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Originally Posted by SIKSUKR
Smith Point brings up an interesting point.I always thought of water as being the most accurate level.Over a long span I guess it's not but, what is level?Relative to the curvature of the earth apparently.Level actually is not a straight line then is it?Oh boy,thanks RG!
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Earth can be modeled as an ellipsoid (since the equatorial radius is longer than the polar axis by about 23 km). This is caused by Earth's gravity and rotational forces. In the ellipsoidal model the direction of down can be shown to always be perpendicular to the ellipsoid. Thus the ellipsoid must be a surface of constant gravity potential.
Fluids don't flow along the ellipsoid due to gravity. Gravity only pulls perpendicular to the ellipsoid, which is why water doesn't "flow" along the curvatures of the earth. It's not like a ball that you pour water over -- and the water flows down the sides. At any localized point, the water always only has one force -- down -- pulled by gravity.
In the real world, there are lots of other influences -- wind and currents, for example. But these are local and temporary, and don't affect the general nature of gravitational/rotational forces.
The actual story is a lot more complicated than this, of course. Scientists describe this phenomenon in terms of Geoids and Level Surfaces, for example. In fact, they call the level surface that represents mean sea level in the open ocean the Geoid. This is a surface of constant gravity potential - a level surface.
But in a word -- it's all about gravity. And don't ask me to explain gravity -- way beyond my abilities.