Why is the Cavendish experiment sometimes described as weighing Earth?

The Cavendish experiment is often described as "weighing Earth" because it was originally devised to determine the planet's mean density.12

By using a torsion balance to measure the incredibly weak gravitational attraction between lead spheres, Henry Cavendish was able to calculate the density of Earth relative to the density of water. Once the mean density of the planet was established, its total mass could be calculated.12

It is a common misconception that Cavendish set out to measure the gravitational constant, G. In fact, the modern formulation of Newton’s law of universal gravitation involving G did not exist at the time of his 1797–98 experiment. While the experiment is now the standard method for determining G, its primary objective for Cavendish was to weigh the Earth by finding its density, which he calculated to be 5.48 times that of water—a figure remarkably close to the modern value of 5.51 grams per cubic centimetre.12