Re: System.Time.Clock Design Issues

On Feb 4, 2005, at 7:00 AM, Ashley Yakeley wrote:

In article <87vf9835x7.fsf@sefirot.ii.uib.no>, Ketil Malde wrote:

...

Isn't UTC prior to the epoch defined as UT1? I seem to remember reading this, but couldn't find the reference atm.

I believe civil time prior to 1972 (which is more a date of establishing UTC than an epoch) was defined as UT1. It was called "Greenwich Mean Time" in those days.

Actually, before 1972 civil time was complicated. "Greenwich Mean Time", referring to the timescale used in celestial navigation, was basically UT1. Civil time in the United States from the 1950s until the introduction of UTC was determined by the time signal distributed by the Master Clock at the US Naval Observatory, with frequent 20 millisecond adjustments (and a few 60 millisecond adjustments) to keep the civil timescale close to UT1. The time signal was broadcast by the radio station WWV. A table of these adjustments can be found in the revised edition of the Explanatory Supplement to the Astronomical Almanac. In essence, civil time in the US before the introduction of UTC had seconds which were usually 1000 SI milliseconds long, some that were 1020 SI milliseconds and a few that were 1060 SI milliseconds. The Naval Observatory Master Clock kept a timescale that closely approximated what was to become TAI, i.e., it kept SI seconds and did not have a phase discontinuity when TAI was introduced in 1972. Best Wishes, Greg

-- Ashley Yakeley, Seattle WA

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