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大家可能注意到近來在日落後，西面天空出現一顆奪目藍鑽─金星。話說遠在400年前，當科學巨人伽利略把望遠鏡指向燦爛的金星，從此改變了西方的宇宙觀。持續的觀測讓伽利略發現到，明亮的金星出現了如月亮般的盈虧現象，尤其當金星在西面天空從耀目的太陽光芒中重現時，更顯現出恍如「滿月」的形態。早在1543年，哥白尼就提出了日心學說，他以較簡單的日心行星軌跡，解釋行星在夜空出現逆行的奇特現象，想不到在半個世紀後，才由伽利略提供了觀測實證，上面的圖解正說明了哥白尼的日心學說能完滿解釋伽利略觀測到金星的盈虧現象。如果你有一支小型天文望遠鏡，或者一副超高放大倍率的雙筒望遠鏡，你大可以在日落後，望向西面的天空，親身體驗400年前伽利略的觀測歷程。
Galileo's observations of Venus
Currently Venus reigns the western sky for about three hours after sunset. Just four centuries ago, Galileo turned his telescopes to the heavens and overturned nearly all of the medieval astronomy. Galileo was astounded to find that Venus was not a star and it exhibited phase changes similar to those of the moon. In particular, Venus exhibited a gibbous shape when it first emerged in the western sky as an evening star. This effect was predicted by Copernicus in 1543 when he suggested that Earth and the other planets orbit the Sun (see illustration). Galileo's observations could not be explained in the context of the geocentric model which had the Sun and planets orbiting Earth. With a small telescope or a pair of very high power binoculars, you can now re-experience Galileo's observations. ■甄枝強 資深天文愛好者
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