First, use this as an opportunity. Why do you believe the Earth is round? Because someone told you, and you accepted it? Because you saw a picture once? Because you learned it in school? Self-reflection is always good, and opportunities like this should never be wasted. Examine your own beliefs and your own reasons for those beliefs. Take a moment to read up on it. Learn about horizons and shadows and eclipses and orbits and seasons and astronomy. Deepen your own knowledge, not to bring more ammo to a later fight, but for your own curiosity.

And second, please remember that the person you’re talking to, the person you’re arguing with, is…a person. A human. Probably a good person. You may think they’re wrong about this (and you have evidence to back it up), but empathize. They have their reasons for why they believe the things they do. Some of those reasons may be totally nonsensical, some of those reasons you might agree with. Find some common ground. Have a dialog, not an argument.