How often do we say birkat ha chama and why? Acording to our science/astronomy teacher the sky has changed in the last 5000 years, enven though it takes a long time, the planets, moon and even the sun are not in the same place when birkat ha olam happened. Is this bracha really connected to birkat ha olam?

Birkat Hachama is said once in 28 years (see Shulchan Aruch Orach Chayyim 229). The significance of the bracha is linked to a particular aspect of the Creation. In other words, it is metaphysical and is not linked only to the simple astronomical calculation.This can be seen by the fact that Chazal in other halachot adopted a different calculation than the one used in birkat hachama.