英语时差：磁极

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.