Ascended not (ou̇̇anebē). It is more emphatic than that: For not David ascended into the heavens. Peter quotes Psa 110:1 as proof. No passage in the O.T. is so constantly quoted as Messianic as this. “St. Peter does not demand belief upon his own assertion, but he again appeals to the Scriptures, and to words which could not have received a fulfilment in the case of David” (Knowling).
Sit thou (kathou). Late Koinéš form for earlier kathēso, present middle imperative second singular of kathēmai.