Earth’s core is a solid iron sphere surrounded by molten iron and nickel. The churning movements of that liquid metal turn the entire planet into a giant bar magnet. Its magnetic field can be depicted in the style of a school textbook: Lines emerge near the south pole, curve around the globe, and reenter near the north pole. This geomagnetic field is always present. It doesn’t change across the day or through the seasons. It’s not affected by weather or obstacles. Consequently, it is a boon for travelers, who can always use it to establish their bearings. Humans have done so for more than a
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