Variations in Magnetic Declination
Declination is not fixed — it varies with time and location. Variations are classified as:
a) Diurnal Variation
- Daily variation (~10 minutes) over 24 hours.
- Greater during daytime.
- Influenced by: Locality. Season and Time of day
b) Annual Variation
- Yearly change from mean value (~1 to 2 minutes).
- Depends on geographical location.
c) Secular Variation
- Long-term, irregular variation over decades.
- Largest in magnitude.
Example: Paris (1680): 11° East, Paris (1820): 22° West
d) Irregular Variation
- Sudden, unpredictable changes.
- Caused by earthquakes, solar storms, etc.