Comment »Posted on Wednesday 31 December 2008 at 11:59 pm by Jacob Aron
In Physics, Space & Astronomy

As another year draws to a close, you’ve got just 61 seconds left of 2008. That’s right, 61. This year, official timekeepers are adding a “leap second” on to the end of the last minute of the last hour of 2008. You might not notice the extra second flit past as you Auld Lang Syne your way in to 2009, but it serves an important purpose.

Our measurement of time used to be based on the movement of the Sun; you got up when it rose and went to bed when it set. Advances in technology meant that timepieces had to become more and more accurate. It starting with the need for coordinated train timetables across a country, meaning that local time just didn’t cut it any more. In the mid-19th century, railway companies around Great Britain adopted Greenwich Mean Time, the familiar GMT. Use by all soon followed.

GMT was still based on the movement of the Sun however, and this is where we hit a problem. The Sun, of course, does not actually move across the sky; it only appears to because the Earth is rotating. The Earth’s rotation is not constant though; changes in the atmosphere or the planet’s molten core can cause it to speed up and slow down.

With the introduction of technology such as GPS positioning and the internet, even more accurate time was needed. Physicists found that oscillation of caesium atoms could be used to define a second; and in 1967 the International System of Units (SI) decreed the duration of 9,192,631,770 such oscillations to be exactly one second.

These so-called atomic clocks are now the de facto standard of time, known as Coordinated Universal Time (UTC). The trouble is, this highly accurate measure doesn’t have any connection to the Earth’s rotation, so if we still want noon to occur when the Sun is at its highest point, corrections to UTC must occasionally be made. Enter the leap second.

Without the occasional leap second (the last was three years ago) UTC would gradually drift away from what we might perceive was “real” time. Eventually, the position of the Sun would have no relation at all to the time, and we can’t be having that. Earlier this year, some scientists proposed that rather than adding a leap second every few years, we should add a leap hour every 6 centuries. This doesn’t sound like the best idea to me – adding a few seconds here and there is easy to slip past people with out too much fuss, but an entire hour? No thanks.

What would we even call such an hour? For those of you with timepieces connected to an atomic clock (like this one, perhaps) might notice the strange occurrence of 23:59:60 before it flicks over to 00:00:00 and the new year. Would a leap hour run from 24:00:00 to 24:59:59? Surely it would cause nothing but problems.

No, a leap second seems to be the way to go. Even though you’re probably not reading this at 11:59pm (and let’s be honest, the sever is posting it at this time, not me!), join me in the rather unusual New Years countdown of 5, 4, 3, 2, 1, 1…


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