Quick Answer
Magnetic declination varies by longitude due to the Earth's magnetic field being weaker at the equator and stronger at the poles, causing a difference in the angle between magnetic and true north.
Earth’s Magnetic Field Variations
The Earth’s magnetic field is not a perfect dipole, but rather a complex field with variations in strength and direction. The magnetic field lines are weaker at the equator and stronger at the poles, causing a difference in the angle between magnetic and true north. This variation results in the magnetic declination, which is the angle between magnetic north and true north, changing depending on the longitude.
Declination by Longitude
The declination is measured in degrees and can range from 20 to 30 degrees in some areas. In general, the declination is more pronounced near the poles and decreases as you move towards the equator. For example, in the Arctic Circle, the declination can be as much as 20 degrees, while near the equator, it can be as little as 1-2 degrees. To accurately navigate, it’s essential to have a reliable way to determine the declination for your specific location.
Navigation Implications
In navigation, magnetic declination can cause significant errors if not accounted for. A compass reading that is not adjusted for declination can lead to incorrect bearings and positioning. This can be particularly problematic in wilderness navigation or when using GPS devices that rely on magnetic fields. To avoid errors, it’s essential to determine the local declination and adjust your navigation tools accordingly. A general rule of thumb is to use a declination of 3-5 degrees per 100 miles of latitude, but this can vary significantly depending on your location.
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