Empirical Earth · Shape, Curvature & the Horizon

“Flat” vs “level” — not the same thing

The claim

“Still water is flat and a spirit level reads flat, so the Earth is a flat plane.”

What is true

A spirit level does read true across a room, a field, or a building site. Builders rely on that every day and it never lets them down.

What is measured

“Level” means at a right angle to gravity (an equipotential ), not “geometrically flat.” Still water settles onto the curved geoid, which is why the oceans wrap around the Earth. A plumb line pulled sideways by a mountain is the Schiehallion experiment.

What would show this is wrong

two plumb lines hundreds of km apart found to be exactly parallel, or a continuous still-water surface shown to be truly flat over a long, controlled distance.

Sources

  1. World Geodetic System 1984 (WGS 84). NGA Standardization Document, defining ellipsoid parameters (a, f, b). National Geospatial-Intelligence Agency. WGS 84 link
  2. Refraction, Snell's law & refractive index of air/water. Standard optics; refractive index of air via the Ciddor/Edlén equations. E. Hecht, Optics ; NIST refractive-index references. Snell's link
  3. The geoid and “level” as an equipotential surface. Still water settles onto a surface of constant gravitational potential — the curved geoid. NOAA / NGS. NOAA: geodesy / geoid link
  4. The Schiehallion experiment (1774). Nevil Maskelyne measured a plumb line deflected about 11.6 arc-seconds toward the mass of the Scottish mountain Schiehallion, by comparing the apparent ve link
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