physical effects;
- local conditions can affect measurements;
- scientific models require dates and locations when the system changes over time.
Go Beyond Primary Science
| Primary idea | Higher-resolution science |
|---|---|
| Needle turns | Magnetic torque and dipole moment |
| Earth behaves magnetically | Core geodynamo |
| Compass has a north direction | Horizontal geomagnetic-field vector |
| Compass north differs from map north | Magnetic declination |
| Needle can tilt | Magnetic inclination |
| Field changes through years | Secular variation and WMM spherical harmonics |
Deep Science Window — A Compass Is a Vector Instrument
The needle does not answer the verbal question “Where is north?”
It physically aligns with one component of a vector field.
Humans then interpret that vector using geography and a model.
sensor output → physical vector → model correction → navigational meaning.
Deep Science Window — Earth’s Magnetic North Moves Because the Source Moves
Earth’s outer core is a moving conductive fluid. Its flow patterns do not remain fixed.
The resulting magnetic field evolves, so the locations where particular field conditions occur can move across Earth’s surface.
This is why a magnetic navigation model has an expiry date while the Geographic North Pole does not drift in the same way.
Evidence Boundaries
- Earth resembles a dipole ≠ Earth is a literal bar magnet.
- Compass points magnetic north ≠ compass aims directly at the magnetic dip pole.
- Declination is modelled ≠ the model predicts every short-term magnetic disturbance.
- WMM2025 is current ≠ it should be used forever. It is periodically updated.
- Phone compass uses geomagnetism ≠ its reading is immune to nearby electronics and metal.
- Magnetic north differs from true north ≠ one is “wrong.” They are different reference directions.
Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK
KNOW: magnetic field, compass needle, horizontal component, true north, declination and inclination.
CONNECT: Earth’s core → geomagnetic field → local horizontal component → needle alignment → magnetic bearing → declination correction → true bearing.
EXPLAIN: a compass is a local magnetic-field detector, not a direct pointer to the geographic North Pole.
APPLY: hiking, maps, marine navigation, aircraft headings, smartphones and geomagnetic surveys.
CHECK: ask which north, which location and which date.
Where to Go Next
Teaching Guide for Parents, Tutors and Teachers
For the people who teach because somebody depends on them.
Begin with the incorrect picture that the compass “points at the North Pole.” Replace the point target with a local vector field.
Central Reasoning Model
Earth produces a magnetic field → local field has horizontal and vertical components → magnetized needle aligns with horizontal component → magnetic north emerges → compare with rotation-defined true north → declination is the angular difference.
Why Ørsted and the World Magnetic Model Are Here
Ørsted shows that a compass can reveal magnetic fields from electric currents. WMM2025 shows the same instrument embedded in modern global measurement: the field is mapped, modelled, updated and operationally corrected.
Teach in This Order
- Let a compass settle.
- Disturb it with a small magnet.
- Establish the needle as a magnet.
- Introduce Earth’s field.
- Split the field into horizontal and vertical components.
- Define true north separately.
- Introduce declination.
- Add time variation and WMM.
- Only then open into geodynamo and spherical harmonics.
Questions That Reveal Understanding
- What physical thing is the needle responding to?
- Why can a nearby magnet change north?
- Why is true north not defined magnetically?
- Why does the correction depend on date?
- Why does a compass struggle near a magnetic pole?
If the Child Is Stuck
Draw a local magnetic-field arrow through the compass. Rotate the arrow and ask what the needle must do. Only after that add the map arrow for true north and measure the angle between them.
If the Child Is Ready for More
Increase resolution into magnetic dipole moments, field-vector components, declination/inclination charts, geodynamo theory, crustal anomalies, geomagnetic storms and WMM spherical-harmonic coefficients.
The strange claim must become more true as it is explained, not less.
Research Sources and Further Reading
- NOAA NCEI — Magnetic Declination
- NOAA NCEI — Geomagnetism Frequently Asked Questions
- NOAA NCEI — World Magnetic Model 2025
- NOAA Science On a Sphere — Earth’s Magnetic Declination
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