How fast does light run? Seven and a half times around the earth in one second! But hundreds of years ago, many people thought that just "whoosh" was enough, and they had no concept of time at all. Scientists don't believe this, they use a moon of Jupiter, and they just reveal the "old bottom" of light!

In 1676, the Danish astronomer Ole Romer stared at Europa for a long time and noticed something strange. Both Earth and Jupiter orbit the Sun, and when they are close, Europa goes from "invisible" to "visible" in a slightly shorter time; When you are far away, it takes a little longer. This is not scientific! If the speed of light is infinitely fast, time should be fixed. Romer's mind turned, and he understood: the speed of light is certainly not infinite!

Why? Romer's explanation is particularly clear. When the Earth is far away from Jupiter, the light emitted by Europa "appears" and has to travel a longer distance to reach Earth. The speed of light is limited, so if you run slower, the time will naturally be longer. Conversely, when the Earth is close to Jupiter, the path of light is short, and the time is short. How clear is this logic! When the scientists heard this, they nodded their heads and felt that this statement was reliable.

這可是人類第一次證明光速不是無限快！羅默用木衛一的“隱身術”，直接給光速定了性。後來，荷蘭的天文學家惠更斯更牛，算出了光速大概是每秒21.4萬公里。雖然跟現在的30萬公里有點差距，但在那個年代，能算到這個數，已經是神操作了！

Romer is also worthy of praise. He didn't just look at the stars, he had to calculate the orbit and keep track of the time, and there were no computers at that time, so he relied on handwritten notes and telescopes with the naked eye. Who can have the patience now? His discovery is like lighting a lamp for the scientific community, illuminating many unknown paths.