We have all heard of a leap year - that extra day added to the calendar every four years. It may give us a longer February but it is also essential in keeping our calendars and seasons accurate. If the extra day is not added to a leap year then eventually (admittedly after over a century) the Winter will begin in July and the summer will start around Christmas (and vice - versa in the southern hemisphere) because the Earth takes an extra six hours longer than the 365 days of a year to circle the sun.



A leap year may be a bit of a fudge but the alternative would be to have a quarter day at the end of the year which would of course throw our days and nights out of sync with each other (and could you imagine just having a six hour day - some of us struggle to get things done in 24!).

We have of course always measured time in relation to the movement of the Earth - a day being an entire revolution, a year an orbit of the sun. However, as our way of measuring time became more and more accurate it soon became apparent that there were more irregularities in the Earth's rotation than just the extra six hours in a year.

GMT (Greenwich Mean Time) was developed because there was a need for a time scale where the mean position of the sun at noon, averaged throughout the year, is above the Greenwich Meridian (zero longitude) and daylight saving hours are added or taken away depending on the time of year.

However, in 1955 the first atomic clock went into operation following the discovery of the stability of the caesium-133 atom which vibrated at an exact rate (9,192,631,770 a second). Impressed with this accuracy, The International System of Units of Measurement (SI) decided that a second should be defined as this number of oscillations of the caesium-133 atom.

Following the SI second a time scale called International Atomic Time (TAI - from the French Temp Atomique International) which was a simple count, in seconds, for the 24 hours of our day. Conversely as TAI is not related to the movement of the Earth, it was soon discovered that TAI and atomic clocks were far more stable and reliable than the Earth itself (in fact an atomic clock is 1,000,000 times more accurate than the Earths rotation).

Generally the Earth is continually slowing in its rotation (although, inexplicably, every now-and-then it seems to speed up) so TAI is of little use for those that wish their clocks to be in step with the Earth (astronomers being by far the most vocal of these).

So another time scale was developed called Coordinated Universal Time (UTC - again from the French - Temp Universel Coordonne). This was based on atomic time (TAI) but small adjustment are made to keep it in step with GMT (which incidentally is now commonly referred to as UT1 or depending on time zone UT+1 UT+2 UT+3 etc)

UTC is adjusted by the insertion of extra seconds, called leap seconds, as necessary to keep it within a second of GMT (or UT1). It is possible a second may have to be removed in the future but that hasn't happened as yet. UTC is essential in modern industry and technology where computers are synchronised to UTC time, usually through a NTP server (Network Time Protocol) - to allow international time sensitive transactions.

A leap second is normally inserted at the end of December in the last hour (although occasionally it has been done in June, March and September). The decision as to whether a leap second is required is taken by the Earth Orientation Centre of the International Earth Rotation and Reference Systems Service (IERS), who monitor the Earth's rotation and suggest the adjustment about six months in advance. When a leap second is added there becomes 61 seconds in that final minute of the year. The familiar 'six pips' radio signal gains an extra pip and even London's famous Big Ben is held back a second before it bongs (but not an extra bong as they are meant to represent the hours)

There have been 33 leap seconds added to UTC since 1972 (although the first ten were added retrospectively) but as the Earth's rotation is continuing to slow it is estimated that over the next millennia or two leap seconds will have to be added each month.

Copyright (c) 2008 Richard Williams