Flatness refers to the shape of a liquid's free surface. On planet Earth, the flatness of a liquid is a function of the curvature of the Earth, and from trigonometry, can be found to deviate from true flatness by approximately 19.6 nanometers over an area of 1 square meter, a deviation which is dominated by the effects of surface tension. This calculation using the Earth's mean radius at sea level, however a liquid will be slightly flatter at the poles.
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