I once heard in the name of Rav Belski Shlita, on this topic, that when you look in the New York Times you see ‘sundown’, ‘sunrise’. Actually, if you look at any astrolgy text you’ll see that, too. The fact is you cannot calculate a zenith by your head point driving past the entity.
By the way, all those Gaonim/ Rishonim you mention are actually talking about this Gemara specifically, or Meklala Itmar?