VAR! 

In the early morning hours #OTD in 1923, Edwin Hubble took a photo plate of M31 showing a Cepheid variable star.

Using Henrietta Swan Leavitt’s distance-luminosity relationship, Hubble concluded that M31 is located outside the Milky Way.

This observation established that there are other galaxies besides our Milky Way, that our little island is not the whole Universe.

Image: Carnegie Observatories

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 “Sure, of course the leopards are going to eat THAT guy’s face. But I am a fellow leopard, so surely they won’t eat MY face.” 

 Nature desk-rejected their paper!

This is right up there with the way the Nobel Committee used to tweet happy birthday to Russell Hulse, then mention that it also happened to be the anniversary of Jocelyn Bell Burnell discovering pulsars.

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 Updating the Richard Scarry books to reflect modern careers and workplaces.

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 Enrico Fermi, one of the foremost physicists of the 20th century, was born #OTD in 1901. While most physicists focus on either experiment or theory, Fermi excelled at both. 

Now, estimate how many new physicists will be born today.

Image: New York Public Library

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 Happy 118th birthday to special relativity!

Albert Einstein introduced special relativity in the paper "On The Electrodynamics Of Moving Bodies," published #OTD in 1905 in the journal Annalen der Physik.

Manuscript: https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.19053221004
English Translation: https://www.fourmilab.ch/etexts/einstein/specrel/www/

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 Mathematician Bernhard Riemann was born #OTD in 1826.

He made deep contributions to complex analysis and number theory, but physicists remember him for his work on the foundations of geometry. These insights provide the mathematical framework for general relativity.

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 Gravitational waves were first detected by LIGO #OTD in 2015, 100 years after they were predicted by Einstein.

This discovery, announced the following February, marked the beginning of gravitational wave astronomy.

The description of what was discovered and how it happened fits in a short post. It is simultaneously so amazing and so preposterous that some people who hear it will want to spend the rest of their lives understanding how it could be true and what we can learn from it:

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