Astronomical Archaeology and the 1919 Eclipse Expeditions

Regular readers of this blog (Sid and Doris Bonkers) will know that in the past year I’ve written some articles and given some talks this year about the total solar eclipse on May 29 1919 at which an experiment was performed to test Einstein’s theory of general relativity.

I recently found out about another artefact of that expedition which has turned up in Denmark. More specifically it was discovered in the basement under the Niels Bohr Institute building on Juliane Maries Vej in Copenhagen. In the archive that is situated there they found records of astronomical observations that go more than 120 years back in time recorded on thin glass photographic plates. You can read more about these discoveries here.

Anyway, one of the plates that turned up in Copenhagen shows this image:

A copy of one of the Sobral Eclipse plates

This image is a low-resolution version from a high-resolution scan of the plate (kindly sent to me by Johan Fynbo) concerned which I believe to be a contact copy (rather than an original) of one of the plates made by Andrew Crommelin’s team in Sobral in 1919. I know that a number of such copies were made in the aftermath of the experiment and similar plates have turned up in several locations.

If you look carefully you can see a number of dark rings (one of them quite clear because of the contrast with the solar corona).  These rings surround the stars used to measure the gravitational deflection of light by the sun; you can see the others more clearly if you click on the image to make it larger.

I think this plate illustrates one of the difficulties of this measurement: the gravitational deflection is larger for lines of sight close to the Sun, but the corona is likely to be in the way precisely for those same stars.


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