# Re: [LEAPSECS] Longer leap second notice

From: Ed Davies <ls_at_edavies.nildram.co.uk>
Date: Wed, 04 Jan 2006 23:14:51 +0000

Rob Seaman wrote:
> ....
> Little support - and again, to a certain level of precision (easily
> better than a second per day), all parties must certainly agree that
> civil time (as we know it) IS mean solar time. ....

We've got a bunch of time scales here:

- apparent solar time.
- mean solar time.
- local civil time.
- international civil time.

The main requirements for local civil time for the bulk of its
users are that:

1. local civil time matches apparent solar time roughly (e.g., the
sun is pretty high in the sky at 12:00 and it's dark at 00:00).

2. the relationship between local civil time and apparent solar
time is constant enough in any one place (e.g., if it's
sometimes noon at 10:45 local somewhere in eastern China then
it should be noon around 10:45 every day there, +/- half an
hour or so).

3. the rate of local civil time is constant at least to the
precision of most clocks and watches.

4. the relationship between local civil time and international
civil time should be predicatable and easy to calculate with
(e.g., round numbers of hours or, if somebody insists, minutes).

Points 1 and 2 are similar but not quite the same. If, for
example, the whole world used GMT then 2 would be true everywhere
but 1 would only be true around the prime meridian.

Mean solar time isn't really of much interest to anybody, except
indirectly. It acts as a more or less constant rate approximation
to apparent solar time (requirements 2 and 3).

If we lived on a planet with an equation of time which gave much
wider swings in the difference between apparent and mean solar
time we might have clocks with variable rates - like they had for
the stage coaches in Britain before GMT except you'd use the
different rates in spring and autumn or whenever, not when you're
going east or west. We'd do that if we thought requirements 1
and 2 trumped 3.

In summary - it is convenient to have a civil timescale which
is both constant rate and a reasonable approximation of apparent
solar time. Mean solar time fits that bill if you don't look
too closely at the constant rate aspects but saying that it "IS"
civil time is probably a bit strong.

Ed.
Received on Wed Jan 04 2006 - 15:18:24 PST

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