On This Day in Space! Nov. 21, 1676: Astronomer (Accidentally) Discovers Speed of Light

Space.com ^ | Nov 21, 2018 | Hanneke Weitering, Space.com Staff Writer

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Welcome to "On This Day ... in Space!" where we peer back in our archives to find historic moments in spaceflight and astronomy. So enjoy a blast from the past with Space.com's Hanneke Weitering to look back at what happened on this day in space!

On Nov.  21, 1676, the Danish astronomer Ole Rømer discovered the speed of light . Before Rømer figured it out, scientists thought that light travels instantaneously, or infinitely fast.

Rømer disproved this almost by accident when he was studying Jupiter's moon Io. 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.

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"Rømer’s determination of the speed of light was the demonstration in 1676 that light has a finite speed, and so does not travel instantaneously.

The discovery is usually attributed to Danish astronomer Ole Rømer (1644–1710),[note 1] who was working at the Royal Observatory in Paris at the time.

By timing the eclipses of the Jupiter moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth’s orbit around the Sun. This would give light a velocity of about 220,000 kilometres per second, about 26% lower than the true value of 299,792 km/s.

Rømer’s theory was controversial at the time he announced it, and he never convinced the director of the Paris Observatory, Giovanni Domenico Cassini, to fully accept it.

However, it quickly gained support among other natural philosophers of the period, such as Christiaan Huygens and Isaac Newton. It was finally confirmed nearly two decades after Rømer’s death, with the explanation in 1729 of stellar aberration by the English astronomer James Bradley."

https://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_of_light

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[reed13k]

Wonder if he also accounted for the effect of the earth being closer/farther to Io having an additional/less pull by R^2

Would likely be miniscule-but the calculations they used to do were amazing considering they had no computers.

[Vermont Lt]

But it must be on a table top millions of miles away. Because it’s all flat. Isn’t it? //sarc

[Red Badger]

Smart dude.

Then, a century and a half later, Edgar Allen Poe explained why space is dark instead of light................

[Red Badger]

They had the ultimate computer, the human mind..................

[Hot Tabasco]

What's amazing is that this was discovered over 300 years ago ...Just who educated these guys?

MIT wasn't around back then........

[A strike]

This is ridiculous !

EVERYone Knows that Muhammad discovered the speed of light !

[ETL]

Then, a century and a half later, Edgar Allen Poe explained why space is dark instead of light...

Is this what you're thinking about?

Olbers’ paradox

In astrophysics and physical cosmology, Olbers’ paradox, named after the German astronomer Heinrich Wilhelm Olbers (1758–1840), also known as the “dark night sky paradox”, is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe.

The darkness of the night sky is one of the pieces of evidence for a dynamic universe, such as the Big Bang model.

In the hypothetical case that the universe is static, homogeneous at a large scale, and populated by an infinite number of stars, then any line of sight from Earth must end at the (very bright) surface of a star and hence the night sky should be completely illuminated and very bright.

This contradicts the observed darkness and non-uniformity of the night.[1]

https://en.wikipedia.org/wiki/Olbers%27_paradox

[Telepathic Intruder]

It was timing, apparently, not computing power that was important here. The pendulum clock was invented in the mid 17th century, and was unrivaled in accuracy until the advent of the atomic clock. In any case, a computer is only as smart as the person who programs it. Mathematically, Isaac Newton could do with quill and parchment what any modern supercomputer can do. It would just take a lot longer.

[Red Badger]

Yep!....................

[Red Badger]

They didn’t have politically Correct teachers....................

[chajin]

EVERYone Knows that Muhammad discovered the speed of light !

No, he discovered the zero. When complimented on the feat, he said it really was nothing.

[libertylover]

the demonstration in 1676 that light has a finite speed

That's amazing considering that Pink Floyd's "Dark Side of the Moon" wasn't released until 1973.

[hecticskeptic]

What's amazing is that this was discovered over 300 years ago ...Just who educated these guys?

A few nights ago, I happened upon an old-time radio program that sounded like something made pre-1950s but I may be off a bit on that. Regardless, it was an announcer who was asking all kinds of science, math, arts etc. questions of a group of smart kids of varying ages. It was simply stunning to hear these kids talk their way through complex mathematical questions, recite Shakespeare at length, know history at a level few would today and go into all kinds of details on scientific matters. The kids all sounded like they were between say 6 and 12 years old. I'd like to know what the program was so that I could shoot it off to my kids to let them know what the 'expectations and standards' were years ago.....

[Eagles6]

Just some Dead White Guy.

They ain't done nuthin'./sarc

[TalBlack]

Geez, should a light year be called a Romer???

[Conan the Librarian]

I’ve heard of MIT.

It’s the Georgia Tech of the North isn’t it?

[imardmd1]

That was "The Quiz Kids." I remember listening to it in the 1940s. These were not just ordinarily educated Public school youths.

[imardmd1]

Correction:

"The Quiz Kids."