Empirical Earth · Shape, Curvature & the Horizon

The abyssal plains are the smoothest on Earth — and still curved

The claim

“The abyssal plains are the flattest places on Earth, so the sea floor is flat.”

What is true

They are the flattest large surfaces on Earth, with slopes under a foot per thousand.

What is measured

Abyssal plains have slopes below 1:1000 (under about 1 m per km). They are extraordinarily smooth, and they follow the curved geoid. Smooth is not the same as flat-plane.

What would show this is wrong

an abyssal plain shown to be a true flat plane (zero curvature) over its full thousands-of-km width, rather than just low-slope at every local patch.

Sources

  1. 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
  2. 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
  3. Abyssal plains — the smoothest, lowest-relief large surfaces on Earth (slope < 1:1000). Sediment-blanketed deep-ocean floor at 3,000–6,000 m; smooth locally, conforming to the curved geoid. link
  4. The geoid undulates by ~±100 m. Mean sea level follows the geoid, an equipotential of Earth’s gravity that departs from a smooth reference ellipsoid by up to about ±100 m because mass is une link
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