Pre-1972 UTC seconds and days

From: Ed Davies <ls_at_EDAVIES.NILDRAM.CO.UK>
Date: Thu, 26 Jun 2003 12:15:09 +0100

Prior to 1972-01-01 UTC days were not an integer number of SI
seconds in length. E.g., in the four years immediately prior
to this date:

  TAI-UTC = 4.213 170 0 + (MJD - 39 126) 0.002 592s


This means that a UTC day was roughly two and a half milliseconds
longer than 86 400 SI seconds.

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?

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?

This question is mostly just for curiousity but it is slightly
relevant when people define time scales supposedly based on an
epoch such as 1970-01-01T00:00:00Z. More on this later.

Received on Thu Jun 26 2003 - 04:26:04 PDT

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