Eclipse Post Special: How Solar Eclipse Observation Helped Relativity Theory

9th March 2016 was supposed to be Nyepi or Seclusion Day – a Hindu holy day commemorated by secluding oneself from life’s hustle and bustle, especially in Bali where it is strictly regulated – in Indonesia, but evidently an astral phenomenon in the form of a solar eclipse have eclipsed that day. I rarely posted something with relevance to current media sensation, but I happened to have an interesting story I wanted to share; the story of a daring expedition of Dyson and Eddington on 1919 to hunt for a solar eclipse.

Some people traveled to specific spots in the world to get a good view of eclipse. Many of them motivated by curiosity, simply just following contemporary fad, or just simply wanted to see it…to appreciate the beauty? Not really confident on that. But Sir Arthur Eddington and Sir Frank Dyson had a mission to prove a theory; Einstein’s relativity theory. What did eclipse have anything to do with relativity theory? In short, Einstein’s theory states that light will bend around a massive object, but it was not widely accepted at that time since no one really knew how to prove it. Well Dyson and Eddington had an idea. Sun is a massive object. Stars emit light that traverses the space to the Earth so we can see them. When that light traversed near the sun, will it bend?

They devised expeditions to observe the 1919 solar eclipse at Sobral, Brazil and Principe, Africa, to prove relativity theory’s prediction. They only traveled to Principe though and assigned other team to Sobral. I will try to give simple explanation of what they did. First, they took a picture of the stars at night when the sun was not an issue. Then, they needed to take a picture once again when the sun was between the stars on the sky, presumably to bend the starlight. But the sun is too bright to enable picture-taking. That’s how eclipse played its part. Eclipse ensured that the sun’s bright light would not unable the picture-taking process. Then, they compared both pictures to see whether there was distance gap of the stars positions. There was. By superimposing the eclipse picture on the night picture, a gap between stars on the eclipse image and the night image was evident and measurable.

an image that might help explaining what the expedition's result (source:
an image that might help explain the expedition’s result

Their observations got published the following year and was reported in newspapers all over the world as a conclusive proof of Einstein’s theory. Please note though that Newton’s gravity theory also predicts the bend, but according to Einstein’s theory, it will bend twice as much as Newton’s gravity theory predicted. According the observation, the bend was more accurately predicted by Einstein’s theory. Suffice to say, I might not be capable to elaborate clearly regarding this matter and thus preferred to make it simpler. A movie titled Einstein and Eddington might give a better explanation about it.


So, maybe for the next eclipse you can emulate this particular observation rather than merely uploading photos of the dark sky in your social networking service account ¯\_(ツ)_/¯.

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