Walter Ritz’s emission theory stated that some fraction of the velocity of an object was added to the speed of light emitted from it. Willem de Sitter pointed out a problem with this in the case of a binary star. Suppose there is a conveniently placed binary star whose two members spend part of their orbit coming directly at us and part of it going away from us. According to Ritz, the light emitted when the star is coming towards us will be travelling faster than the the light emitted when it’s travelling away. De Sitter reasoned that this could result in light from different parts of the orbit arriving out of sequence, that light emitted at the 'coming towards' part of the orbit might arrive sooner at earth than light emitted earlier but at the 'going away' part of the orbit.

In a 1913 paper de Sitter names two binary systems (delta Equulei and zeta Herculis) which are appropriate for visual observation of radial velocities in order to demonstrate that this doesn’t happen and therefore that Ritz is wrong.

And my question is: was his telescope able to resolve the two components of the binary star so that he could see that they appeared to move in orbits around each other and not jump around to different parts of the orbit, or was his telescope not able to resolve them optically and therefore he drew his conclusion from just looking at the spectra?


1 Answer 1


A quote from de Sitter's paper

Moreover in many cases the orbit derived from the radial velocities is confirmed by visual observations (as for Equulei, ζ Herculis, etc.) or by eclipse-observations (as in Algol-variables).

ζ Hercules was identified as a visual binary in 1826 by F.G.W. von Struve.

δ Equulei was instead classified as a visual binary by his son O.W. von Struve in 1852.

Therefore, de Sitter was likely referring to actually resolved observations of the binaries.


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