Upside down or downside up?

I’ve often wondered how come we know that Australia is “down under” and we are “up top”. Map projections would suggest it to be the case, but how to we actually know which way up the earth is relative to space?

Last night I remembered that the man in the moon is upside down in Australia. I hung my head upside down and he looked exactly as he does in the UK. So there’s your proof. I’ll wonder no more.