The mass of an object represents the concentration of matter in it. Mass is a constant value, as opposed to weight. Weight is actually a measure of gravity, and that changes from place to place.
Various methods have been used to determine Earth's mass. Today a sensitive instrument called a torsion balance is generally used for this calculation. The attraction of a large ball of known mass to a small ball is compared with the attraction of Earth to the small ball. According to a recent estimate, the mass of Earth is 6.59 × 1021 tons (5.98 × 1021 metric tons).
To determine Earth's density, its mass in grams is divided by the volume in cubic centimeters. The volume of Earth is 1.083 × 1027 cubic centimeters. When 5.98 × 1021 is divided by 1.083 × 1021, the result is approximately 5.5 grams per cubic centimeter. This is the figure for the density of Earth. Said another way, if the planet's air, water, and rock were all mixed together, the mixture would weigh about 5.5 times the same quantity of water. Earth is therefore the densest of all the planets.
