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

[ETL]

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.

[outofsalt]

Ping me if you figure it out.

[outofsalt]

https://www.otrcat.com/p/quiz-kids

Seems it’s available on CD

[Paal Gulli]

There was a young lady named Bright
Whose speed was far faster than light;
She set out one day
In a relative way
And returned on the previous night.

— Arthur Henry Reginald Buller

[JAKraig]

While the pendulum clock was marketed in the mid 17th century it was actually discovered that a pendulum had a constant moment in the very early 16th century by Galileo and others. He started work on a clock but got too busy to finish it.

Pendulum clocks were extraordinarily accurate. While my living room grandfather clock loses about 1 minute a month the most accurate pendulum clocks lose about 1 second every 12 years.

It is amazing what people did in the past that we forget about now and think of them as backwards.

[SunkenCiv]

Thanks ETL.

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[Telepathic Intruder]

Not many would understand but I so love being corrected. Galileo is uncredited in many ways.

[hecticskeptic]

Excellent and thanks..... what a great program that was.

[outofsalt]

Thanks belong to you and imardmd1!
I was happy to learn about the show and imardmd1 filled in the gap for me to find it on the interwebs. I’m going to buy the CD’s myself as the price is very reasonable and I will probably check out the catalog for other programs.