Ashley Yakeley writes:

...

So UTC and UT1 seem to be more-or-less equivalent, in that they both include leap seconds.

Well, they don't drift apart if that's what you mean. There are no leap seconds in UT1, instead UT1 seconds are of variable length.

Isn't UTC prior to the epoch defined as UT1? I seem to remember reading this, but couldn't find the reference atm. -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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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On Feb 4, 2005, at 8:34 PM, Ashley Yakeley wrote:

In article <6c0017bff5beac04cf224af544c840b4@comcast.net>, Gregory Wright wrote:

...

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.

Did they ever have shorter seconds, 980 or 940ms? UT1 wobbles all over the place.

Yes, on five occasions shorter seconds were used to advance the timescale. These used seconds with lengths 940 ms, 950 ms, 900 ms and 999.8 ms (yes, a 200 microsecond advance). The retarded seconds had lengths of 1020 ms, 1005 ms, 1100 ms and 1001 ms. (There was no retarded second of 1060 ms as I mentioned in my last note; the 60 ms was a clock advancement and I added when I should have subtracted.) Before 1960 20 ms steps were common, happening almost every month. After 1960 until the adoption of UTC 100 ms steps were most common. (The objective was to keep broadcast time signals within 100 ms of UT2. This procedure was followed from 1962 until the introduction of the leap second scheme with UTC in 1972. More info in the Explanatory Supplement to the Astronomical Almanac, section 2.57 et seq.) Greg

-- Ashley Yakeley, Seattle WA

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