An Earth mass (denoted as or , where ⊕ is the standard astronomical symbol for Earth), is a unit of mass equal to the mass of the planet Earth. The current best estimate for the mass of Earth is , with a relative uncertainty of 10−4. It is equivalent to an average density of . Using the nearest metric prefix, the Earth mass is approximately six ronnagrams, or 6.0 Rg. The Earth mass is a standard unit of mass in astronomy that is used to indicate the masses of other planets, including rocky terrestrial planets and exoplanets. One Solar mass is close to Earth masses. The Earth mass excludes the mass of the Moon. The mass of the Moon is about 1.2% of that of the Earth, so that the mass of the Earth+Moon system is close to . Most of the mass is accounted for by iron and oxygen (c. 32% each), magnesium and silicon (c. 15% each), calcium, aluminium and nickel (c. 1.5% each). 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 G product for the Earth is called the geocentric gravitational constant and equals . It is determined using laser ranging data from Earth-orbiting satellites, such as LAGEOS-1. The product can also be calculated by observing the motion of the Moon or the period of a pendulum at various elevations. These methods are less precise than observations of artificial satellites. The relative uncertainty of the geocentric gravitational constant is just , i.e. times smaller than the relative uncertainty for itself. can be found out only by dividing the product by , and is known only to a relative uncertainty of (2014 NIST recommended value), so will have the same uncertainty at best. For this reason and