Moses speaks to the Israelites as if they were one man, so except where noted the words "you" and "your" here are singular. (See: [[rc://en/ta/man/jit/figs-you]])
Possible meanings are 1) Moses is calling on all those who live in heaven and earth to be witnesses to what he says, or 2) Moses is speaking to heaven and earth as if they are people, and he is calling them to be witnesses to what he says (See: [[rc://en/ta/man/jit/figs-metonymy]] and [[rc://en/ta/man/jit/figs-personification]] and [[rc://en/ta/man/jit/figs-apostrophe]])