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Question
What is the secular variation of magnetic declination?
 
A.
Magnetic meridian swings like a pendulum
B.
Change in the declination at a place over a period of one year
C.
Change in the declination at a place in 24 hours
D.
Variation caused due to a magnetic disturbance or storm
Answer ( Option A)

Magnetic meridian swings like a pendulum

Solution

VARIATIONS 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. Daily Variation: This variation, also called diurnal variation, 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 taken by the magnetic meridian with respect to the
true meridian.
 
3. Secular Variation: 
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 amount or 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 sun spots, and when
auroral displays occurs.

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