Ken Tapping, October 27 2010

Sometimes something strange happens at midnight on December 31. Instead of counting to midnight as 11:59:58, 11:59:59, 00:00:00, "Happy New Year," the electronic clocks acting as international time standards count 11:59:58, 11:59:59, 11:59:60, 00:00:00. An extra second has been sneaked into the time, a "leap second."

From practically the beginning of human history, we have needed to have some understanding of the date and time and, as our societies became increasingly complex, we needed to grasp them more accurately. We found the means to do this in the sky, with the movements of the Sun, Moon and stars. Making a clock and calendar that worked reliably and consistently was not easy, and now, thousands of years later, we are still working on it.

The problems we encounter are due to the year itself, the time taken for the Earth to make a complete orbit around the Sun, which is not actually a whole number of days. Our ancestors knew nothing about the Earth's orbit, but they did see the rhythm of the constellations, which depend upon the Earth's orbit. A day is the time taken for the Earth to rotate once compared with the direction of the Sun.

We needed the date and time in order to stay in step with nature, knowing when to sow, when to reap, when the Nile was going to flood, and when game animals would be passing through or the salmon coming back. Since we like our calendar to have a whole number of days and consistent dates for events such as the beginning of spring, the winter solstice and festivals like Christmas, we needed a method to make that happen. One of the first modifications to our calendar was made by Julius Caesar in 46 BC. He defined a 365-day year with an additional day every four years, making an average year of 365.25 days. In fact, a year is roughly eleven minutes less than that, so errors slowly accumulated. By mediaeval times the calendar had drifted by almost ten days. Needing to keep religious festivals in harmony, Pope Gregory made yet another change to the calendar in 1582. He added days and built upon Caesar's rule by adding one day every four years unless the year is divisible by 400. This change largely dealt with previous errors until we developed atomic clocks, which keep accurate time without depending upon motions of celestial bodies. Pope Gregory's rule was amended by adding a "leap second" as needed to keep the clock and calendar in sync with Nature. This worked well, but then along came satellite navigation systems, like GPS.

GPS satellites are basically orbiting clocks. A GPS navigation device establishes where it is located by looking at how long the signals take to get from several satellites to the receiver. However, when this system was put together, no allowance was made for adjusting the clocks by adding leap seconds, with the additional proviso that doing it wrongly could create chaos or serious accidents.

Now we are faced with a big question - whether to let time separate further from the rhythm of days and years, or to do something to solve the GPS problem. The argument goes on.