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Time References
| Fundamentals | |
|---|---|
| Title | Time References |
| Author(s) | J. Sanz Subirana, JM. Juan Zornoza and M. Hernandez-Pajares, University of Catalunia, Spain. |
| Level | Basic |
| Year of Publication | 2011 |
Introduction
Our everyday life follows the alternation of day and night, and seasons of year, and then the initial conception of time was based on the motion of sun. Nevertheless, as science and technology evolved, more precise uniform and well-defined time scales were needed. Several time references are currently adopted, based on different periodic processes associated with earth rotation, celestial mechanics or transitions between energetic levels in atomic oscillators. Table 1 summarises the different times systems used, with their associated periodic process.
Universal and Sidereal times are associated with earth daily rotation. Universal time (solar time) uses the sun as a reference. Sidereal time uses the Vernal Equinox (the Aries point[footnote 1]). This leads to the fact that, in a year, both times differ by 24 hours (one lap) and, thence, the sidereal day is shorter than the solar day by 3m 56s4. That is:
<blocktext>1 mean sidereal day = 1 mean solar day − 3m56s· 4</blocktext>
Universal time UT (UT0, UT1, UT2) is not completely uniform2. So, the Atomic Time TAI was introduced to achieve a more uniform time scale. TAI and UT accumulate a drift along time. Due to this, UTC was defined, which is an atomic time that keeps itself within 0.9 seconds from UT1. It is done by the systematic introduction of a certain number of seconds called Leap Seconds3. This causes that, along time, the difference between UTC and TAI varies in integer leaps of 1 second. UTC is generated by the Bureau International des Poids et Mesures (BIPM), located in Paris. The leap seconds are determined by the International Earth Rotation and Reference Systems Service (IERS).
UTC is obtained from about 250 cesium clocks and hydrogen masers located at about 65 different laboratories, distributed around the world, and applying a set of algorithms to assure an uniform time. It is not determined in real time, but generated with a delay of about half month. Real-time estimates of UTC are computed and provided by different centres, as the UTC(USNO), from the United States Naval Observatory (USNO), the UTC(NIST), from the US National Institute of Standards and Technology (NIST), and the UTC(SU) from Russia (SU stands for the former Soviet Union). The following relations are met (see details in article appendix Transformations between Time Systems:
Notes
- ^ Aries point is a fictitious direction pointing to the Aries Constellation about 2000 years ago. Due to the equinox precession, Aries point is continuously moving over the ecliptic (apparent orbit of sun relative to earth) by about 50′′26 each year, therefore in 2000 years it goes over an arc of 30° in a retrograde way. Currently, Aries point is going out of Pisces Constellation and entering Aquarius.