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Dear George,



Thank you for your reply.

I think that I hear what you say. And it does make a lot of sense.

*******

Still, and after further thought on this subject, I am not 100 % sure or 
convinced that your explanation covers every aspect of the subject I 
submitted to our community for the following reasons.

To the best of my knowledge, Astronomers in the past have always endeavoured 
to keep CONTINUOUS time scales so that in any future period, former 
observations could be easily "reworked" (this was Point 7 of [NavList 11087] 
Section I ). For all historical "newtonian" theories, their designers seemed 
to use extremely regular time scales very close to the later scales 
subsequently used (Temps Uniforme by Leverrier, ET by Newcomb, or TT to-day). 
Anyway, did they really have any other choice ???

Actually, I just recently made a number of "historical sampling" computations 
on the BDL Server - 
http://www.imcce.fr/page.php?nav=fr/ephemerides/formulaire/form_ephepos.php 
-(choose DE406, INPOP06, VSOP87 or ELP-2000 for remote periods). On these few 
dozen examples starting by 1700, I have observed that when checking to-day 
the accuracy of the data printed in ancient volumes of Connaissance des 
Temps, the "best fit" to these ancient data almost always happens when you 
nowadays select delta-T = 0 for these remote periods.

This is essentially because this "Best Fit" - just recently performed with 
extremely accurate theories - seems to (almost) always happen with delta-T=0 
that I submitted all my previous comments in [NavList 11087].

Maybe should we probably should run more examples - preferably from ancient 
UK/US Astronomical Almanachs this time - to better ascertain that this 
"best-fit" is still achieved with delta-T=0 for the English Publications.

*******

Other comments welcome :-)   !


*******

Best Regards, and thank you again George for your precious comments


Antoine


Antoine M. Cou�tte


*****************

One last note Regarding your account about Newcomb's "choice" of delta-T.

As regards the value of delta-T being null @ 1895 - a few years later actually 
- I would rather think that it might have been more of a coincidence, rather 
than a deliberate choice by Simon Newcomb. Historically, continuity between 
sucessive time-scales has been regarded as extremely important and I am not 
sure that for this reason Newcomb had some kind choice - if any - to assign 
any value then to delta-T. Additionnally, was it also not just a bit early to 
assign such a value while its concept had not been not fully proven then ? 
Newcomb did definitely suspect that UT irregularities would account for a 
number of unexplained discrepancies. There were so many and of such nature 
that one could hardly attribute them to insufficient theories. Newcomb did 
create himself the momentum for the subsequent huge research undertaken after 
him. Only by the late 1930's - 17 years after his departure - was it 
conclusively demonstrated that UT is not a sufficiently regular timescale. 
All this is extremely well explained in Chapter VII of "Astronomie G�n�rale" 
by Andr� Danjon. Unfortunately for most of us, it is written in French. I was 
not aware of such an excellent treaty on Astronomy until in 1981 or 1982 when 
the USN Nautical Almanach Director himself, Dr. Leroy E. Doggett personnally 
recommended that I should get a copy of this book. Never any regret since for 
this purchase.

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