What is the rounding?

Answer 1

Answer:
To a first-order approximation the Earth is round. This
is due to gravity. Gravity pulls with equal strength in all
directions; therefore any variations from a spherical
shape will lead to gravitational forces that
bring the shape back into that of a sphere.

This is without considering the rotation of the earth,
however. The rotation of the earth adds centrifugal
effects, which cause the earth to bulge slightly at
its equator and flatten slightly at its poles.
(This is like twirling a rock on the end of a string
and then letting go--the rock flies away from the
twirler.) Because of these centrifugal effects, the
distance from the center of the earth to the surface
of the earth is about 0.33% shorter at the poles
compared to the equator.

Answer 2

Rounding is estimating.

Answer 3

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