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The Day When The Speed Of Light Was Discovered

Back in the day, scientists believed that light was infinitely fast; it traveled instantaneously. But 344 years ago, on November 21, 1676, a Danish astronomer named Ole Rømer disproved that. When he studied Io, one of Jupiter’s moons, he discovered that light did not travel instantaneously, and light had a finite speed.

He was trying to figure out how long it takes Io to orbit Jupiter in hopes of using it as a cosmic clock. He watched Io disappear behind Jupiter and reappear on the other side. He did this over and over every 42 hours for years. 
To his surprise, the timing of the eclipses was not consistent. When Earth was closest to Jupiter, the eclipses happened 11 minutes early. Likewise, when the two planets were farthest away, the eclipses were 11 minutes behind schedule. 
Rømer figured out the pattern and made an accurate prediction for Io's eclipse on Nov. 9, 1676. Then on Nov. 21, he took his findings to the Royal Academy of Sciences and explained that a finite speed of light must be responsible.

What an intelligent man.

(Image Credit: NASA/ Wikimedia Commons)

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The freaky thing about GPS is that those accurate timers in those satellites are not on our 24-hour cycle. Earth's gravity affects the satellites less than it does down here on Earth (duh), and that speeds up the passage of time. The timers have to run a little bit slow to make up for the difference caused by low gravity.
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The speed of light is grade school educational material now, but at one point or another we blamed illness on foul odors and "knew" Earth was the center of the universe.
Thankfully, we're fortunate enough to live in times where we can see atoms, viruses, and bacteria.We even use invisible light at a whim to control our TVs. Radio signals from satellites provide such precise measurement that we use it for navigating the planet(GPS).
Our ability to measure things is better than ever and we're still finding surprising things from time to time. I wonder what we'll discover in the next 50 years.
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