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Emily LakdawallaSeptember 21, 2005

A debate about time

I received a press release in my inbox this morning that made me think. It came from the Royal Astronomical Society, and was titled "RAS Statement on Proposed Abolition of Leap Seconds." It seems there's a debate going on between many different stakeholders -- most of whom I hadn't heard of -- on how exactly to keep time on Earth. There are already two distinct standards that scientists use for timekeeping. One of them is the one I use in the heading to each daily weblog entry, Coordinated Universal Time (UTC). UTC is essentially a high-precision ordinary clock time. It is synchronized to the rotation of the Earth. Anybody who studies phenomena on the surface of Earth, or in Earth's skies (which brings in meteorologists, astronomers, and anyone working with Earth-orbiting satellites) uses Coordinated Universal Time. All civil clocks are based on UTC.

The problem with UTC is that it's based on a changing standard. Earth's rotation rate is not constant. What's more, the tidal interaction between the Moon and Earth is slowing down Earth's rotation rate by about 2 milliseconds per day per century. As a result, the timekeepers responsible for UTC have to insert "leap seconds" from time to time in order to keep UTC synchronized with Earth's rotation rate. The Royal Astronomical Society statement says, "There have been 21 leap seconds since 1972 and the next is planned at the end of 2005. Their use is determined by the International Earth Rotation Service." (Which makes me wonder who works for the International Earth Rotation Service. That would be a funny thing to have on one's business card.)

The other standard is International Atomic Time (TAI). According to the Royal Astronomical Society statement, this is a "contuinuous, uniform time scale, derived from atomic clocks, which are accurate to within one second per three million years. It is the ideal time scale for scientific use, but it is not practical for everyday use since it is not linked to the rotation of the Earth and the actual length of the day. Since it has not been changed since 1958, there is now a 32-second time difference between UTC and TAI." It's nice to have a timekeeping standard that doesn't have fits and starts like UTC does, especially if you care about millisecond-level precision timing.

So what has the Royal Astronomical Society in a tizzy is that the International Telecommunications Union is proposing to abolish leap seconds. This would help those who care about precision timing applications but would cause UTC -- and all of the clocks in the world -- to drift with respect to mean solar time. Most of us on the ground wouldn't notice the difference right away, but those seconds would build up over the decades, and our civil time scale would slowly diverge from the rotation of the Earth. This would be a tremendous headache for astronomers, who could certainly base their work on TAI, but for whom the heavens would be moving at a different rate than the clocks on their walls and computers.

As astonishing as this proposal sounds, I think it's true that civil time scales are drifting with respect to solar time scales now. More and more, people are working at early or late hours. Whole industries have sprung up in Asia that work at night to do telephone services to North America and Europe during their days. I myself frequently have to rise very early in order to connect with folks in the U.K. or Germany or Russia or Sri Lanka. As there is more global cooperation, we have to be prepared to work at all hours. We had to work at all hours to support the rovers in their own, separate local solar time zone on Mars. It's a funny idea, that human civilization could one day drift free of solar timekeeping. But farmers and astronomers will always care what the local solar time is!

Read more: Earth

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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