Deflection of Light by the Sun

[nanjo]

TL;DR

Which three points did Eddington use to define the deflection angle of 1.75"

I'm having difficulty finding the definition of Eddington's deflection angle. Can anyone tell me precisely which three points were used to define the deflection angle of 1.75" confirmed by Eddington's experiment in 1919?
John

 

[renormalize]

TL;DR: Which three points did Eddington use to define the deflection angle of 1.75"

Can you better describe the points you're asking about by making reference to the diagram below?

(from Shadow of the Moon and general relativity: Einstein, Dyson, Eddington and the 1919 light deflection)

 

[nanjo]

Thanks, Berkeman, for the most informative diagram and interesting link. You have confirmed the angle star/observer/apparent star I wasn't sure about, and provided a bit more information. I didn't realise an angle ϵ1 was involved which, for a star obscured by the Sun, is effectively the angle subtended at the observer by the relevant part of the Sun's diameter. This has solved a big problem for me. Thanks again.
John

 

[berkeman]

Thanks, Berkeman @renormalize for the most informative diagram and interesting link.

Fixed that for you.