Mass
The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
Earth mass
The mass of Earth is measured indirectly by determining other quantities such as Earth's density, gravity, or gravitational constant.
The first measurement in the 1770s Schiehallion experiment resulted in a value about 20% too low.
The Cavendish experiment of 1798 found the correct value within 1%.
Uncertainty was reduced to about 0.2% by the 1890s, 
to 0.1% by 1930.