Surrounding this is the outer core, a 2,000 km thick layer of iron, nickel, and small quantities of other metals.
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5,700☌, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid. Right at the heart of the Earth is a solid inner core, two thirds of the size of the Moon and composed primarily of iron. Our planet’s magnetic field is believed to be generated deep down in the Earth’s core. If off the arc, it must be added to sextant alt. If the reading is a positive angle (greater than 0°-00.0′) the error is said to be “on the arc.” If the reading is less than zero, or a negative angle, it is aid to be “off the arc.” If the error determined is on the arc it must be subtracted from sextant alt. If not, note the sextant reading when the horizons are aligned. If the sextant reads zero when the horizons are aligned, there is no error. Index error is determined by setting the sextant near zero, pointing it at the horizon and then turning the micrometer drum slowly until the actual and reflected images of the horizon are aligned.
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Because sextant readings can be effected by changing temperatures, which tend to expand or contract the metal parts of a sextant, the navigator should determine the amount of index error at least once each day. Of all these errors index error is the most important because it is an error which directly effects altitude measurements. This error results when the index arm is not pivoted at the exact center of curvature of the arc.