For as long as we’ve kept track of time, we’ve needed astronomy. A year was defined by the time taken for Earth to complete one revolution of its orbit around the Sun, a day was defined by the time it takes the Sun to make one full circuit of the sky, and all other units were derived from these two. An hour is one twenty fourth of a day, a minute is a sixtieth of an hour, and a second is a sixtieth of a minute, all of which gives us 86400 seconds in a day. This is still the common understanding of what a second is.
But scientists and engineers today are able to use atomic clocks to define time a little differently. The SI unit for time is the second, and it is defined as the duration of 9,192,631,770 periods of radiation from the transition between two hyperfine levels of the ground state of the cesium-133 atom. Atomic clocks are the most exquisitely sensitive timepieces we have, and we use these as the basis for all timekeeping on Earth. Unfortunately, because the Earth’s motions are slightly irregular, it gradually falls out of sync with our standard time. These tiny variations add up until we start noticing that Noon is no longer happening at the time we expect. As these discrepancies add up, we’ve traditionally inserted a ‘Leap Second’, which means the atomic clocks get set forward or backwards by a single second as required to keep them in line with the astronomical definitions. This causes a lot of grief for people who have to work with such precise measures of time over long periods, as they have to keep track of how many leap seconds have been inserted over the course of their work and remember to factor it into their calculations. Modern digital timepieces and computers also have a problem tracking leap seconds – after all, inserting a leap second means that there will be a single minute with either 59 or 61 seconds – how do you make a clock count that while still showing correct time?
This might all change in January 2012 if the International Telecommunications Union (the organ of the United Nations responsible for defining and regulating matters regarding to communication and also, for historical reasons, timekeeping) agrees to change the definition of Coordinated Universal Time (UTC). The problem they face is that if they choose to simplify the technological problems with leap seconds by simply eliminating them, then standard time will be allowed to tick along unmolested while the physical world we live on will gradually drift off sync. While this has a certain elegance, and allows our measures of time to more truly reflect the chaotic nature of reality, we will start to notice physical consequences gradually accumulating. Over time we’ll stop associating 12am with midnight, international date lines will need to shift. If you thought daylight savings time was bad, well it’s going to have to get worse!
It certainly isn’t an easy decision, but international standards bodies have to juggle pro’s and con’s like this all the time. I’d bet my money on them deciding to go ahead with decoupling standard time from astronomy, and letting the human consequences sort themselves out.