Light does curve because of gravity, but it requires a large force. In terms of the earth gravity it doesn't bend light for us to be able to discern it, to us it would look like it's traveling in a straight line, but as you mentioned light is refracted and reflected and these things have been studied and can be studied but to discern exactly the extent of the force at any particular image/location requires very precise equipment to know the exact distance I imagine and it would only tell you about the mirage, not about the curvature. There are other simpler ways to figure out with a fairly accurate figure the radius or diameter of the earth, like you mentioned, and like it has been done.

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