I previously posted the view of the Klausenberger that we today don't go back to the underlying facts to decide a dispute between Rishonim or to dispute an assertion made by a rishon. In the following quote from the Gra - that is exactly what he is doing. In dealing with the dispute between the Geonim and Tosfos he resolves the issue by saying that Tosfos' position does not fit into observable reality.
Gra (O.C. 261:2): … All of this is according to the view of Tosfos. However it is not so, because that would mean that the time from alos hashachar until netz would be equal to the time from shkiah until tzais hakokovim. In truth it is not - as we wrote before. Sensory perception rejects this assertion - for all those who see - because in fact the time from alos hashachar is a much greater time than tzais after shkiah. …
Gra (O.C. 261:2): … All of this is according to the view of Tosfos. However it is not so, because that would mean that the time from alos hashachar until netz would be equal to the time from shkiah until tzais hakokovim. In truth it is not - as we wrote before. Sensory perception rejects this assertion - for all those who see - because in fact the time from alos hashachar is a much greater time than tzais after shkiah. …