Magnetic Declination 磁偏角 Glossary

The physical angle between the direction a compass needle points (magnetic north) and true north — a quantity that drifts with both place and date.

Cípiānjiǎo · 磁偏角 (simplified) · 磁偏角 (traditional)

Source credibility (S/A/B/C/X): S    Rule computability (A/B/C): A for WMM-based values; the "declination fossil" theory of the needles is C — a belief statement

What it is

A compass needle aligns with the local geomagnetic field, which almost nowhere points at the geographic pole. The difference is declination, and it changes over decades: in Beijing it was about 1.9° east in 1971 and is about 7.5° west in 2026 (WMM2025, epoch 2026.0) — the sign has flipped in living memory. Across China today it ranges from roughly 0.4° west (Lhasa) to 12.7° west (Mohe). The historical eastward dip is anciently attested — Shen Kuo in the 11th century wrote that the magnetised needle "always inclines slightly east, not due south" （常微偏東，不全南也）.

The practical trap is a popular narrative that the luopan's sewing needle （縫針） — offset 7.5° from the correct needle — was designed as a declination correction. The numbers happen to be close in 2026 Beijing, but the sign is opposite: today's declination is west, the sewing needle rotates the ring as if compensating eastward. Applying the "sewing needle = declination fix" reading today stacks the errors to roughly 15° instead of cancelling them. The needle offsets are plate construction (fixed ring rotations for the San He water and sand methods); declination is a physical quantity that must be computed per place and date.

School differences

One lineage of interpretation (via Needham) reads the three needles as "fossil traces" of declination as it drifted east then west across Chinese history; the celestial/constellation schools insist the needles are a heaven–earth–human coordinate scheme with no relation to declination. The project rates the fossil theory C — a traditional claim, not computable and not adopted as engine behaviour. There is also no unified written correction standard across mainland, Taiwan and Hong Kong practice that the collation could find. Either way, the engine's answer is the same: expose both reference norths explicitly. Context: School Dispute Register.

How this engine computes it

• Declination is computed from the World Magnetic Model (WMM2025, 12th-order spherical harmonics) by coordinates and date; the implementation matches all 12 official test vectors bit-for-bit and 14 overseas reference coordinates to within 0.00°.
• "Three needles" and "declination correction" are two independent switches in the compass interface, each declaring its own reference north; they are never merged or substituted.
• A magnetic-north / true-north toggle lets the user state which reference their reading was taken in; the mountain table is always applied in the declared frame.
• Uncertainty is kept honest: the model's own uncertainty at Chinese mid-latitudes (≈0.32°) is far smaller than a phone magnetometer's sensor error (±3°–5°), so declination is corrected but never presented as the dominant error source.

Related entries

Luopan · 24 Mountains · Xia Gua / Ti Gua

Back to the glossary index.