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Weighing water from space

7 May 2019, 08:00 UTC
Weighing water from space
(200 words excerpt, click title or image to see full post)

Imagine the sea on a still day, calmer than you have ever seen it, with no wind to stir its surface, and no currents or tides to disturb its depths. Now imagine that the sea has risen to cover the whole face of the planet, submerging the continents and even the highest mountain peaks. What you are seeing approximates the “geoid” – a surface that joins all of the points on the Earth where the strength of gravity is the same. The geoid is the level that a hypothetical global ocean would attain in the absence of forces such as tides, winds and currents, influenced only by gravity and the rotation of the Earth.
You might expect the surface of such an ocean to be a nearly perfect sphere, albeit bulging a little at the equator due to centrifugal forces arising from its rotation. However, this hypothetical ocean is not uniform. Given that the gravitational field varies slightly across the Earth, this ocean would – in finding its natural level – flow towards areas where gravity is strongest. It would pile up over the heavy spots to produce a watery planet with a lumpy, undulating surface.
With only a few ...

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