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Magnetic declination varies both from place to place and with the passage of time. As a traveller cruises the east coast of the United States, for example, the declination varies from 16 degrees west in Maine, to 6 in Florida, to 0 degrees in Louisiana, to 4 degrees east in Texas.
The local magnetic variation is indicated on NOAA nautical charts at the center of the compass rose. The magnetic variation is indicated along with the year of that variation. The annual increase or decrease of the variation is also usually indicated, so that the variation for the current year can be calculated.
Isogonic lines, dashed magenta lines, indicating magnetic variation for a given year; Indications of flashing lights (star with Fl) or marine lights (black dot); Local operations such as parachuting, glider, and ultralight (magenta parachute or glider symbol with letter G, H (hang glider), U (ultralight));
The magnetic information was used for safe surface and air navigation, special Navy requirements and general scientific research. [7] Magnetic Variation Charts were published on a five-year schedule by the Naval Oceanographic Office and later by the Defense Mapping Agency, now the National Geospatial Intelligence Agency.
Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation. Over hundreds of years, magnetic declination is observed to vary over tens of degrees. [13] The animation shows how global declinations have changed over the last few centuries. [34] The direction and intensity of the dipole change over time.
Turtles can detect all Earth-strength magnetic fields, ranging from around 25,000 nanoteslas to 65,000 nanoteslas — a measure of magnetic field intensity, according to Goforth.. To understand ...
A common compass rose as found on a nautical chart showing both true north (using a nautical star symbol) and magnetic north with magnetic variation. Also notice the correspondence between the 32-point rose (inner circle) and the modern 0–360° graduations. Compass rose with the eight principal winds.
3 – Magnetic north, which differs from true north by the magnetic variation. 4 – Compass north, including a two-part error; the magnetic variation (6) and the ship's own magnetic field (5) 5 – Magnetic deviation, caused by vessel's magnetic field. 6 – Magnetic variation, caused by variations in Earth's magnetic field.