From: 3blue1brown
Observations of Jupiter’s moons were instrumental in one of the most significant discoveries in the history of physics: the finite speed of light [00:00:00].
Observing Io’s Eclipses
Io, one of Jupiter’s moons, regularly falls into Jupiter’s shadow, causing it to go dark every cycle [00:00:04]. As it emerges, it becomes bright again [00:00:11]. By watching Io through a telescope night after night, observers could mark the exact time it reappeared [00:00:14]. Initially, these reappearances appeared to occur in precise 42-hour increments [00:00:20].
The Unforeseen Discrepancy
However, this seemingly precise “clockwork” was not exact [00:00:26]. At different times of the year, Io’s reappearances were either ahead of schedule or behind schedule [00:00:28]. When Earth was on the same side of the Sun as Jupiter, Io’s reappearance was approximately 20 minutes earlier [00:00:46]. Conversely, when Earth was on the opposite side of the Sun from Jupiter, it took about 20 minutes longer for Io to reappear [00:00:49]. This indicated a delay in the orbit [00:00:53].
Huygens’ Groundbreaking Realization
After performing precise calculations, Huygens realized that this 20-minute discrepancy was due to light taking time to traverse the extra distance between Earth and Jupiter [00:00:55]. Specifically, light was observed to take 20 minutes to cross a distance equivalent to two astronomical units [00:01:00].
Early Estimations and Their Impact
At the time of Huygens’ discovery, the true size of an astronomical unit was not accurately known [00:01:13]. Consequently, the initial estimates for the speed of light based on this observation were not numerically impressive by modern standards [00:01:16].
However, the more profound realization was that light possessed a finite speed at all [00:01:22]. For most experiments conducted on Earth, light appeared to travel instantaneously [00:01:24]. Yet, at astronomical scales, it became evident that light traveled quite slowly [00:01:30]. This ingenious method of measuring light’s speed, despite its initial imprecision, laid the essential groundwork for more accurate experiments performed on Earth once a rough understanding of light’s velocity had been established [00:01:31].