Re: Pre-1972 UTC seconds and days

From: Steve Allen <>
Date: Thu, 26 Jun 2003 08:36:57 -0700

On Thu 2003-06-26T12:15:09 +0100, Ed Davies hath writ:
> Here's the question: was a UTC second the same as an SI second
> so that days were a non-integral number of UTC seconds or was
> the UTC second slightly longer than the SI second?

The best table that I've seen for this applies only to the WWV
transmissions in the US, and it is located on pages 86-87 of the
Explanatory Supplement to the Astronomical Almanac edited by P.K.
Seidelmann. It shows the changes in the frequency of WWV (before
1960) as well as UTC (after 1960) and the >= 200 ms steps that were
introduced. If this table is online it would be nice to know.

When the CCIR stepped in and created leap seconds instead of the
older scheme was one of the technically most brilliant acts ever
done, even though it did ignore all the cries of the scientific
unions for consensus.

> Putting the same question another way: was the MJD date
> considered to be an integer or a real day number? I.e., did
> TAI-UTC vary during the course of a single UTC day?

For practical purposes it still does. See

Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064      Voice: +1 831 459 3046
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93
Received on Thu Jun 26 2003 - 08:37:08 PDT

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