Of Course

• It looks as if the sun goes round the earth, but of course the earth really goes around the sun.

Of course is not a polite reply to a statement of fact.

Compare:

• ‘Could you help me?’ ‘Of course.’
• ‘It is cold.’ ‘It certainly is.’ (‘Of course it is’ would be quite rude, because it would suggest that the first speaker said something too obvious to be worth mentioning.)