<P> The gravity model of migration is a model in urban geography derived from Newton's law of gravity, and used to predict the degree of interaction between two places . Newton's law states that: "Any two bodies attract one another with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them ." </P> <P> When used geographically, the words' bodies' and' masses' are replaced by' locations' and' importance' respectively, where importance can be measured in terms of population numbers, gross domestic product, or other appropriate variables . The gravity model of migration is therefore based upon the idea that as the importance of one or both of the location increases, there will also be an increase in movement between them . The farther apart the two locations are, however, the movement between them will be less . This phenomenon is known as distance decay . </P> <P> The gravity model can be used to estimate: </P>

When was the gravity model of spatial interaction made