This is centrifugal force, but Newton regarded it as absolute motion.
Literal
This [topic-は] centrifugal-force is-but, Newton [topic-は] absolute-motion [as-と] considers.
The が after です pivots between two related claims. 見なす means 'regard as,' taking と to mark the attributed identity — a different role of と from the conjunctive one. Two は-marked topics in sequence (これは...ニュートンは) is characteristic of expository writing that introduces a subject then reframes it from another angle.