Magnetic declination is the horizontal angle between magnetic north (the direction indicated by a compass) and geographic north (the direction of the Earth’s North Pole). This angle varies according to geographic location and slowly changes over the years.
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How to measure it in class
With the FizziQ app, it is possible to demonstrate magnetic declination.
Steps:
- Open FizziQ and note the direction of magnetic north (magnetometer or phone compass)
- Use GPS to determine geographic north (direction of increasing latitude)
- Compare the two directions
- Look up the local declination on a specialized website
- Discuss its importance for navigation
- Track the evolution of declination over the years (historical data)
Scientific activities on this topic
Possible extensions with FizziQ: measuring the local declination and studying the historical evolution of Earth’s magnetic field.
Learn more
Origin of declination:
The magnetic North Pole does not coincide with the geographic pole. Long located in the Canadian Arctic, it has left Canada and is now migrating rapidly toward Siberia (Russia), at a rate of several tens of kilometers per year.
Typical values:
| Location | Declination (2024) |
|---|---|
| Paris | ≈ 0° |
| London | -1° (West) |
| New York | -13° (West) |
| Sydney | +12° (East) |
In metropolitan France, the declination is currently close to 0°.
Conventions:
- Positive declination (East): magnetic north is east of geographic north
- Negative declination (West): magnetic north is west of it
Declination correction:
True heading = Magnetic heading + Declination (with signs)
Formula
Geographic azimuth = Magnetic azimuth + D
where:
- D: magnetic declination (positive to the East, negative to the West)
Annual variation (France): approximately +0.1° per year currently
Application examples
- Topographic maps indicate the local declination and its annual variation
- Aircraft pilots correct their magnetic headings
- GPS uses geographic north, not magnetic north
- Historical maritime navigation required declination tables
FAQ
Q: Why doesn’t the smartphone compass indicate true north? A: Most smartphones automatically correct for declination and display geographic north. Check the settings of your compass app.
Q: Does declination change over time? A: Yes, the magnetic pole moves. In Paris, the declination changed from -8° in 1900 to approximately 0° today.
Q: How do I find out the declination at my location? A: Websites such as NOAA or NGDC provide models of Earth’s magnetic field. FizziQ can compare the magnetic and GPS directions.
Q: Are there places without declination? A: Yes, the points where D = 0° form lines called “agonic lines”. One agonic line currently passes near France.
Related concepts
Earth’s Magnetic Field - Compass - Magnetometer - GPS Geolocation