topherchris

Leap seconds are necessary because time is measured using stable atomic clocks, whereas the rotation of Earth slows down continually, though at a slightly variable rate. Originally, the second was defined as 1/86400 of a mean solar day. This is determined by the rotation of the Earth around its axis and its orbit around the Sun; time was measured by astronomical observations. But the solar day becomes 1.7 ms longer every century, due mainly to the friction associated with tides.