Posted by: Kash Farooq | January 3, 2011

How Eddington demonstrated that Einstein was right

I recently watched “Einstein and Eddington”. The storyline is about how the English scientist Eddington became interested in Einstein’s General Theory of Relativity, and how he helped “release it to the world”. This all happened around the time of World War I. The Great War provides a continuous backdrop to the story; the scandal that an English scientist was promoting the work of a German-born scientist.

Einstein and Eddington

David Tennant and Andy Serkis play Eddington and Einstein in the BBC drama.

I’m unsure how historically accurate the storyline is (for example, did Eddington really write to Einstein to ask if his theories could predict the orbit of Mercury that could not quite be explained by Newton’s Theory of Gravity?). Nevertheless, I enjoyed the film and would recommend it.

So, how did Eddington demonstrate that Einstein was right?

One of the things that General Relativity predicts is that light will bend around a massive object (such as the Sun). Newtonian gravity also predicts this. However, General Relativity predicts that light will bend twice as much as the value predicted by Newtonian gravity.

So, we have something that can be tested. If we can measure how much light bends around the Sun, then the value obtained will show which prediction was right: Einstein’s or Newton’s.

An experiment could be devised to test this.

There are plenty of light sources that can be used to see how much light bends when passing by a massive object; there are millions of stars that we can see in the night sky. The problem is the “night sky” bit of that last sentence. We see the stars at night when the Sun isn’t there. How can we check if the Sun is bending starlight when the Sun is too bright to allow the stars to be seen?

Simple: run the experiment during a total eclipse.

So, Eddington and his colleagues performed some calculations to find a suitable location to observe the next solar eclipse, and determined which stars would be close to the Sun during the eclipse.

They calculated that the tiny island of Príncipe off the coast of West Africa would be an excellent place from which to observe and photograph the next solar eclipse on 29 May 1919. Eddington himself would go to Príncipe and he also despatched one team to Sobral in Brazil.

Stamp commemorating Eddington's eclipse expedition

Stamp commemorating Sir Arthur Eddington’s expedition to the island of Príncipe. Image credit Ian Ridpath ( The designs are copyright of the issuing authorities.

The Hyades open cluster was selected as the stars that would be measured. This is how it looks in the night sky:

Hyades open star cluster

Hyades, the star cluster Eddington measured, is the nearest open cluster to the Solar System at a distance of 151 light years and consists of a group of 300 to 400 stars that share the same age, place of origin, chemistry, and velocity. The four brightest member stars of the Hyades are all red giants. All are located within a few light years of each other. Image credit: Joe Robert (

Eddington’s experiment would attempt to image this star cluster during the eclipse. An image taken during the eclipse would then be superimposed on top of an image taken of Hyades at night (i.e. when the light from stars in the cluster was nowhere near the Sun). Both Einstein and Newton’s gravitation theories predict that stars in the eclipse image that were close to the Sun would appear to be shifted away from the corresponding stars in the night time image – i.e. there would be a gap between stars on the eclipse image and the night image. The theories predict different values; the size of this gap would determine which prediction was right.

Negative of the 1919 solar eclipse

Negative of the 1919 solar eclipse taken from the report of Sir Arthur Eddington. Eddington highlighted the stars he used in the comparison with horizontal marks; these can be seen at 2 o’clock on the image.

When the eclipse and night images were compared a gap was found. And when measured, it confirmed that Einstein’s prediction was right. Something that the General Theory of Relativity (and its explanations for gravity) had predicted had now been confirmed experimentally.

The results were reported by the worldwide press. The Illustrated London News explains the experiment well with a nice graphic:

22 November 1919 edition of the Illustrated London News

Illustrated London News, 22 November 1919

The New York Times got a bit “tabloidy”:

New York Times, 10 November 1919

New York Times, 10 November 1919

Almost overnight, Eddington had turned Einstein into an international celebrity.

The rest is history.


  1. I’m so clueless with this kind of stuff that I find it remarkable how people just come up with such fundamental ways to test theories like Einstein’s, especially in 1919 for goodness sake. Wow. Great post dude, you certainly feed my astronomy bug 🙂

  2. Great explanation Kash.

    And a great example of how science doesn’t always progress in the way most people think. It doesn’t fit with a probabilistic confirmation: try getting a p-value from that. Neither does it fit Kuhn’s “paradigm shift” (which he thought was always an a-rational act that no experiment could inform).

    Now explain to me why Newton’s theory predicts half as much effect as Einstein’s!

  3. Top work Kash, although I now have yet another addition to the mountain of ‘TV I must catch up with’.

  4. Were the stars that were involved with Eddington’s measures close enough to the sun for their light to have been affected by the sun’s atmosphere? If the light was affected, has anyone determined how much?

  5. Pretty interesting, huh? Which type of scientist would you agree with… Theoretical or Experimental?

  6. i had minor doubt in order to superimpose one image on another, where first image was taken during eclipse or during night sky

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