To mark the centenary of the 1919 solar eclipse that confirmed Einstein's theory of relativity, we'll take you along on the Joint Permanent Eclipse Committee's expeditions to Sobral, Brazil and Principe. Follow along in No Shadow of a Doubt, out 4/30.…
100 years ago today, Arthur Eddington and the rest of the team traveling to Sobral and Principe met at Burlington House in Piccadilly, London to finish planning for the expedition. The eclipse was just 3 months away.…
Before setting off on this journey, meet the scientists leading the Principe and Sobral teams: Arthur Eddington and Frank Dyson.
Arthur Eddington, director of the Cambridge Observatory and a keenly intelligent astrophysicist, headed up the expedition to Principe. While the Cambridge Observatory didn’t focus its research on eclipses, Eddington made it his mission to prove that light had weight.
Though Frank Dyson did not travel with the Sobral team, he was integral to the experiment’s success. Dyson, the Astronomer Royal, “was a well-respected and widely known public figure” who was able to analyze the results and share them with the world.
Accompanying Eddington to Principe was Edwin Cottingham, a clockmaker who would keep the measuring instruments finely tuned and running properly. Andrew Crommelin and Charles Davidson operated the equipment in Sobral.
On February 14th, 1919, the Joint Permanent Eclipse Committee (JPEC) met for the last time to go over the travel arrangements. They would depart for Liverpool in three weeks.
Stay tuned for more expedition updates as we approach the 100th anniversary of what author Daniel Kennefick calls “probably the most important eclipse in history.” @UArkansas
We’re picking up on this thread that follows the JPEC’s 1919 #solareclipse expedition from start to finish. Join us!
#OTD in 1919, Dyson sent the equipment for the expedition to Liverpool, ahead of the team, so that the astrographic lenses would be aboard the RMS Anselm well in advance of the ship’s departure.
The lenses were valuable: each one was worth about £1000. Losing or breaking a lens would be detrimental to the experiment.
Due to the aftermath of the Great War, the JPEC had limited access to British equipment, so they used mirrors and lenses from the @RIAdawson and @RDS.
Here’s a look at one of the lenses used in the experiment, currently housed at the @DIASDunsink in Dublin.
More updates on the expedition to come! Keep following along, and don’t forget to snag a copy of No Shadow of a Doubt on 4/30 to learn more about the #eclipse.…
Let's check in again on the JPEC as we near the 100th anniversary of their departure from Liverpool!…
It was during his stay at the Royal Observatory, Greenwich that Cottingham the clockmaker learned about the theory behind the #eclipse experiment. He seized upon it with excitement, according to Daniel Kennefick in No Shadow of a Doubt.
Would Eddington see the deflection? Would Cottingham indeed have to return to England alone, leaving a dismayed Eddington behind on Principe? They would soon find out, as the Anselm would set sail in two days and kick off the first leg of the voyage.
Check back with this thread on Friday, 3/8, when the first leg of the expedition begins! #1919eclipse #NoShadowofaDoubt
It’s time to set sail aboard the Anselm, from Liverpool to Madeira! Join the JPEC observers (Eddington, Cottingham, Crommelin and Davidson) on the 100th anniversary of the start of their expeditions. #OTD #NoShadowofaDoubt…
@UArkansas We're picking up on this thread to follow the expedition's progress! #OTD in 1919, the JPEC made landfall on Madeira. It was fortunate timing, because Portugal’s recent outbreak of civil war ended before the Anselm docked in March 1919. #1919eclipse #NoShadowofaDoubt
@UArkansas Of course, the Great War and the Portuguese Civil War had disrupted travel by sea, and it left Eddington and Cottingham unsure if and when they would be able to board another ship in time to reach Principe.
@UArkansas Davidson and Crommelin said their goodbyes over lunch that afternoon, and then promptly re-boarded the Anselm to Belém, Brazil. They would carry out Dyson’s experiment from the South American continent.
@UArkansas Meanwhile, Eddington and Cottingham waited indefinitely for a ship—any ship—to carry them to Principe. There was little else to do but wait around and write letters home (though, Eddington did take long hikes to pass the time).
@UArkansas How long would it be before a ship arrived to take Eddington and Cottingham to Principe? Would they make it to the island in time to set up the eclipse experiment?
@UArkansas Keep following this thread (and the hashtags #1919eclipse & #NoShadowofaDoubt) to find out! And don't forget to pre-order Daniel Kennefick's book, available on 4/30, for a deeper dive into the expedition. @UArkansas @e_crahan…
An #OTD from 3/23: Davidson & Crommelin docked in Belém, a port city in Brazil. They still had plenty of time to reach Sobral, with two months left until the eclipse.
Since the Brazilian observatory director, Henry Morize, wasn’t at the site yet, the two men decided to travel up the Amazon aboard the Anselm.
Check back in soon to find out if Eddington & Cottingham were able to board a ship to Principe, and follow the hashtags #1919eclipse and #NoShadowofaDoubt. @KennefickDaniel…
We're resuming our #NoShadowofaDoubt thread! #OTD in 1919, Eddington and Cottingham boarded the Portugal steamship to Principe, after 25 days of waiting for a vessel to take them to the expedition site. #1919eclipse
Eddington had been finding creative ways to pass the time until a ship arrived, turning to entertainment at Madeira’s casino. Though Eddington was a Quaker, he spent time “‘go[ing there] for tea’” and noticing that Portuguese authorities turned a blind eye towards gambling there.
Said Eddington to his mother, "I found the main doors, which lead out through the dancing saloon, were fastened... the reason was that the Chief of the Police had come up for the dancing and he was supposed not to know what was going on the other side of the door.”
Though grateful to have found entertainment while waiting for transport to Principe, Eddington was relieved to be able to continue the journey.
He wrote to his mother after the boat they’d expected to take to Principe, the SS Quelimane, failed to arrive on April 3rd: “it was going direct to St. Thomé, only a hundred miles from Principe, but did not call at Principe.”
On April 6th, he wrote again, relieved: “I think our time here is nearly up. We are to go on by the steamer Portugal which is due here on Wednesday, April 9th and should reach Principe on the 23rd.”
“I expect my next letter will be from the Cape Verde Islands,” he wrote. “I shall be glad to be progressing again; but I have enjoyed the whole of my stay here immensely. It has been a splendid holiday.”
Check in on 4/23 to see when Eddington and Cottingham made landfall on Principe! #NoShadowofaDoubt #1919eclipse @KennefickDaniel @e_crahan @UArkResearch @UAFulbright…
We're picking this thread back up to check in on the JPEC expedition's progress! Two weeks after setting sail from Madeira, Eddington and Cottingham reached Principe on April 23rd, 1919. There was roughly one month left to prepare for the eclipse and set up equipment.
In the weeks that followed, the JPEC teams built huts and screens to protect the equipment from the Sun, as temperature changes during the eclipse would likely impact the focus and image quality and make it impossible to accurately measure the distance between the stars.
Here’s a lens under a temporary hut in Sobral. This protected the equipment from the intense heat and direct sunlight. (Courtesy of Observatório Nacional, Rio de Janeiro, Brazil.)
The teams also realized they would need to stay on site for several months past the eclipse to get accurate comparison plates. If Eddington couldn’t observe the star field on May 29th, that meant he would have to stay on the island for a few months until it was possible again.
Check back in a few weeks for the eclipse centenary, and don’t forget to pre-order No Shadow of a Doubt, which is available next week! #1919eclipse #NoShadowofaDoubt…
Happy eclipse day! 100 years ago #OTD, the JPEC observed the total solar eclipse on Principe and in Sobral, Brazil. It would ultimately back up @AlbertEinstein’s theory of relativity.
@AlbertEinstein Both Principe and Sobral were in dry season by the time the eclipse began, making for favorable conditions to observe it. (But Eddington, unluckily, discovered that dry season on Principe meant heavy cloud cover during the day.)
@AlbertEinstein Yet just in time for the eclipse to start, Principe experienced a thunderstorm that “‘helped to clear the sky,’” per Eddington’s letter back home.
@AlbertEinstein There was just enough cloud cover on Principe that kept the plates from being overexposed.

Dyson received a telegram from Eddington to share the good news: “Through cloud, hopeful.”

The Sobral team reported: “Splendid.”
Good news from Principe #OTD in 1919!

Dyson joked that if the plates didn't show light deflection, "Eddington will go mad, and [Cottingham] will have to come home alone."

On May 30th, Eddington eagerly examined the plates and told Cottingham, “You won’t have to go home alone.”
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