The “eight inches per mile squared” is a rule of thumb to determine the drop height due to the curvature of the Earth. It does not account for the observer’s height and atmospheric refraction. And therefore, the rule is unsuitable to determine the amount of obstruction of a distant object due to Earth’s curvature.

Many Flat-Earthers often use the “8 inches” rule to reach the conclusion similar to “X is visible, but at the distance of Y miles, X should be Z feet below the horizon, so the Earth is flat.” They are wrong. The “8 inches” rule is the wrong tool for the purpose as it does not account for the height of the observer and atmospheric refraction.

Using the “8 inches” rule to calculate the obstruction due to Earth’s curvature would give us a much higher value compared to the real-world situation. Using the rule to calculate the obstruction from Earth’s curvature will erroneously lead us to the wrong conclusion. It can appear that the object is visible, but should have been completely obscured.

After accounting for the height of the observer and atmospheric refraction, everything will be consistent with the spherical Earth model.

The “8 inches” rule is also an approximation. It remains usable until up to about 100 miles. After that, it deviates from the correct value very quickly.

## References

- Flat Earth Follies: How to derive 8″ per mile squared and why it’s wrong – Flat Earth Busted