Robert McNees
Deflection of starlight by the gravitational field of the sun was observed during a solar eclipse #OTD in 1919. This was an important piece of evidence supporting a central prediction of Einstein's 1915 theory of general relativity.
Image: Arthur Eddington
Robert McNees
In 1911, before general relativity, Einstein used the equivalence principle and Newtonian gravity to show that the path followed by light should bend a bit as it passes by a massive body. Light from a distant star that just grazes our sun would be deflected about 0.9 arcseconds.
The moon covers about half a degree on the sky. There are 60 arcminutes per degree, and 60 arcseconds per arcminute. So 0.9 arcseconds is about one two-thousandth of the angular width of the moon. A tiny effect!
Robert McNees
Later, after developing general relativity, Einstein realized that the curvature of space near the sun would make the effect twice as large as his earlier prediction. The deflection should be about 1.75 arcseconds. Still tiny!
But to see this miniscule effect, the light must pass very close to the sun. And the sun, being pretty bright, makes it very hard to see...stars. The only hope of measuring the deflection of light by the sun is to wait for an eclipse.
❌: ★ ☀️ 🔭 🌍 🌗
✅: ⭐☀️ 🌗 🔭 🌍
Robert McNees
Einstein wrote to astronomer George Hale in 1913, asking if it might be possible to observe stars passing near the sun without an eclipse.
Hale patiently explained that, yes, the eclipse really was necessary.
https://hdl.huntington.org/digital/collection/p15150coll2/id/1846/
Robert McNees
The Royal Society organized two expeditions to the path of totality for the 1919 eclipse. Andrew Crommelin led a team to Sobral, in northern Brazil; Arthur Eddington went to the island of Principe, off the west coast of Africa.
The teams measured small shifts in the expected positions of a dozen stars near the edge of the eclipsed sun. The data from the Sobral expedition suggested a deflection of around 2 seconds of arc; the Principe data gave about 1.6 seconds of arc.
https://royalsocietypublishing.org/doi/10.1098/rsta.1920.0009
Robert McNees
This dramatic result — close to the prediction of general relativity, and significantly different than what would be expected from just Newtonian gravity and the Equivalence Principle — was a major triumph for Einstein's theory.
And it drew the attention of the public, as well. When the results were reported at a joint meeting of the Royal Society and Royal Astronomical Society that November, it generated sensational headlines. "Lights All Askew in the Heavens," read the NYT.
Robert McNees
That bit at the end! "Hey, uh, you know the stars? So, I know you’re used to them being in the right place, like, all the time. But... they definitely are not. We checked and they’re for sure in the wrong place. No, don’t worry, it’s okay, that Einstein guy says it’s fine.”
Later, in his book "Space, Time and Gravitation: An Outline of the General Relativity Theory," Eddington captured the intensity and excitement of those five minutes of totality on May 29, 1919.
https://archive.org/details/spacetimegravita00eddirich
Robert McNees
"Our shadow-box takes up all our attention. There is a marvelous spectacle above, and, as the photographs afterward revealed, a wonderful prominence-flame is poised a hundred thousand miles above the surface of the sun. We have no time to snatch a glance..."
"...We are conscious only of the the weird half-light of the landscape and the hush of nature, broken by the calls of the observers, and the beat of the metronome ticking out the 302 seconds of totality."
Robert McNees
@Victang Something similar happened in 1780, during the Revolutionary War. Harvard wanted to send astronomers to observe an eclipse, but the path of totality was behind British lines in Maine. John Hancock sent the British commander a letter, and said "Though we are political enemies, yet with regard to Science it is presumable we shall not dissent from the practice of all civilized people in promoting it?”
Victor Tangent
An interesting sidebar to this is how WW I almost ended the quest to measure the phenomenon.
Robert McNees
@Victang And the Brits agreed! The commander was like “Of course. We’re at war, but that’s no reason to stop people from doing science.” There were limits to where they could go, so they didn’t get full totality, but they still observed the eclipse.
Robert McNees
@SpaceLifeForm It’s a pretty standard term for something being diverted from an expected straight line path.
SpaceLifeForm
Is deflection the best descriptive term?
Would not gravitational bending be a more appropriate description?
Deflection would seem to imply that something bounced off of something.
Veda Dalsette
I'll bet the women of science weren't surprised.
Robert McNees
@VedaDalsette it’s a very of-its-time (sexist and unnecessarily gendered) headline
Robert McNees
@peterrelph There appear to be some faint star trails in it? I can't tell if it's that, or if it's artifacts. In either case, I think this is just an image of totality, and not a plate used to measure the position of the stars.
Peter Relph
@mcnees@mastodon.social
Is there any evidence in that image?
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