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By calculating how old the rocks are where we find these ancient compasses, we can figure out how long ago the magnetized parts were locked in. That tells us which direction was magnetic north tens of thousands of years ago. Surprisingly, it’s not always the same as north is now. Occasionally the poles change places. What is the reversal period like? So, did the north pole drift slowly southward, so that at some point magnetic north was in, say Mexico city? No. The rocks tell us that, during pole reversals, there is a several thousand year period during which the earth’s magnetic field is either gone or very much weakened. At these times north and south become arbitrary terms. Eventually the magnetic field re-establishes itself, and it seems to be a matter of chance which side comes out north and which south.
通過計算在發現古老指南針地點的巖石年齡，我們能計算出被磁化的部分已經被封存了多久。那會告訴我們數萬年前那個方向是北極。令人驚奇的是，北極并不總是在一個方向。有時候南北極會調換。磁極調換期間會是什么樣子？北極會緩慢地變為南極，所以在某個階段，磁極可能在墨西哥城？不是的。巖石告訴我們，在磁極變換過程中，有幾千年地球的磁場要么是消失不見了，要么是被大大削弱。在這期間，南北極變得十分隨意。最終磁極重新恢復，那端變為北極，那端成為南極都是偶然的。
