Terrestrial magnetic declination · epoch 2025.5
North is
a lie
Your compass points at the magnetic pole, not the geographic one. The gap between the two is declination — and it is different at every point on Earth. Move across the chart and watch the needle swing.
How to read the lie
Isogonic lines
Each blue curve joins every place where the compass is wrong by the same angle. Stand anywhere on one line and your needle is deflected identically — five degrees east in Lisbon is five degrees east all the way down the curve.
The agonic line
Follow the gold line and declination falls to zero. This is the one seam across the globe where the compass tells the truth — magnetic north and true north stack exactly. Step off it in either direction and the lie begins.
The wandering pole
The magnetic pole is not nailed down — it drifts kilometres a year. As it wanders the whole field re-knits, so every line on this chart is slowly crawling. A bearing you trust today is a fraction of a degree off by next season.
The model under the map
The field is drawn from a tilted geocentric dipole — the first, coarsest approximation of the geomagnetic field. At any point the horizontal field runs along the great circle toward the geomagnetic pole, so the local declination is simply the compass bearing from that point to the pole. Trace the set of points that share a bearing and you have an isogonic line; the bearing of exactly zero draws the agonic thread. The pole then wanders on a slow arc and every contour is recomputed, live, in the browser.
where Δλ = λpole − λ · positive D deflects the needle east, negative west