A lot harder than it sounds

I wanted the purpose of this question to be the idea that we can only truly know what we can explain to others. In this experiment you cannot bring anything from the present day to further your arguments.

How would you go about it? (assume that your friends in Ancient Greece have read Aristotle and are au fait with heliocentrism)

A tough one for a Monday afternoon - or morning/evening wherever you are in the world!