Instead of saying "we should believe that the Earth is round because it looks round when seen from the space", just say "the Earth is round because it looks round when seen from the space". The latter sentence is clearer and doesn't lack anything important which the former has.

The latter sentence parses as either malformed or false, to me. The earth appears to be round from space because it's round, not vice versa; the earth is round because of the forces that were involved in its creation.

I do agree that the 'should' needs to go, but I think the formulation should look something like 'it is rational to believe [thing] because [evidence]' or 'I/we believe [thing] because [evidence]'.