Why does a compass needle point north?
― The Earth itself is a giant magnet
A hiking or camping compass will settle in exactly the same direction (north) no matter where you let go of it. You haven't placed a magnet nearby — so why does the needle unerringly find north? The answer lies in a simple fact: the ground we stand on is itself acting as a giant magnet.
Put a compass on your palm, and no matter which way it starts, the needle spins around and finally settles pointing the same way every time. This holds true wherever outdoors you stand.
You may already know that a magnet points north because of the attraction and repulsion between magnets — but you may never have stopped to ask: "So what, exactly, is pulling on the compass?"
The answer lies far below your feet, deep underground.
The needle is made of magnetized metal and is mounted so it can rotate freely to align with the surrounding magnetic field lines.
Processes happening deep inside the Earth generate a planet-scale magnetic field. This field is what aligns the compass needle.
Let's work through the fact — rarely noticed in everyday life — that "the Earth is a magnet."
A compass needle is itself a tiny magnet
A compass needle is made of a slender piece of pre-magnetized metal. A magnet has two distinct ends, called the N pole and S pole, and like poles repel each other while N and S poles attract.
The needle is mounted on a pivot so it can rotate almost frictionlessly. So wherever there is a magnetic field around it, the needle turns to align with that field and settles there.
The Earth is a giant magnet, built by motion deep inside it
Temperature and pressure inside the Earth rise the deeper you go. Near the center is a core made mostly of iron, and its outer layer (the outer core) is thought to be in a molten, liquid state.
This liquid iron is kept in constant churning motion by the Earth's rotation and by convection driven by internal heat. When an electrically conductive metal moves on this kind of large scale, it generates electric currents, and those currents generate a magnetic field. This process is called the geodynamo. The geodynamo is thought to be what produces the planet-scale magnetic field that wraps around the entire Earth.
The Earth itself, beneath your feet, is quietly exerting its pull.
Magnetically speaking, it's actually the "S pole" that attracts the needle north
Here's a slightly confusing but interesting fact. As we said, in magnets N and S poles attract each other. The end of a compass needle that points north is called its "N pole." That means the magnetic field near Earth's North Pole, which attracts that needle, is technically an "S pole."
If you've ever pictured "the North Pole as the magnet's N pole," that isn't quite right, strictly speaking. The name "the pole that points north" and the actual magnetic polarity are easy to mix up.
Magnetic north and map north are slightly different
The north a compass points to (magnetic north) is known to be slightly offset from map north (true north, the Earth's rotational axis). This offset is called magnetic declination, and its angle varies by location. What's more, because Earth's magnetic field itself keeps shifting, gradually and somewhat unpredictably, along with changes in the geodynamo, the position of magnetic north is said to change year by year too.
Even on a planetary scale, Earth's magnetism is actually quite weak compared with an everyday magnet. Because of this, bringing a smartphone, metal objects, or a strong magnet close to a compass can throw off its reading. The next expandable section works out, with real numbers, just how big that difference is.
Try it yourself
- Place a compass on a flat surface, well away from metal or magnets, and confirm the needle settles pointing north
- Slowly bring a fridge magnet, for example, toward the compass
- Once it gets close enough, confirm the needle swings to point at the magnet instead of north
- Move the magnet away, and confirm the needle settles back pointing north
You'll feel directly how a nearby magnet can overpower Earth's own magnetism at close range, despite the Earth's being far larger overall.
Summary
A compass needle points north because the needle is itself a small magnet, and it rotates to align with the planet-scale magnetic field created by the motion of liquid iron inside the Earth (the geodynamo). There's also a slightly twisted relationship at play: the end of the needle that points north is called its N pole, yet the region near Earth's North Pole that attracts it is, magnetically speaking, an S pole. Map north and magnetic north are slightly offset, and this magnetic field itself is thought to keep slowly changing.
A compass needle isn't looking off toward the distant Pole Star.
It's sensing the heartbeat of the Earth, far below your feet.
So what about that Pole Star? It's actually not "the brightest star." Take a look at Is the Pole Star the brightest star? too.
Want to know more? ― Terms, numbers, and how this connects to the textbooksWe label which level each idea belongs to, from middle-school science to open research questions
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics"
- HS+Covered in high-school "Earth Science," or treated as advanced/sidebar material in textbooks
- Univ.Not covered in high school — university-level specialist content (geophysics)
- ResearchNot yet settled even at university level — an active research question
MSTerms: vocabulary around compasses and geomagnetism
- Magnetic field: the region of space where magnetic force acts.
- Field lines: lines used to visualize the direction and strength of a magnetic field.
- Geomagnetism: the magnetic field belonging to the Earth itself.
- Magnetic declination: the angle between map north (true north) and the north a compass points to (magnetic north).
HSWorking it out: how much weaker is Earth's magnetism than an everyday magnet?
Let's compare the overall strength of Earth's magnetic field with that of an everyday magnet.
| Strength of Earth's magnetic field (near the surface, typical) | A typical estimate is around 50 microtesla |
| Strength of a fridge magnet (near the surface, typical) | A typical estimate is around 5000 microtesla (5 millitesla) |
Magnetic field strength is measured in tesla. A microtesla is one-millionth of a tesla.
| Convert 5 millitesla to microtesla | 5 × 1000 = 5000 |
| Magnet's strength (in microtesla) | 5000 microtesla |
| How many times stronger is the magnet than Earth's field? | 5000 ÷ 50 = 100 |
| How many times stronger is the magnet than Earth's field? | About 100 times |
Let's also work out what percentage of the magnet's strength Earth's field is.
| What % of the magnet's strength is Earth's field? | (50 ÷ 5000) × 100 = 1 |
| What % of the magnet's strength is Earth's field, roughly? | About 1% |
The calculation shows that even an everyday fridge magnet is, near its surface, roughly 100 times stronger than the field generated by the entire Earth. Put the other way round, Earth's magnetism is only about 1% as strong as an everyday magnet. Earth's field spans a magnificent, planet-wide scale — yet its actual strength close-up is surprisingly weak. That's exactly why bringing a magnet near a compass so easily overwhelms the effect of Earth's own field.
※ Magnet strength varies widely by type; the figures here are representative estimates.
HS+Magnetic north is moving over time
The geodynamo that generates Earth's magnetic field is not constant — its behavior keeps changing gradually. Observations confirm that, as a result, the position of magnetic north, which a compass points to, is also slowly shifting year by year. In mapmaking and surveying, this change in magnetic declination must be taken into account.
Univ.Earth's magnetic field has flipped many times
Geophysical research, by studying traces of past magnetic field orientation recorded in seafloor rock (paleomagnetism), has shown that the N and S poles of Earth's magnetic field have swapped places many times over geological timescales. This is called geomagnetic reversal. Seafloor rock is known to preserve a record of these reversals in stripe-like patterns, making it one of the key pieces of evidence supporting plate tectonics.
ResearchWhat's still unclear
- Research is ongoing to simulate the geodynamo's detailed behavior more accurately and predict future changes in the magnetic field. There have also been recent reports that magnetic north is moving faster than it used to, and explaining why remains a research question.
- Exactly when and how the next geomagnetic reversal will happen cannot yet be accurately predicted. It has been suggested that Earth's magnetic field may temporarily weaken during a reversal, and research into its effects is also underway.
- Comparing how the magnetic fields of other planets arise is also considered an important research theme for deepening our understanding of Earth's own field.
Behind the simple swing of a compass needle lies a vast, still-unfolding story about motion deep inside the Earth.
Connections to the textbooks (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science: magnets and compasses | Basic terms: field, field lines, geomagnetism |
| HS | Basic Physics: magnetism | Comparing Earth's field to an everyday magnet's strength |
| HS+ | Earth Science: Earth's magnetic field | Movement of magnetic north and declination |
| Univ. | Geophysics / paleomagnetism | Geomagnetic reversal and seafloor records |
| Research | Geophysics (ongoing research) | Geodynamo simulation, predicting the next reversal |
- Japan Meteorological Agency (気象庁), explanatory materials on "geomagnetism."
- Basic Physics textbook explanations of magnetic fields and field lines.
- Geophysics textbook explanations of geodynamo theory.
- Research review in paleomagnetism on seafloor striped magnetic anomalies and geomagnetic reversal.
- Geospatial Information Authority of Japan (国土地理院), explanatory materials on "magnetic declination."
※ Field-strength figures are representative estimates; actual values vary by location and product.
※This article is a general-audience science explainer. For proper compass use during hiking or other outdoor activities, and for how to correct for magnetic declination, please consult a specialist organization or the Geospatial Information Authority of Japan.