| We have all heard of a leap year - that extra day | | | | atomic clock is 1,000,000 times more accurate than the |
| added to the calendar every four years. It may give us | | | | Earths rotation). |
| a longer February but it is also essential in keeping our | | | | Generally the Earth is continually slowing in its rotation |
| calendars and seasons accurate. If the extra day is | | | | (although, inexplicably, every now-and-then it seems to |
| not added to a leap year then eventually (admittedly | | | | speed up) so TAI is of little use for those that wish |
| after over a century) the Winter will begin in July and | | | | their clocks to be in step with the Earth (astronomers |
| the summer will start around Christmas (and vice - | | | | being by far the most vocal of these). |
| versa in the southern hemisphere) because the Earth | | | | So another time scale was developed called |
| takes an extra six hours longer than the 365 days of | | | | Coordinated Universal Time (UTC - again from the |
| a year to circle the sun. | | | | French - Temp Universel Coordonne). This was based |
| A leap year may be a bit of a fudge but the | | | | on atomic time (TAI) but small adjustment are made to |
| alternative would be to have a quarter day at the end | | | | keep it in step with GMT (which incidentally is now |
| of the year which would of course throw our days | | | | commonly referred to as UT1 or depending on time |
| and nights out of sync with each other (and could you | | | | zone UT+1 UT+2 UT+3 etc) |
| imagine just having a six hour day - some of us | | | | UTC is adjusted by the insertion of extra seconds, |
| struggle to get things done in 24!). | | | | called leap seconds, as necessary to keep it within a |
| We have of course always measured time in relation | | | | second of GMT (or UT1). It is possible a second may |
| to the movement of the Earth - a day being an entire | | | | have to be removed in the future but that hasn't |
| revolution, a year an orbit of the sun. However, as our | | | | happened as yet. UTC is essential in modern industry |
| way of measuring time became more and more | | | | and technology where computers are synchronised to |
| accurate it soon became apparent that there were | | | | UTC time, usually through a NTP server (Network |
| more irregularities in the Earth's rotation than just the | | | | Time Protocol) - to allow international time sensitive |
| extra six hours in a year. | | | | transactions. |
| GMT (Greenwich Mean Time) was developed | | | | A leap second is normally inserted at the end of |
| because there was a need for a time scale where | | | | December in the last hour (although occasionally it has |
| the mean position of the sun at noon, averaged | | | | been done in June, March and September). The |
| throughout the year, is above the Greenwich Meridian | | | | decision as to whether a leap second is required is |
| (zero longitude) and daylight saving hours are added or | | | | taken by the Earth Orientation Centre of the |
| taken away depending on the time of year. | | | | International Earth Rotation and Reference Systems |
| However, in 1955 the first atomic clock went into | | | | Service (IERS), who monitor the Earth's rotation and |
| operation following the discovery of the stability of the | | | | suggest the adjustment about six months in advance. |
| caesium-133 atom which vibrated at an exact rate | | | | When a leap second is added there becomes 61 |
| (9,192,631,770 a second). Impressed with this accuracy, | | | | seconds in that final minute of the year. The familiar |
| The International System of Units of Measurement (SI) | | | | 'six pips' radio signal gains an extra pip and even |
| decided that a second should be defined as this | | | | London's famous Big Ben is held back a second |
| number of oscillations of the caesium-133 atom. | | | | before it bongs (but not an extra bong as they are |
| Following the SI second a time scale called International | | | | meant to represent the hours) |
| Atomic Time (TAI - from the French Temp Atomique | | | | There have been 33 leap seconds added to UTC |
| International) which was a simple count, in seconds, for | | | | since 1972 (although the first ten were added |
| the 24 hours of our day. Conversely as TAI is not | | | | retrospectively) but as the Earth's rotation is continuing |
| related to the movement of the Earth, it was soon | | | | to slow it is estimated that over the next millennia or |
| discovered that TAI and atomic clocks were far more | | | | two leap seconds will have to be added each month. |
| stable and reliable than the Earth itself (in fact an | | | | |