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# What is a leap second?

An occasional leap second adjustment ensures our clocks reflect the Earth's rotation speed as accurately as possible.

## Keeping up with the rotation of the Earth

The rotation of the Earth on its axis has served as the basis for timekeeping since the dawn of history. The day was divided into 24 hours, each of 60 minutes, each of 60 seconds. Because the length of the apparent solar day (as shown, for example, by a sundial) varies in a regular way during the year it became necessary to average-out this effect and define a mean solar day. This explains the name Greenwich Mean Time (GMT), a time scale in which the mean position of the sun at noon, averaged over the year, is above the Greenwich meridian (longitude zero).

Over the centuries the accuracy of time measurement has steadily improved enabling us to see irregularities in the Earth's rate of rotation. In effect, duration of the second of Universal Time (UT1, as GMT is now officially known) varies slightly to keep in step with the changes in the Earth's rotation.

### Measuring time using an atomic clock

In 1955 the caesium atomic clock was brought into operation at NPL. It was a much more regular timekeeper than the Earth itself, or indeed any other type of clock then in existence. Constructed by Louis Essen and Jack Parry, it was based on measurements of a particular vibration of the caesium-133 atom. Over the next few years, the frequency of the NPL caesium clock was compared with the astronomical second calculated by the United States Naval Observatory (USNO). As a result of this work, in 1967, the second was defined in the International System of units of measurement (SI) as the duration of 9,192,631,770 periods of the chosen vibration of the caesium-133 atom.

The SI second is used to make a time scale called International Atomic Time (TAI), which is a simple count of SI seconds labelled conventionally using hours, minutes and seconds. As TAI is not linked to the Earth's rotation, a clock based on TAI would gradually become more and more out of step with UT1 and the rotation of the Earth.

### Ensuring a co-ordinated approach

The solution adopted was to construct a second atomic time scale called Coordinated Universal Time, which is abbreviated in all languages as UTC, as the basis of international timekeeping. It combines the regularity of atomic time with most of the convenience of UT1 (or GMT), and many countries have adopted it as the legal basis for time. UTC is adjusted occasionally by small amounts to keep it close to UT1.

### What is a leap second?

From 1 January 1972, the seconds of UTC have been exactly the same length as those of TAI, and they occur at the same instants. UTC is kept always within 0.9 seconds of UT1 by the insertion of an extra second when necessary. These are known as positive leap seconds.

When a leap second is inserted, it is done in the last minute of either December or June, or exceptionally March or September, immediately prior to midnight or 00:00:00 hours UTC. The decision as to whether a leap second is required is taken by the Earth Orientation Center of the International Earth Rotation and Reference Systems Service (IERS), approximately 6 months in advance.

### Could there be a negative leap second in the future?

It could happen that a second would need to be removed from UTC (a negative leap second). So far, all leap seconds have been positive, but increases in the Earth’s rotation rate in recent years have allowed astronomical time (UT1) to catch up with UTC. If this trend continues, a negative leap second might be needed to bring UTC back into alignment with UT1.

### What is a leap day?

We add a leap day on February 29 every year that can be divided by 4 (unless it is also divisible by 100 but not 400). The leap day is an extra day that we add to the shortest month of the year, February, in a leap year. For example, 2020, 2024 and 2028 are all leap years.

It takes the Earth approximately 365 days, 5 hours, 48 minutes and 45 seconds to circle once around the Sun. However, the Gregorian calendar has only 365 days in a normal year. If we didn't add a leap day on February 29 almost every four years, each calendar year would begin about 6 hours earlier than the previous one in relation to Earth's revolution around the Sun.

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