By calculating how old the rocks are where we findthese ancient compasses, we can figure out howlong ago the magnetized parts were locked in. Thattells us which direction was magnetic north tens ofthousands of years ago. Surprisingly, it’s not alwaysthe same as north is now. Occasionally the poleschange places. What is the reversal period like? So,did the north pole drift slowly southward, so that atsome point magnetic north was in, say Mexico city? No. The rocks tell us that, during polereversals, there is a several thousand year period during which the earth’s magnetic field iseither 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 chancewhich side comes out north and which south.

通过计算在发现古老指南针地点的岩石年龄，我们能计算出被磁化的部分已经被封存了多久。那会告诉我们数万年前那个方向是北极。令人惊奇的是，北极并不总是在一个方向。有时候南北极会调换。磁极调换期间会是什么样子?北极会缓慢地变为南极，所以在某个阶段，磁极可能在墨西哥城?不是的。岩石告诉我们，在磁极变换过程中，有几千年地球的磁场要么是消失不见了，要么是被大大削弱。在这期间，南北极变得十分随意。最终磁极重新恢复，那端变为北极，那端成为南极都是偶然的。

calculate vt.计算，核算

magnetized (magnetize的过去分词)意思相当于形容词，指“磁化了的”

reversal n. 翻转

weakened (weake的过去分词)意思相当于形容词，指“削弱了的”

arbitrary adj. 任意的

re-establish v.恢复