Without flying into space.. how do we KNOW that our planet is spherical?
• Because we can observe and study other planets and considering why they are (visibly) spherical, helps us to understand why Earth is too.
Note: The answer here pertains to planetary mass.
• Because we can measure the lengths of shadows simultaneously over great distances, with greater certainty now in this age of communication than ever before.
• Because constellations appear one way in The Northern Hemisphere (head up, feet down) and another way in Southern Hemisphere (head down feet up).
Note: The existence of hemispheres can only support the existence of a sphere.
• Because we have no precedent for any other planetary shape.