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對於遠古的旅客來說，「北極星」是最可信賴的方位指標，所謂「北極星」是指最接近天北極的光亮恆星。原來地球在繞日運行的同時，亦以約每24小時一圈的速度自轉，轉動的軸心稱「地軸」。在北半球，地軸指向的天區即為天北極。地球自轉時，從地球的角度看，天空的眾星就像繞著天極運轉，以肉眼觀察，最接近天北極的亮星成為迴轉的中心，「而眾星拱之」（《論語》）。地球自轉時，會好像陀螺般迴旋擺動（左圖），在不同的年代，地軸因擺動而指向不同的天區（右圖的橙色圈），譬如在周代（距今3千年前後），最接近天北極的恆星是「帝」（小熊座β星），公元前2,800年，北極星是天龍座的α星。現今我們一般都把小熊座α星（勾陳一）看作北極星，織女星則於14,000年後成為北極星。(圖片：鳴謝Tau'olunga)
North Stars through the ages
The North Star refers to the prominent star that lies closest to the north celestial pole. It has been used by man to find the north since time began. Different stars had held this title in the past. Currently, Polaris is our North Star. The earth spins on its axis once every 24 hours, approximately. The north celestrial pole is the region of the northern sky at which earth's rotation axis is pointing. But the earth spins and wobbles like a top (top left). While it wobbles in a 26,000-year cycle, its axis draws a circle in the sky defining the positions of the pole through different ages (top right). 3000 years ago, during the Zhou Dynasty, α Draconis was the North Star. The Chinese named it the ‘Emperor Star’. 14,000 years from now, Vega of the constellation Lyra will have its turn. ■甄枝強 資深天文愛好者
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