Soldner, Einstein, gravitational light deflection and factors of two

Abstract

The Newtonian value of 0." 84 for the gravitational deflection of a ray of light from a distant star, grazing the rim of the Sun, was derived already in 1801 by Johann Georg von Soldner. The same value was obtained by Albert Einstein in 1911 on the basis of the equivalence principle alone. Four years later, Einstein predicted twice that value on the basis of the full theory of general relativity, a value that was later confirmed by observation. A direct comparison of Soldner's and Einstein's works is obscured by a confluence of various factors of 2, arising both from different conventions and from printing errors.