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Frank:

Thanks as always for your cogent and intelligent comments.

> You and George were talking at cross-purposes here.

So it would seem....  ;-)

> namely your comment that we no longer keep time astronomically. I 
> think you're saying this because UTC is allowed to fluctuate from mean 
> solar time by up to 0.9 seconds.

Correct, which is what I meant by "disconnected from solar time" (i.e. the 
atomic timebase is constant for all practical purposes, while the Earth's 
rotation rate isn't).

> But the point which you considered your primary point, noted again 
> above, is certainly correct. Any time scale that we can calculate by a 
> known algorithm from UTC will serve our needs.

Simplify that to say just that ANY time scale that's agreed on by the users 
will work, and I'd agree with that (I think I used a somewhat extreme example 
of calling Noon 1600, or something similar). 

Point is (and it was only intended as a minor side-comment, not something that 
would spark a mini-flame war) that as long as the timescale (however it's 
derived) used for taking sights matches the timescale that's used for the 
sight-reduction data source (almanac, software, whatever), any relation to 
whatever the rest of the world considers "real time" is totally irrelevant. 
After all, time is just one of the starting points for deriving the necessary 
data (i.e. GHA and DEC) to solve the celestial navigation triangle.

And to hopefully close out this part of the, ermmm..... "spirited" discussion, 
my contention is that whether or not we use leap seconds (to correct an 
atomic timescale to "Earth time") is totally irrelevant for our purposes AS 
LONG AS both our source for a time reference for our sextant sights and the 
time published for the sight reduction data are on the same timescale. 
Hopefully we can all agree on that (perhaps abstract) point, if not I'm not 
going to continue flogging what seems to be a dead horse for at least some of 
the list members... ;-)

--
GregR




--- On Sun, 1/11/09, frankreed@HistoricalAtlas.com  wrote:

> From: frankreed@HistoricalAtlas.com 
> Subject: [NavList 6997] Re: Leap seconds
> To: NavList@fer3.com
> Date: Sunday, January 11, 2009, 6:31 PM
> Greg, you wrote:
> "Are you reading this? Because this is EXACTLY what I
> was referring to earlier (i.e. that whatever time scale we
> use as navigators is totally irrelevant - as long as we can
> correlate it to the time in an almanac, or whatever is used
> to obtain the date/time for a celestial event)."
> 
> You and George were talking at cross-purposes here. I
> don't think George objected to the point you're
> making above. He was picking at one small issue: namely your
> comment that we no longer keep time astronomically. I think
> you're saying this because UTC is allowed to fluctuate
> from mean solar time by up to 0.9 seconds. Myself, I would
> have to say, "yes and no" on this point. Clearly
> since it does fluctuate by that amount, and we know that it
> is "wrong" by that amount, then we are not exactly
> keeping astronomical time anymore. It's those banks of
> atomic clocks casting their ballots that determine the exact
> time to the fraction of a second. But at the same time,
> it's the deviation from astronomical time that is used
> to correct UTC so in that sense the Earth's rotation
> casts the final ballot. 
> 
> But the point which you considered your primary point,
> noted again above, is certainly correct. Any time scale that
> we can calculate by a known algorithm from UTC will serve
> our needs. Indeed, it would be easy today to construct
> watches that keep, for example, Greenwich Apparent Time. In
> the days of mechanical clocks, this was considered very
> difficult, and rather pointless, but it's trivial in
> software, so why not? Then we could go back to publishing
> the almanacs as they were before 1834. You would read the
> GAT off your watch and enter the almanac for that time to
> take out your ephemeris data. Functionally the same as
> today. This would let us drop one column of data since the
> GHA of the Sun is then equal to the GAT (converted from
> hours and minutes of time to degrees and minutes of arc...
> in other words, at 0300 GAT, the GHA of the Sun is 45
> degrees exactly). So should we do this? Almost certainly not
> because of the "old book effect". We want to keep
> the vast storehouse of information on navigation published
> within the past fifty years relevant and useful, especially
> since very little is being published new today. So if they
> DO eventually drop leap seconds, rather than using UTC
> (which would differ from GMT at a rate of about 0.5 seconds
> per year), it would probably make good sense to publish an
> annual "DT" correction to add on to UTC as
> broadcast. Then enter the tables published for GMT as usual.
> The alternative would be to do the calculations in UTC
> without leap seconds, which is easier in some way, and then
> you would have to adjust GHA values by DT (converted to arc)
> and also some of the other planning data like sunrise times
> would eventually have to be adjusted.
> 
> -FER
> PS: In your second paragraph, you wrote: "I still
> don't know if you were being troll-ish earlier"
> 
> I think it's best to avoid net-isms like
> "trolling" (I never liked that one anyway). This
> community is very much Internet-lite. Many people here would
> not have the slightest idea that trolling refers to a
> particular type of posting game, and many probably don't
> even know that such a thing exists. Additionally, since
> there are many non-native English speakers, and many others
> who speak English natively but from somewhat different
> cultures, like those crazy upside-down Australians
> (kidding!), it's always a good idea to use language that
> is culturally neutral, even to the point of being dull.
> That's the best way to avoid accidentally offending
> people.
> 
> 
> 
> 

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