You don’t need to be in space to know the earth is curved.

The sun doesn’t appear to get smaller or change shape as it sets. If the light comes from an object, that object generally does not change distance from its observer, or if it does, that change is too negligible to be seen.

So we can say that the sun travels across the sky as if it were on a sphere. (the sphere is the shape of constant distance in three dimensions.)

Now consider that different people across the world experience different times of day at the same time. Easily verifiable with modern communication.

If someone somewhere sees the sun straight up while we see the sun to the side, the earth must be curved.

You don’t need to be in space to know the earth is curved.

The sun doesn’t appear to get smaller or change shape as it sets. If the light comes from an object, that object generally does not change distance from its observer, or if it does, that change is too negligible to be seen.

So we can say that the sun travels across the sky as if it were on a sphere.

Now consider that different people across the world experience different times of day at the same time. Easily verifiable with modern communication.

If someone somewhere sees the sun straight up while we see the sun to the side, the earth must be curved.

You don’t need to be in space to know the earth is curved.

The sun doesn’t appear to get smaller or change shape