In 1956 , the second was redefined in terms of a year ( the period of the Earth 's revolution around the Sun ) for a particular epoch because , by then , it had become recognized that the Earth 's rotation on its own axis was not sufficiently uniform as a standard of time .
In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun for a particular epoch , because by then it had become recognized that the Earth 's rotation on its own axis was not sufficiently uniform as a standard of time .