Did June 30th seem just a little longer than usual? Well, it actually was. It was exactly one second longer than a normal 24 hour day. If you were looking at the website for the National Institute of Standards and Technology (NIST) where the official time for the United States is kept on June 30 at around 7:00 p.m. (Eastern Standard Time or midnight GMT) you would have seen something very strange. As the time counted from 6:59 to 7:00 p.m. – just as it was about to change to 7:00 p.m. – an extra second occurred. The clock read 6:59:58 . . . 6:59:59 . . . 6:59:60 and then 7:00:00. One whole second was slipped in just before 7:00 p.m.!
Why did they do that? What’s going on? Did we slip into some sort of time warp?
Well, no. What they actually did was correct our clocks. It seems that our clocks gradually lose time because the Earth’s rotation is slowing down a little bit. This braking of the Earth’s rotation is mostly caused by the Moon as tidal friction slows down the spin of the Earth. Consequently our days get a tiny bit longer every day.
Atomic clocks are almost one million times more accurate at measuring time than the old fashioned method of measuring time using the Earth’s rotation. The variation in earth’s rotation can be caused by more than just the moon, however. Major Earthquakes, the melting of ice caps, and even gigantic building projects like the filling of China’s Three Gorges Dam can all change the speed of rotation of the Earth. Consequently, if official time keepers didn’t correct our clocks every now and then noon might eventually switch places with midnight. Also other things that require absolute precision in time keeping like stock markets and GPS could begin to have growing difficulties. However, a growing number of concerned scientists are beginning to complain that we should quit playing with time and just skip future leap seconds as we simply adjust over thousands of years to the natural slowing of the Earth.