I've only taught this 4 or 5 times, so it's hard to be specific about (insufficient experience to make a better judgment.)
Most people think satellites (like the earth round the sun) are in circular orbits, Kepler showed this isn't true, they are ellipses.
Students often want to know 'if the sun is at one focus of the ellipse, what's at the other?' The answer is nothing (unsatisfactory, but true.)
Students also struggle with the geometry if they didn't like it at elementary level.