Correct Answer - Option 3 : Seasonal variation
Concept:
Variation in magnetic declination:
The changes in direction of the magnetic meridian at any given place are not constant. Variations in magnetic declination can be categorized as daily, annual, secular, and irregular.
1. Diurnal Variation:
- This variation is also called daily variation.
- It is an oscillation of the compass needle through a cycle from its mean position over a 24-hour period.
2. Annual Variation:
- Another form of periodic swing is taken by the magnetic meridian with respect to the true meridian.
3. Secular Variation:
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Secular variation is a slow, gradual, but unexplainable shift in the position of the earth’s magnetic meridian over a regular cycle.
- The meridian swings like a pendulum in one direction for about 150 years and gradually come to a stop and then swings back in the opposite direction.
4. Irregular Variation:
- This type of variation is uncertain in character and cannot be predicted as to the amount of occurrence.
- At high altitudes, they usually amount to a degree or more.
- Irregular variations are most likely to occur during magnetic storms and disturbances associated with sunspots, and when auroral displays occur.
