Magnetic declination — how does declination behave at a given place over time?

Introduction / Context:
Magnetic declination is the horizontal angle between magnetic north and true north. Surveyors must account for it when converting magnetic bearings to true bearings and vice versa. Understanding its temporal behavior prevents systematic errors when older maps or bearings are reused.

Given Data / Assumptions:

• Declination is influenced by Earth’s geomagnetic field and local anomalies.
• We are discussing the general trend over days to years.
• Survey adjustments often require epoch-specific values.

Concept / Approach:
Declination does not remain constant. It exhibits long-term secular variation (years to decades), annual changes, and short-term daily (diurnal) fluctuations. While these variations are typically gradual rather than abrupt, they are significant enough to require periodic updates from isogonic charts, models, or local observations. Therefore, the most accurate general statement is that declination varies with time and is not constant.

Step-by-Step Solution:

Verification / Alternative check:
Geomagnetic models (e.g., WMM/IGRF) are updated every 5 years, reflecting that declination changes measurably over time.

Why Other Options Are Wrong:

• Remains constant: False in practice.
• Fluctuates: Partly true but incomplete; the broader statement “does not remain constant” captures all time scales.
• Changes abruptly: Not generally; changes are usually gradual.

Common Pitfalls:
Using declination values from old field books without updating to the present epoch.

Final Answer:
It does not remain constant (varies with time)