As the Earth is spherical, the horizon is below the eye-level (or the astronomical horizon). The angle between the eye-level and the horizon is the dip of the horizon. The angle becomes larger as we go higher.

Flat-Earthers often claim that “the horizon always rises to eye-level”, and thus ‘proving’ the flat Earth claim. Despite their insistence to use a water level to ‘prove’ water is flat, the same device can be used to demonstrate the dip of the horizon, proving the water surface has curvature, and consistent with the spherical Earth model.

A water level exploits the fact that water always finds its level. Looking at the surface of water formed in two vessels, we can aim at the horizon and determine the projected eye-level. If we are at a sufficient altitude, we should be able to observe that the horizon —the line separating the sky and the ground— lies below the eye-level.

As we climb higher, the larger is the dip. Conversely, as we approach sea level, the dip approaches zero.

Flat-Earthers often use water level as ‘proof’ that the surface of the water is flat. They are wrong. The surface of the water is level, or equipotential, but not perfectly flat.

## Reference

- Flat Earth Water Level Test – Jon McIntyre – YouTube
- Flat Earth Water Level Test #2 – Jon McIntyre – YouTube
- Horizon Drop at Varying Altitudes. Flat Earth Debunked. – madmelon101 – YouTube
- Water level (device) – Wikipedia