civil time = solar time

From: Rob Seaman <>
Date: Wed, 4 Jan 2006 23:56:35 -0700

I said:

> all parties must certainly agree that civil time (as we know it) IS
> mean solar time.

Ed says:

> saying that it "IS" civil time is probably a bit strong.

"Probably a bit strong" is not precisely a staunch denial.

The argument again:

        A) Pick an unacceptably large amplitude for DUT1. WP7A asserts one
        B) Pick an unacceptably short period to reach that amplitude.
Suspect we could agree on a century, but relax this constraint to a
        C) Divide. 3600 seconds per 3652 days

        QED) Civil time must track mean solar time to better than 1s/day

Or civil time = solar time to better than 0.1s/day if B is a
century. Those who think a secular drift of an hour over a
millennium is too large would set a limit of < 0.01s/day. The raging
lunatics (like myself) who think allowing any significant secular
drift over any historical period is unacceptable would, of course,
reduce this by orders of magnitude.

This is simply a classic exercise in applying epsilon constraints.
In the limit, civil time can be brought arbitrarily close to mean
solar time. More to the point, to ONLY mean solar time (unless the
entire concept of civil time changes and then we're back to day
turning into night).

> 1. local civil time matches apparent solar time roughly

> 2. the relationship between local civil time and apparent solar
> time is constant enough in any one place

> 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

Add those up and you end up back at mean solar time. Numbers 1 & 2
imply that we care about solar time and that annual variations must
average out. (See attached plot.) The precision of typical clocks
is well matched to MST, of course, since the clocks were developed to
convey MST. And 121 years have established that Greenwich Mean Time
is a fine basis for international civil time.

> Mean solar time isn't really of much interest to anybody

(I won't, I won't, I won't... trot out my rant on periodic versus
secular effects again...)

Rather, the often repeated canard that civilians don't give a fig for
the actual position of the sun in the sky implies that it is
precisely apparent solar time that only queer ducks like astronomers
care about. Mean solar time is what civilians DO care about.

Another handle on this is that the "real" day is the sidereal day,
that is, how long the Earth takes to rotate 360 degrees. An
apparent solar day wanders closer and farther from this baseline. A
mean solar day, on the other hand, is simply a constant offset above
the baseline. Finally, the whole reason we're having this discussion
is the 2 ms/century slowing of the Earth's rotation. That slowing is
realized as a slope to the mean solar (or sidereal) baseline. It is
exactly mean solar time that provides us with the tools to reconcile
the lengthening day.

Rob Seaman
National Optical Astronomy Observatory

Received on Wed Jan 04 2006 - 22:57:08 PST

This archive was generated by hypermail 2.3.0 : Sat Sep 04 2010 - 09:44:55 PDT