Global Explorations

Some unknown early Arabic or European merchant caravans returning from China to Turkey along the famous Silk Road undoubtedly brought the compass design to the eastern Mediterranean region mariners. Loadstone (or lode-stone), an early word for "leading stone," was the name for the natural magnets used as compasses, at least by the late-twelfth-century Mediterranean seamen. Geologists now call this loadstone material magnetite. A Frenchman, Pierre de Maricourt (better known as Padres Peregrinus), had apparently seen the compass used in his ship travels to the Holy Land during (or immediately following) the Seventh Crusade of 1248-1254. In 1269 he wrote about his scientific investigation of pivoted magnets and a spherical loadstone. Peregrinus described the pole locations, how the opposite magnetic poles of a compass attract each other, and how similar poles repel (Figure 1.2). He found that the magnetic poles always occurred in pairs of opposite signs (unlike electric charges, for which the negative and positive charges can be separated). The northward pointing pole has been named the magnet's north pole. That means the Earth's pole in the northern Arctic region is really a magnet's south pole because it attracts the compass magnet's north pole. But if it were given that name, there would be even greater confusion.

By the early fifteenth century, the compass was a typical fixture on Spanish and Portuguese ships trading along the West African coastline. Christopher Columbus's navigation experience came from such trips. The extensive ship's log that Columbus kept of his 1492 westward voyage of discovery to the Americas describes his curious observation of a growing misalignment between the magnetic north direction and the North Star. For our Western culture, this was probably the first documented observation of the change in

FIGURE 1.2 ► Force must be used to overcome the attraction of opposite magnetic poles. This force is a measure of the magnetic field strength and is the most direct evidence of the magnetic field's existence.

FIGURE 1.3 ► In the time of Columbus, the compass direction, when measured from the west coast of Spain and Portugal, pointed slightly west of the geographic north. Measured from the West Indies, the compass magnetic northward direction pointed considerably east of geographic north. Along an agonic line, magnetic northward coincides with the geographic northward.

FIGURE 1.3 ► In the time of Columbus, the compass direction, when measured from the west coast of Spain and Portugal, pointed slightly west of the geographic north. Measured from the West Indies, the compass magnetic northward direction pointed considerably east of geographic north. Along an agonic line, magnetic northward coincides with the geographic northward.

declination (the angle between magnetic and geographic North Pole directions) with changing longitude.

We now know that the Earth's magnetic North Pole is offset from the Earth's spin axis (geographic North Pole) by about a dozen degrees. It is presently tilted toward eastern North America and is gradually circling westward around the geographic pole, about once every two to three thousand years. In Columbus's time, if one looked northward from the eastern Atlantic Ocean, the two poles were essentially in alignment with the compass, pointing no more than a few degrees west of true north. That was fine for the Portuguese and Spanish merchant ships plying a narrow longitude sector along the west coast of Africa. However, as Columbus sailed to distant westward longitudes, the angular difference in alignment of the two locations at first went to zero, and then became greater, with the compass pointing drastically east of north as he entered the Caribbean Sea (Figure 1.3). His log indicated so much concern regarding this strange behavior that he fabricated excuses about

FIGURE 1.4 ► William Gilbert (1540-1603) produced the first scientific book about magnets and the Earth's magnetic field.

the North Star (rather than magnetic pole) location to his crew. The seamen were already distressed by the overdue landfall (Columbus's predicted Earth circumference was much too small).

In those days everyone knew that the compass pointed northward to the fixed north pole location of a "magnetic mountain." Now, after the year 2000, due to the westward drift of the magnetic pole location, an alignment of magnetic and true north (agonic line) passes through eastern United States and the western Caribbean Sea rather than the east Atlantic Ocean of Columbus's time.

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