[thepoodlebites]

Tell me...what is the curvature at 3 miles? Still waiting.
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Why are you arguing about the curvature of the Earth when you believe that the Earth is flat? Your estimate using a parabola is approximate for three miles but quickly loses accuracy for longer distances.
For celestial mechanics, trying to fit a parabola to a circle does not provide the precision for scientific calculations.
You can find several sources for the answer to your question. I looked up one source for you. [1]

1. https://www.quora.com/When-NASA-tells-us-that-the-Earths-curvature-is-approximately-8-inches-per-mile-squared-what-is-the-true-and-exact-value

[philman_36]

Why are you arguing about the curvature of the Earth when you believe that the Earth is flat?
Because you will have to verify it and it easily disproves what you believe and can be easily verified by anyone without relying on anything but their own senses.

Your estimate using a parabola is approximate for three miles but quickly loses accuracy for longer distances.
I didn't use a parabola formula and 8" per mile² is good at any distance.