 放大圖片
近來在日落之後，總會看見一高一低兩顆亮星在西南方的低空。較高的是木星，低的一顆，古人稱之為「長庚」，即現今的「金星」。古時金星見於傍晚稱為「長庚」，破曉出現則叫「啟明」，亦有稱金星為「太白」。金星是行星，行星一詞始見清咸豐末年，意指會在星野間遊走的星體。行星真的會遊走嗎？你可以親自去證實一下。附圖顯示最近10天內，金星和木星於黃昏7時的位置。在這10天裡，你可以親眼看見金星每一晚都比前一晚距離地平線高了些。如果用雙筒望遠鏡觀看，可察覺金星的左邊有幾顆亮星，只要用紙筆把它們和金星的相對位置畫下來，每天比較，就不難發現金星果然是會遊走的。怎麼不覺木星在遊走呢？木星需時12年才繞天空一圈，所以難以察覺木星每天的移動量。古人又把周天分成十二「地支」，觀察木星所在的「支」來紀年，故古人稱木星為「歲星」。
Two planets in the evening sky
The word‘planet’means‘wandering star’. Two planets, Jupiter and Venus, are now hanging low in the southwest after sunset. The above pictures mark their positions at 7 p.m. in the next 10 days. You will notice that Venus climbs higher and higher each day. If you care to look at Venus through a pair of binoculars, you can actually see a few stars to its left. Compare the position of Venus with these stars each evening and you will know why Venus is a‘wandering star’. Jupiter, on the other hand, is moving much more slowly. So slow that you would not be able to notice any shift within the span of a few days. ■甄枝強 資深天文愛好者
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放大圖片
