Common sense tells you the earth is round..  1. if the earth was flat--> the sun and moon would hover at dawn and stay there for a while and then speed over head to sunset and 'rest' there. Just like a plane flying over your head. 2. The face of the moon would look different to each part of the world. The moon doesnt rotate so the face would look different throughout the night/day. 3. the different moon shapes would not exist at the angle they do. The moon shapes come from the sun as 'below' the horizon. ( <-- would show the sun is above horizon. Any rotation would show the sun is below the earth.