THE EARTH'S MAGNETIC FIELD
To a first approximation, the Earth's magnetic field resembles that of an enormous bar magnet. The field lines emerge from the southern half of the earth and re-enter in the northern half.
Several features of the Earth's field vary in a predictable way across the surface of the globe and might, in principle, be used in assessing geographic position. For example, at each location on the Earth, the field lines intersect the Earth's surface at a specific angle of inclination. On the diagram above, note that near the equator, the field lines are approximately parallel to the Earth's surface; the inclination angle in this region is said to be 0°. As one travels north from the equator, however, the field lines become progressively steeper. At the magnetic pole, the field lines are directed almost straight down into the Earth and the inclination is said to be 90°. Thus, inclination angle varies with latitude. As a consequence, an animal that has the ability to distinguish between magnetic inclination angles has a mechanism that it might be able to use to approximate its latitude. As we will discuss, hatchling loggerheads have been shown to have this ability....More. Read on....http://www.unc.edu/depts/oceanweb/turtles/
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