J.A. Garcia-Barreto
Time
Universal Time (from U.S. Naval Observatory)
Rotation of Earth around its own axis is not uniform: some evidence is a) Earth rotates slower
in such a way that the current day is aproximately 16 milliseconds shorter than 1000 years ago;
b) the positions of the north and suth pole change year after year and could contribute upto
30 milliseconds; c) there are regular and irregular fluctuations in the relative inclination
of the Earth axis of rotation that would introduce upto milliseconds per year.
All of these facts contribute to the idea that Earth is an irregular time keeping machine and
a definition of a Universal Time has been developed. Greenwich mean time or Universal Time as is also
known, is the mean solar time as measured on the Greenwich meridian. Greewich meridian is that
half of the great circle on the surface of the Earth passing through the north and south poles
and through the reference point in Greewich, England. It is taken as the line of
longitude 0 degrees. Over the years there are four definitions of Universal Time; those are:
- UT0 is the scale of time generated taken into account the mean solar day.
UT0 takes into account the inclination of the Earth axis and the translation movement around
the Sun
- UT1 is UT0 taking into account the position of the poles (nutation)
- UT2 is UT1 taking into account the slow desacceleration and acceleration of Earth during
different seasons, that is, at different positions on its orbit around the Sun
- UTC is UT2 based on atomic time: it is called Universal Coordinated Time
Local time (Mexico City: Central Standard time)
Click on update to update time (and run another 10 seconds)
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(Created: August 19, 1998. Updated: Dec. 09, 2002)