Types of Variation
In declination, the following type of variations is observed:
- Secular variation,
- Annual variation,
- Daily variations, and
- Irregular variations.
Secular Variation
The magnetic meridian swings such as a pendulum to the left and to the right of true meridian. Its period of variation is about 250 years.
Annual Variation
It is observed that declination varies from 1′ to 2′ in a year.
Daily Variation
Daily variation of magnetic declination is as much as 10′. This variation is also called as 'Dirunal Variation'. Following explained factors influence its magnitude:
(1) It is more in summer and less in winter
(2) It is more in day and less in night.
(3) It is more near magnetic poles and less near equator.
(4) The amount of variation changes from year to year 13.6.4 Irregular Variation
Due to volcanic eruptions and earthquakes, magnetic storms tale place, resulting into changes in magnetic meridian. Such changes are from 1° to 2°.