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Discover and read the best of Twitter Threads about #OTD

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The Voyager 1 spacecraft, 7.25 million miles away and speeding towards Jupiter, looked homeward #OTD in 1977 and captured the first picture of the whole Earth and Moon together in the same frame.
Image: NASA / JPL
Yes, by all means, if you’re a billionaire with the opportunity please send artists to the moon. Send poets, painters, and writers. Send folks who’ll see the moon growing large, the Earth small and flawless, and find perspectives that frame our place in the vast, lonesome cosmos.
Twelve-and-a-half years passed between this view, and Sagan and colleagues thinking to turn Voyager 1’s cameras back towards Earth to capture a solar system mosaic that included the Pale Blue Dot photo.
Images: NASA / JPL
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The mathematician Bernhard Riemann was born #OTD in 1826. He made deep contributions to complex analysis and number theory, but is best remembered by physicists for his work on the foundations of geometry that would provide the mathematical framework for general relativity.
Riemann was the star pupil of Gauss, who described Riemann's PhD thesis on complex variables as the work of someone with “a gloriously fertile originality.”
A few years later, when Riemann was up for a faculty position, Gauss tasked him with reformulating the foundations of geometry. Riemann's lecture "On the Hypotheses Which Lie at the Foundations of Geometry" was delivered in 1854. Here's a thread:
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Glashow submitted "Partial-Symmetries Of Weak Interactions"#OTD in 1960, an important step towards electroweak unification. It answered the question: How can there be a symmetry between electromagnetism and the weak interaction if one conserves parity but the other does not?
Electromagnetism is based on the exchange of the photon, a massless and electrically neutral vector boson. Schwinger had proposed in the late 1950s that weak interactions were mediated by the exchange of two charged and massive vector bosons called the W^{+} and W^{-}.
Electromagnetic interactions (as well as gravity and the strong interaction, as far as we know) conserve parity. That means the rules work the same way and produce the same results if we replace everything with its mirror reflection.
Read 7 tweets
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Epidemiology pioneer John Snow, who doubted the prevailing miasma theory for the spread of a cholera outbreak in London, traced infections and spoke to locals to pinpoint a public water pump as the source. He removed its handle #OTD in 1854, helping put an end to the outbreak. Portrait of John Snow by Thomas Jones Barker
You can see the famous “ghost map” (which was made later) of the outbreak here, a foundational document of data visualization pinpointing the pump as the culprit. @EdwardTufte
matrix.msu.edu/~johnsnow/book…
Unfortunately, Twitter rules require you to RT before you can make a Game Of Thrones reference.
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Physicist James Van Allen was born #OTD in 1914. His 1958 satellite experiments (Explorer 1 and 3, Pioneer 3) revealed the Van Allen Radiation Belts: donut shaped regions where charged particles captured from the solar wind are trapped by Earth’s magnetic field.
Images: AAS, NASA
A lesser known fact is that, for some reason, my family used to have a picnic table built by Van Allen and some other scientists. His name was carved on the bottom with a wood-burning tool. Idk where we got it, but when my parents sold their lake cabin the table went with it.
Van Allen was also an early advocate for getting scientific experiments on satellites and rockets: first with captured V-2 rockets after the war, then his rocket-assisted balloons dubbed “rockoons“, and finally satellites launched by the US space program.
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The space probe Voyager 1, now the most distant human-made object, was launched from NASA’s Kennedy Space Center at Cape Canaveral #OTD in 1977. It is currently 13.3 billion miles from Earth, racing away from us at over 38,000 miles per hour.
Image: NASA/KSC
NASA released these posters last year to commemorate the 40th anniversary of the Voyager missions. However, it turns out you can STILL look at them EVEN TODAY. Go enjoy these posters.
voyager.jpl.nasa.gov/downloads/
Image: NASA/JPL-Caltech
"I'd love to hear more about what's happening back on Earth, like the extremist shadow government running a coup against the amoral toddler in the White House. But the thing is, I'm only like 13.3 billion miles away, and frankly that doesn't feel far enough. Byeeeee." –Voyager 1
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The astrophysicist Dr. Jaqueline Hewitt, known for her pioneering work on gravitational lensing, was born #OTD in 1958. In 1987 she led the team of observers at the Very Large Array that discovered the first Einstein ring.
Images: Hewitt et al, Nature 333, 537 (1988)
We see an Einstein ring when light rays from a distant source are gravitationally lensed by a massive object in the foreground, so that the source of the light appears ring-like in shape. It requires a delicate alignment of the source, the lensing object, and the Earth.
Einstein himself was skeptical about the prospects of seeing a ring. He imagined both the source and the lensing object to be pointlike. In that case, the precise alignment needed to form a complete ring just seemed too unlikely.
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John Dalton made an entry into his logbook #OTD in 1803 that introduced a set of symbols to represent elements. His system, which was the first to use symbols, was replaced about a decade later by the modern notation introduced by Berzelius.
It is an unconscionable failure of modern science, that there is no metal with chemical symbol “Af”
Scientist 1: What do the tests say? How metal is the compound?

Scientist 2: Well, it is metal Af.
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The first solar flare observations were made #OTD in 1859 by Carrington & Hodgson, preceding one of the largest geomagnetic storms on record. Both amateur astronomers made their observations independently, and published them simultaneously in MNRAS.
adsbit.harvard.edu/cgi-bin/nph-ia…
The solar storm that followed was one of the most intense on record. Auroras were reported all over the world and there were mass telegraph outages. Currents induced in the telegraph lines were strong enough to melt contacts, shock operators, and set some kinds of paper on fire.
A “Carrington-class” solar storm occurred in 2012, but the coronal mass ejection from that one just missed the Earth.
Image: NASA/STEREO
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During a solar eclipse #OTD in 1932, Karl Jansky saw no change in the intensity of the faint radio hiss he'd been monitoring. This ruled out the sun as a source. He soon attributed what he called "star noise" to large clouds of ionized gas near the center of the Milky Way.
There wasn't much interest from other scientists at the time. Even Jansky didn't really appreciate that he had invented the field of Radio Astronomy.
Image: Bell Telephone Laboratories
A little more background: Jansky was working for Bell Labs, studying sources of radio static that interfered with trans-Atlantic telephone transmissions. He identified three different kinds of noise. The first two were related to nearby and distant storms.
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Physicist Ernest Rutherford was born in Brightwater, New Zealand #OTD in 1871. He discovered the nucleus, proton, α, and β particles; explained nuclear decay; and was the first person to successfully transmute one element into another.
Image: George Grantham Bain Collection, LOC
Here is a thread from last year about Rutherford’s career, including the many notable scientists he mentored and trained.
And here is a thread explaining what Rutherford actually claimed to have done when he triggered what was later understood to be the process ¹⁴N + α → ¹⁷O + p.
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Michael Faraday first demonstrated the phenomenon of electromagnetic induction #OTD in 1831. He wound insulated wire coils around two sides of an iron ring, then observed a brief current in one as he connected the other to a battery.
Image: The Royal Institution, Paul Wilkinson
Faraday's ring-coil apparatus is on display at the Royal Institution's Faraday museum. They also have his notebooks. Here is the page from August 29, 1831, where he describes the construction of his apparatus.
Ref: The Royal Institution, RI MS F_2_C
Faraday explained induction in terms of what he called "lines of force." This idea, which he couldn't really formalize mathematically, was ahead of its time. He thought of lines of force as truly fundamental. Everything – all matter and its interactions – were realized this way.
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The mathematician Katherine Johnson, whose orbital mechanics calculations played a vital role in so many NASA missions, was born #OTD in 1918. Happy 100th birthday to the woman who John Glenn trusted more than his onboard flight computer!
Image: NASA
NASA mathematician Katherine Johnson receives the Mational Medal of Freedom from President Obama in 2015.
Image: Kevin Dietsch/UPI
Last year NASA Langley named their new computational research facility after Johnson.
nasa.gov/feature/langle…
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Hitler had planned for his invasion of #Poland to start #OTD on this day in 1939 - however he was momentarily spooked by the Anglo-Polish alliance and the prospect of Mussolini's arbitration and so decided to rescind the invasion order... #WW2
One detachment of #German soldiers, however, had already left their starting positions when the halt order came through, and could not be reached by radio. The group under Lt. Hans-Albrecht Herzner was invading Poland on their own.
Herzner's task was to capture and hold the strategically important rail tunnel beneath Jablunka Pass on the Polish frontier, and wait for the arrival of the German 10th Army, which he assumed would be following hard on his heels. #WW2
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Physiologist & biophysicist Alice Mary Stoll was born #OTD in 1917. She studied the effects of high speed flight and large accelerations on the human body, and her work on exposure to heat & thermal radiation led to new fire-retardant materials.
Image: Society of Women Engineers
Stoll was known for her innovative experiments, which often called for new instruments and measurement apparatus that she would invent as needed.
As far as I can tell she does not have a wikipedia page. (cc @jesswade)
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#TodayInHistory #OTD 1929: 67 Jews murdered in #Hebron, second holy city of Judaism, ending 3000 years of Jewish presence.
False rumors and inflammatory sermons in mosques in the previous weeks had already caused Jewish deaths and wounded in the Old City of #Jerusalem.
The Jewish community of Hebron was around 700 people out of 20,000.
False rumors of a massacre of Arabs in Jerusalem spread by infamous Mufti of Jerusalem Amin Al-Husseini, who would be the host of Hitler during WW II, triggered the #pogrom in Hebron.
The killings started on the 23rd, with the attack by a mob of the Hebron Yeshiva.
On the 24th morning, a mob started attacking Jewish homes, murdering, raping and beating Jewish residents.
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1/ #OTD IN COUNTERINTELLIGENCE & SECURITY HISTORY: On 21 Aug. 1776, the Continental Congress enacts the first law in America against espionage, authorizing the death penalty for the crime. The promulgation of this law was hastened by the continuing imprisonment of...
2/ Dr. Benjamin Church, chief physician of the Continental Army - and a British spy. There was no civilian espionage act and the provisions of military law did not provide a sufficiently strong incentive against committing the crime. Although Congress had actually added the...
3/ death penalty for espionage to the Articles of War in Nov. 1775, the law was not applied retroactively, leaving Church in jail. The new law read, in part that those who "shall be found lurking as spies...shall suffer death..." In Feb. 1778, Congress broadened the definition...
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German astronomers led by Erwin Freundlich and funded by the arms manufacturer Krupp journeyed to Crimea to measure Einstein's predicted deflection of sunlight during a solar eclipse #OTD in 1914. But WWI had broken out. Their equipment was seized, some were arrested as spies.
This was a lucky break for Einstein, who was still predicting the wrong value for the deflection (0.87 arcsec) at the time. In 1915 he realized the effect should be twice as large, about 1.75 arcsec, which was verified during the 1919 eclipse expeditions to Sobral and Principe.
☝️A short (two tweet) thread about Einstein catching a lucky break.
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21 August 1968: The Warsaw Pact [USSR, Bulgaria, GDR, Hungary, and Poland] invade Czechoslovakia, where a reformist leader had taken power in January and in April a reformist government took office, intent on "socialism with a human face". Moscow regarded this as a mortal threat.
Yuri Andropov, the KGB chief, advocated invading Czechoslovakia from early in the "crisis". The KGB made innovative use of Illegals, all of them posing as Westerners, to try to kidnap leading Czech reformists and to disseminate active measures to discredit the Prague Spring.
The KGB line was that "Rightists" and "imperialists" were preparing a coup in Czechoslovakia, hence the need for a preemptive invasion. Stories of American arms caches were disseminated. This despite solid information that the US had nothing to do with the Czech reformists.
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Voyager 2 launched #OTD in 1977, a few weeks before Voyager 1. It is currently more than 11 billion miles from the sun, hurtling through the heliosheath and towards interstellar space at over 34,000 mph with respect to the sun.
Image: NASA/JPL-Caltech
Animation of Voyager 2 images of volcanic plumes on Io, first seen by Voyager 1 a few months before, taken during its flyby in 1979.
Image copyright: Calvin Hamilton
Voyager 2 looks back at a crescent Uranus as it sails past in 1986.
Image: NASA/JPL
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Margaret Hamilton, a pioneer of software engineering who led NASA's in-flight software team for the Apollo mission, was born #OTD in 1936.
Image: NASA
The Apollo 11 Guidance Computer source code — Margaret Hamilton oversaw the project and wrote much of it — is available on github. Chris Garry posted Ron Burkey's transcription.
github.com/chrislgarry/Ap…
It is worth browsing the github repo just for some of the closed issues.
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130 million years ago, when dinosaurs roamed Earth and the first flowering plants were beginning to bloom, two neutron stars spiraled into each other. #OTD in 2017 the gravitational waves arrived and made a mirror here on Earth move about 1/1000 of a proton radius.
Image: LIGO
About the same time one of @LIGO’s detectors was ringing, the @NASAFermi space telescope detected a burst of gamma rays. @LIGO and @ego_virgo’s automated systems determined that the signals were probably related.
The same subatomic shiver was confirmed in the second LIGO detector, and an alert went out that turned telescopes around the world towards the patch of sky that issued the chirp and burst.
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The physicist Hans Christian Ørsted was born #OTD in 1777. In 1820 he demonstrated an unexpected connection between electric and magnetic phenomenon, believed by many to be unrelated, initiating the development of the theory of electromagnetism.
Image: Danmarks Tekniske Museum
Ørsted's discovery reputedly took place on April 21, 1820, as he prepared a demonstration during a lecture. When he connected a wire to a voltaic pile (a battery, or "galvanic apparatus") a nearby compass needle twitched as the current began to flow.
(There is some debate as to whether this was a genuinely unexpected discovery, or he expected something like this from previous experiments.)
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#GDR #OTD 1961 – Construction of Berlin Wall begins - it will stand for the next 28 years. (1)
Berlin was already a divided city, but thousands still commuted daily across the East-West border until 1961 (2)
A huge outflow of refugees from East Germany to the West, as well as problems with currency controls and daily border crossers, prompted the construction of the Berlin Wall (3)
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