Did we celebrate the new year too early?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

01 January 2006

Happy New Year! 2005 is finally behind us and despite all the calamities that befell the world during the last 12 months (Tsunami, Death of a Pope etc), we saw it fit to extend it a little bit. Yes! 2005 was longer than average – by one second.

The International Earth Rotation and Reference System Service (IERS) made the announcement just before Christmas 2005. Thus as midnight last night approached, the clocks should have read as follows: from 23:59:58 December 31, 2005 to 23:59:59 December 31, 2005 then to 23:59:60 December 31, 2005 and then, 00:00:00 January 1, 2006.

Ordinarily, the counting of the seconds would have gone: 23:59:58 December 31, 2005 – 23:59:59 December 31, 2005 and then, 00:00:00 January 1, 2006. Notice that 23:59:60 does not normally appear on a digital clock.

So, why would we bother to adjust our clocks by one second? Answer: because the Earth is slowing down. Every century, the length of a day increases by about 1.7 milliseconds (ms). That is 1.7 thousandths of second.

The main reason for this is slow down is the effect of the moon revolving around the Earth. The exact mechanics of this process are a long story (preferably left for another day); suffice it to say for now that in another two million years, one day on Earth will be 25 hours, and in 50 million years, it will be about 48 hours long!

But, if the slow-down is so small, why add a whole second now? The second was initially defined as the fraction one divided by 86,400 of the average length of one day for the period between 1750 and 1892. Today however, the second is defined in terms of atomic vibrations.

Atomic clocks do not depend on the motion of the planet. So, when the Earth slows down it becomes necessary to adjust our clocks so that time of day remains the same. The 1.7ms per day, per century may seem small but they can accumulate very rapidly.

One century has about 36,525 days. If we do not add the leap seconds regularly, at the end of 100 years, our clock would lag behind the solar movement by 62 seconds - more than one minute. That is, the 1.7ms per day multiplied by 36,525 days.

Thus it is clear that we need about 60 leap second adjustments every century – i.e., once every 588 days. But life is never that easy. Although in the long term the average lag is 1.7ms per day per century, when observed on a year-to-year basis, the deceleration is quite erratic. Indeed, in some years, the Earth has been observed to speed up!

For this reason, the IERS announces the leap second only when the atomic clocks have lagged behind by more than 0.9second since the previous adjustment and the correction is only done either on December 31 or June 30. The last correction was done on December 31, 1998, and before that it was on June 30 1997.

I hope you didn’t pop the Champaign one second too early last night.