Time: GMT, Universal, Civil and Atomic

This is an introduction, or summary - not necessarily detailed enough for serious research. Some more details are available at Nautical time and civil date.

Greenwich Mean Time

• Greenwich Mean Time is a time scale based on the apparent motion of the "mean" sun with respect to the meridian through the Old Greenwich Observatory (zero degrees longitude). The "mean" sun is used because time based on the actual or true apparent motion of the sun doesn't "tick" at a constant rate. The earth's orbit is slightly eccentric and the plane of the earth's orbit is inclined with respect to the equator (about 23-1/2 degrees) hence at different times of the year the sun appears to move faster or slower in the sky. That's why an uncorrected sundial can be "wrong" (if it is supposed to be telling mean time) by up to 16 minutes. So if the mean (i.e. corrected) sun is directly over the meridian through Greenwich, it is exactly 12 noon GMT or 12:00 GMT (Prior to 1925, astronomers reckoned mean solar time from noon so that when the mean sun was on the meridian, it was actually 00:00 GMT. This practice arose so that astronomers wouldn't have a change in date during a night's observing. Some in the astronomical community still use the pre-1925 definition of GMT in the analysis of old data although it is recommended that the term Greenwich Mean Astronomical Time now be used to refer to time reckoned from noon.)
• Mean time on selected meridians 15 degrees apart is generally known as standard time. For example, Eastern Standard Time (EST) is the mean solar time of the meridian at 75 degrees W. and Pacific Standard Time (PST) is the mean solar time of the 120 degree meridian West

Universal Time

• In 1928, the International Astronomical Union recommended that the time used in the compilation of astronomical almanacs, essentially GMT, or what was also sometimes called Greenwich Civil Time, be referred to as Universal Time (UT). The terms "Universal Time" and "Universal Day" were introduced at the various conferences in the 1800's held to set up the standard time system.
• There are actually a couple of variants of UT. UT as determined by actual astronomical observations at a particular observatory is known as UT0 ("UT-zero"). It is affected by the motion of the earth's rotation pole with respect to the crust of the earth. If UT0 is corrected for this effect, we get UT1 which is a measure of the true angular orientation of the earth in space. However, because the earth does not spin at exactly a constant rate, UT1 is not a uniform time scale. The variation in UT1 is dominated by seasonal oscillations due primarily to the exchange of angular momentum between the atmosphere and the solid earth and seasonal tides. In an effort to derive a more uniform time scale, scientists established UT2. UT2 is obtained from UT1 by applying an adopted formula that approximates the seasonal oscillations in the earth's rotation. However, due to other variations including those associated with the secular effects of tidal friction (the earth's spin is continually but gradually slowing down), high frequency tides and winds, and the exchange of angular momentum between the earth's core and its shell, UT2 is also not a uniform time scale.

Civil Time

• The notion of Civil Time is therefore based on UT (Universal Time), corrected to the meridian (multiple of 15 degrees corresponding to one whole hour) so that the sun's zenith is reasonably close to 12:00 noon. This can be modified by "Daylight Savings Time" or "Summer Time" which is often employed to artificially give longer daylight hours during the evening; this has to revert during the winter to normal Civil Time, else work hours (particularly school hours for children) start before sunrise.

Atomic Time

• However, given technological and communications advances over the last few years, rather than base our civil time on the rotation of the earth we now use Atomic Time, time based on the extremely constant frequency of a radio emission from cesium atoms when they change between two particular energy states. The unit of Atomic Time is the atomic second. 86,400 atomic seconds define the length of a nominal "reference" day -- the length of the day as given by the earth's rotation around the year 1900. But because of the variations in the earth's spin the length of the actual day can be shorter or longer than the nominal day of 86,400 seconds. The time scale based on the atomic second but corrected every now and again to keep it in approximate sync with the earth's rotation is known as UTC or Coordinated Universal Time. The corrections show up as the leap seconds put into UTC from time to time - usually on New Year's Eve. With these leap second adjustments, UTC is kept within 0.9 seconds of UT1. Currently, the need for leap seconds is primarily due to the effects of tidal friction. The earth's rotation in space is monitored by the International Earth Rotation Service (IERS) in Paris, France, using a global network of satellite and lunar laser ranging, very long baseline interferometry, and Global Positioning System (GPS) stations. The IERS, in consultation with the Bureau International des Poids et Mesures in Sèvres, France, determine when a leap second is needed.