Newton's law of gravitation is a statement that any particle of matter in
the universe attracts any other with a force varying directly as the
product of the masses and inversely as the square of the distance between
them. In symbols, the magnitude of the attractive force F is equal to G
(the gravitational constant, a number the size of which depends on the
system of units used and which is a universal constant) multiplied by the
product of the masses (m_{1} and m_{2}) and divided by the
square of the distance R (F = G(m_{1}
m_{2})/R^{2}). Isaac Newton put forward the law in 1687 and
used it to explain the observed motions of the planets and their moons,
which had been reduced to mathematical form by Johannes Kepler early in
the 17th century.

*Excerpt from the Encyclopedia Britannica without permission.*