Expert: Courtney Seligman - 12/1/2008

Question
Hi
This problem has been bugging me since I saw the BBC drama about Eddington and Einstein.
This is to do with Alfred Eddington proving Einstein’s theory of relativity by using the eclipse of 1919 to do it;  specifically his theory that a large mass (the sun) would bend light [there was a BBC drama about this on TV recently]. Eddington set about proving this by taking pictures during the eclipse of the Hyades star cluster (not sure if this name is right?) which appeared right next to the sun during this eclipse. Einstein’s theory stated that if this picture, taken during the eclipse, was compared with a picture taken in exactly the same place at night ( i.e. Without the suns gravitational mass interfering with the stars light) then there would be a slight discrepancy as to this actual clusters position when the photographic plates were overlaid with one another – the stars actual position on these two plates would be different proving that the sun does indeed bend light. The problem is this;  I can understand the theory behind this but how did Eddington take a picture, the base plate picture, against which the picture taken during the eclipse was compared? How did he know that the base plate picture he had taken was in exactly the same place, allowing for all the possible discrepancies (angle of telescope, rotation of the earth, actual location the picture was taken on the earth)?
thanks


Answer
I just noticed this question, in the Question Pool. (I presume the individual you sent it to either declined to answer it or was on vacation, or had exceeded their daily question quota.) If you bother to rate it, you should take into account the fact that I wasn't aware of its existence until now, and had nothing to do with the delay in answering you.

All Eddington had to do was calculate where the Sun would be in the sky, relative to the stars, during the eclipse. This doesn't require much knowledge of the details you mention above, because the Earth is so small, compared to the distance to the Sun, that the general area it would be seen in would be the same anywhere on the Earth (and in fact, was calculated in the previous century by Olbers, for all such eclipses within a ten thousand year period). The only question would be what direction to point the telescope in, which could be taken care of just by noting where the Sun was in the sky, at the time of the eclipse.

So all he had to do was take a picture of the area involved a few months earlier, when it was up at night, and could be photographed without any interference from the Sun, or its light. Then, on the day of the eclipse, he took another picture, pointing the telescope at the Sun. On the eclipse picture, he  measured the distance between various stars, scattered around the disc of the Sun; and compared that to the distances between the stars, when they weren't anywhere near the Sun. The result showed that the images of the stars appeared to be slightly farther apart for stars whose images were close to the Sun, but not as much further apart for stars whose images were further from the Sun. The comparison between the various results yielded a calculation of the bending of light for stars whose images would have been at the edge of the Sun, which was in essential agreement with the predictions of Einstein's theory.

Courtney Seligman
Professor of Astronomy
Long Beach City College