你有没有留意过，晚上抬头看星星，几乎所有的星星都在缓缓转动，只有一颗几乎纹丝不动——它就是北极星。几千年来，从航海的水手到沙漠中的商队，无数人靠着它辨认方向，因为无论你走到哪里，只要找到北极星，就知道哪边是北方。

Have you ever noticed that when you look up at the night sky, almost every star slowly moves across it, except for one that barely seems to move at all, the North Star, also called Polaris. For thousands of years, from sailors at sea to caravans crossing the desert, countless people have used it to find their way, because no matter where you stand, once you spot the North Star, you know which way is north.

北极星之所以看起来"不动"，是因为它几乎正好对准了地球自转轴的延长线——地球在转，其他星星看起来也跟着转，但北极星刚好在轴心附近，所以显得格外安静。有意思的是，"北极星"并不是某一颗固定的星星专属的称号——因为地轴会缓慢摆动（这叫"岁差"），大约每两万六千年转一圈，所以在遥远的过去和未来，"北极星"这个称号，会由不同的星星轮流"接班"。

The reason the North Star appears to stand still is that it lies almost exactly along the extension of Earth's rotational axis. As Earth spins, other stars appear to move across the sky, but the North Star sits right near that axis, so it looks remarkably steady. Interestingly, the North Star is not a title that belongs to one single star forever, because Earth's axis slowly wobbles over thousands of years, a motion called precession, completing one full cycle roughly every twenty-six thousand years. So in the distant past and the distant future, different stars take turns holding the title of North Star.

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