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提到光速，很多人會不期然聯想到愛恩斯坦和他的著名程式：E=MC2。原來早在1676年，丹麥天文學家奧勒．羅默(1644-1710)就已經發現光受到速度局限的性質。伽利略在1609年發現了木星的4大衛星，他更注意到這4顆衛星以固定的軌道環繞木星公轉的同時，亦會被木星或木星的影所掩蝕。當時就有科學家提出可利用這些掩蝕現象報時。在1671年羅默追隨著名的卡西尼對木衛掩蝕進行了觀測，他們發現當地球漸遠離木星時，觀測到的木衛一掩蝕時間比預測的時間遲。這個延遲現象被解釋為因木星與地球距離增加，光需要更長時間跨越較長的空間才到達地球，可惜卡西尼後來放棄了這個論點。羅默卻堅持他的信念並於1677年向法國科學院發表論文，因此羅默被認許為首個以現代量度方法測量光速的人。
The discovery of the speed of light
In 1609 Galileo discovered the four big moons of Jupiter and observed that they would occasionally be eclipsed by either Jupiter or its shadow. Scientists thought this could be the most accurate clockwork system in the sky. In order to work out predictions for these eclipse events, Danish astronomer Ole R禦er (1644~1710) went to work with Cassini in Paris. They found that when Earth was moving away from Jupiter, the eclipse events always fell behind the predicted times. They accounted the phenomena as the result of the finite speed of light. Cassini later discarded the idea. R禦er, however, presented the findings to the French Academy of Sciences in 1677. For this reason R禦er is credited as the person to make the first modern measure of the speed of light. ■甄枝強 資深天文愛好者 ■圖片來源：NASA
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