光速每秒钟达到299,792,458米，如果以光速行进，约1.3秒就能到达月球。虽然这个数字令人惊叹，但我们可能会好奇，光速是如何被测定出来的？今天我们就来讨论这个话题。

Its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s). It is exact because, by international agreement, a metre is defined as the length of the path ...