Wrong. For rotation you don't look at mass, but at moment of inertia. That scales like ~M*R^2 or ~rho*R^5 for a density of rho and distance R from the axis of r

Wrong. For rotation you don't look at mass, but at moment of inertia. That scales like ~M*R^2 or ~rho*R^5 for a density of rho and distance R from the axis of rotation. So even if the core is much denser, it contributes a lot less to the earth's total moment of inertia. Earth's rotation is even measurably affected by plants changing throughout the seasons. Glacial melts and earthquakes also have measurable effects that need to be compensated for. See here for the plot: https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/Deviation_of_day_length_from_SI_day.svg/2560px-Deviation_of_day_length_from_SI_day.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/De... And here for a thorough explanation: https://www.sciencedirect.com/science/article/pii/S1674984717300666 https://www.sciencedirect.com/science/article/pii/S167498471...