Years ago, the world's most complicated clock was displayed for the first time in the town hall in Copenhagen, Denmark. It took 40 years and a million dollars to build. The clock was accurate to two-fifths of a second every 300 years. The ten faces of that clock told the time of the day, the day of the week, week of the month, the month of the year, the year of the century, and tracked the movements of planets and suns for 2,500 years. There were parts in the clock that would not move for 2,500 years.

But there was something disturbing about the clock: it was not accurate. It lost two-fifths of a second every 300 years. How did they know that? That clock in the town hall in Copenhagen, Denmark, was measured against the clock of the universe, with all of its myriad parts, from atoms to suns to planets to stars. But that clock is so accurate that every clock on Earth is measured against it.