"...The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal gravitational constant, Newton's constant, and colloquially Big G.[1] It should not be confused with "little g" (g), which is the local gravitational field (equivalent to the local acceleration due to gravity), especially that at the Earth's surface; see Earth's gravity and Standard gravity.
According to the law of universal gravitation, the attractive force (F) between two bodies is proportional to the product of their masses (m1 and m2), and inversely proportional to the square of the distance (r) between them:
The constant of proportionality, G, is the gravitational constant.
The gravitational constant is perhaps the most difficult physical constant to measure.[2] In SI units, the 2006 CODATA-recommended value of the gravitational constant is:[3]
Another authoritative estimate is given by the International Astronomical Union (see Standish, 1995).
http://en.wikipedia.org/wiki/Gravitational_constant
http://physics-history.suite101.com/article.cfm/understanding_the_cavendish_experiment
