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Particle Physics and Astronomy Research Council

Royal Greenwich Observatory

Short Special Information Leaflet No. 15: `Leap Seconds'.

LEAP SECONDS

The passage of time can now be measured with such accuracy that the rotation rate of the Earth can be seen to be variable. This can depend on the seasons (for example as trees grow, this affects the distribution of the mass of the Earth) and can even be dependent on weather conditions such as `El Niño'.

By tradition, the time given by our clocks is related to the position of the Sun in the sky which is determined by the rotation of the Earth. Early clocks used this motion to determine the time and we still have sundials as decorative reminders of this by-gone age.

Since 1955 the most accurate clocks available have used an atomic transition in the gas caesium which defines a very accurately known frequency (there are 919263177 oscillations per second). This frequency is then divided down to give seconds, minutes etc. Several atomic clocks are used to define a local time standard service. There are many separate time services throughout the world and since 1971 a combined mean version of their time measurement is used as International Atomic Time (TAI).

Our normal concept of time is strongly linked to the rotation of the Earth but, unfortunately it does not rotate at a constant rate. In order to keep the apparent position of the Sun in the sky, and hence our concept of the day, in line with Atomic Time a time scale, called Coordinated Universal Time (UTC) has been defined.

UTC is defined as being an integral number of seconds different from TAI so as to keep the Earth's rotation linked with the day. This number of seconds is altered by one each time that the Earth's irregular rotation has produced a difference amounting to a second.

These additional (or perhaps negative) Leap Seconds are added (or subtracted) at either the start of the year (January 1) or half way through the year (July 1).

In recent years leap seconds have been added on the following dates from which the number of seconds differences between UTC and TAI are:

Date TAI-UTC 1990 Jan 1 +25.0 sec
1991 Jan 1 +26.0 sec
1992 Jul 1 +27.0 sec
1993 Jul 1 +28.0 sec
1994 Jul 1 +29.0 sec
1996 Jan 1 +30.0 sec
1997 Jul 1 +31.0 sec

The decision to insert a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS) in Paris. At the time of writing (June 1998) the latest information is that no leap second will be introduced at the end of June 1998 and that until further notice TAI-UTC=+31 sec

Leap seconds are added as an additional second to the last minute of the previous month, ie this minute will have 61 seconds. For example, the successive times each second at the end of 1995 were

1995 Dec 31 23h 59m 59s
1995 Dec 31 23h 59m 60s
1996 Jan 01 00h 00m 00s

So our celebration of the New Year was delayed by one whole second!

See also leaflet on Time. and Sundials

Produced by the Information Services Department of the Royal Greenwich Observatory.
PJA 1996 May 9, revised MJP 1998 June 5

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