text: Precise measurement of the Earth mass is difficult, as it is equivalent to measuring the gravitational constant, which is the fundamental physical constant known with least accuracy, due to the relative weakness of the gravitational force. The mass of the Earth was first measured with any accuracy (within about 20% of the correct value) in the Schiehallion experiment in the 1770s, and within 1% of the modern value in the Cavendish experiment of 1798.
  Unit of mass in astronomy
  The mass of Earth is estimated to be:
  ,
  which can be expressed in terms of solar mass as:
  .
  The ratio of Earth mass to lunar mass has been measured to great accuracy. The current best estimate is:
  The product of  and the universal gravitational constant (G) is known as the geocentric gravitational constant (G) and equals . It is determined using laser ranging data from Earth-orbiting satellites, such as LAGEOS-1.  can also be calculated by observing the motion of the Moon or the period of a pendulum at various elevations, although these methods are less precise than observations of artificial satellites.
  The relative uncertainty of  is just , considerably smaller than the relative uncertainty for  itself.  can be found out only by dividing  by , and  is known only to a relative uncertainty of  so  will have the same uncertainty at best. For this reason and others, astronomers prefer to use , or mass ratios (masses expressed in units of Earth mass or Solar mass) rather than mass in kilograms when referencing and comparing planetary objects.