The gravitational constant refers to an observed physical behavior or quality that explains the level of gravitational pull between objects. Essentially, any two objects with mass will assert some gravitational pull on one another; it is the force that keeps the Earth spinning around the sun rather than drifting off to nothingness. The gravitational constant, known as G, explains the amount of pull or attraction one object has on another, when multiplied by the mass of the two objects and divided by the square of the distance between the two objects.
The hunt for the gravitational constant preoccupied many of the most brilliant minds in science for much of the 17th and 18th centuries. According to legend, the path to discovering the gravitational constant began with a bet between three eminent scientists of the age — Sir Christopher Wren, Edmund Halley, and Robert Hooke — over the orbital paths of planets. Halley, inspired, choose to visit the venerated professor Isaac Newton for assistance, who revealed not only the correct answer, but that he had solved the problem some time ago but somehow mislaid the notes. Halley pressed Newton to look into the matter again, gaining him considerable credit for inspiring the publication of Newton's Philosophiae Naturalis Principia Mathematica, one of the most enduring scientific works of human history.
Though Newton's Principia theorized the presence of the gravitational constant, it did not answer the question of the mathematical value of G. More than 70 years after Newton's death, a brilliant and fascinatingly eccentric scientist named Sir Henry Cavendish inherited a machine meant to measure the density of the Earth. The machine was the design of another scientist, Reverend John Michell, who died before he could complete his experiments. The fabulously complex machine, which was supposedly so sensitive it needed to be observed in operation from another room to avoid contaminating the results, helped produce not only the density results desired, but also led to future calculations of the gravitational constant.
Cavendish's calculations were not exactly correct, but even with 21st century technology, the gravitational constant remains one of the most difficult physical constants to measure. Scientists have revised the calculations several times throughout the interim centuries, arriving in 2006 at a widely-accepted mathematical expression of G= 6.667428 X 10^{-11} m^{3} kg^{-1} s^{-2}, where M=length in meters, kg=mass in kilograms, and s=time in seconds. With centuries of recalculation behind them and the potential for future centuries filled with more refinements, most scientific explanations add that this equation should still include some margin for error.