**by Phill Edwards**

There are several definitions of how time is measured.

Time was measured using the position of the Sun using a sundial. A solar day is the time between noon and noon, where noon is when the Sun is at its highest point in the sky. Solar time was measured using the markings on the sundial. Actually, the length of a solar day varies continously due to the Earth's orbit as described by the Equation of Time.

Once clocks had been invented time was measured in terms of a fixed 24 hour day in place of the varying solar day. The International Meridien Conference of 1884 adopted Greenwich Mean Time globally as the time centred on the Greenwich Meridian at 0° longitude.

Coordinated Universal Time replaced GMT as a global time in 1972. The terms UTC and GMT are often used interchangeably.

UTC is based on the Earth's true rotation. The Earth rotation period is getting longer due to tidal effects of the Moon and Sun. Universal time takes this into account by taking measurements based on then distant quasars cross the meridian rather than the Sun. It is measured by the International Earth Rotation and Reference Systems Service (IERS).

Leap secconds were introduced into UTC took ensure that the difference between UT and UTC is always less than 0.9 seconds. They occur at approximately 18 month intervals on June 30 or December 31. A leap second can either be added or subtracted, though all so far have been added. When added the clocks read 23:59:59 UTC, 23:59:60 UTC, 00:00:00 UTC.

There are moves to stop adding leap seconds as it causes problems with computer software which assumes a 86400 second day. A leap second makes the day 86401 seconds long.

Terrestrial time is based International Atomic Time (TAI) as measured by atomic clocks. It was previously called Dynamic Time (TD). Many still prefer the older name. TT is independent on the variations of the Earth's rotational period. TT is used for astronomical calculations. TT = TAI + 32.184s

Delta t is the difference between Terrestrial Time and Universal Time - Δt = TT - UT. It is required to convert astronomical calculatiosn of event times from TD to UT. The value of Δt in 2005 was +64.7 seconds.

The value of Δt is constantly changing. Power series have been determined from observation data which can be used to determine the value of Δt for years past and future. On 2013-01-01 Δt was 66.91s.

To calculate Δt in seconds for a given year. First calculate t = year -
2000.

For years 1986-2005, Δt = 63.86 + 0.3345t - 0.060374t^{2} +
0.0017275t^{3} + 0.000651814t^{4}

For years 2005-2050, Δt = 62.92 + 0.32217t + 0.005589t^{2}