Dear Hervé, You are right. I made several mistakes in my previous message. (I should not write messages in a crowded train!) I even got the date wrong; in two places I wrote 2009 instead of 2019!
By great good fortune, I have also heard from Gian Casalegno who confirms what you say. The IMCCE and Horizons results are very close; the Sol et Umbra values are a little different but nothing that should worry a sundial user! Your explanations exactly coincide with Gian's. He also notes that the underlying model used by Sol et Umbra is not quite as precise as more recent models. I think we can say that you have fully solved the puzzle! It is true that we have to use a geocentric observatory which is not a good place to visit :-) Very best wishes Frank On Sun, Oct 6, 2019, 02:17 Hervé Guillemet <guillemet.he...@free.fr> wrote: > Dear Frank, > > > > I am not so convinced of a big difference, at least between JPL and IMCCE. > I used the JPL Horizons simulator *with a geocentric position* because if > you choose a town (on the surface) the results are different. I admit that > the center of the Earth is not the best place to watch the Sun. > > The results seem close to those of IMCCE. > > > Horizons simulator gives : > > ************************************************ > > Date__(UT)__HR:MN R.A. (a-apparent)__DEC > > ************************************************** > > 2019-Sep-23 07:40 11 59 58.47 +00 00 09.6 > > . . . . . . . . > > 2019-Sep-23 07:48 11 59 59.66 +00 00 01.8 > > 2019-Sep-23 07:49 11 59 59.81 *+00 00 00.9* > > 2019-Sep-23 07:50 *11 59 59.96* *-00 00 00.1* > > 2019-Sep-23 07:51 *12 00 00.11* -00 00 01.1 > > 2019-Sep-23 07:52 12 00 00.26 -00 00 02.1 > > . . . . . . . . > > 2019-Sep-23 08:00 12 00 01.46 -00 00 09.8 > > > The RA is increasing by 2.99 s in 20 min, which means 0.01 s in 4,01337 > sec). > Calculating it from 07:40, this means that RA reached 12h at 07:50: > *14.04s* for JPL according to the table and IMCCE gives 07:50 *15.58s* > > > The Dec is decreasing by 19.4 " in 20 min from 07:40 to 08:00 which means > 0.1" in 6.18556 sec. > > Calculating it from 07:40, this means that Dec reached 0° at 07:49 *53,81s > *for JPL and IMCCE gives 07:49:*51,80* > > > This is just an extrapolation of data over 20 min and not the best in > terms of accuracy. > > > > There is a difference, indeed (1.5 and 2s) but it is not huge. This could > come from some of the selected parameters (there are many) or/and the > planetary theory used (VSOP or DE406, etc..) that could be different from > JPL and IMCCE. This does not mean that one is better than the other one. > > I have not investigated Sol et Umbra yet, because of lack of time, but > this could come from the selected location not being the center of the > Earth. > > > I am open to any other better explanation. > > > > Very best regards > > Hervé Guillemet > > > > ------------------------------ > *De: *"Frank King" <f...@cl.cam.ac.uk> > *À: *"Hervé Guillemet" <guillemet.he...@free.fr> > *Cc: *"Sundial list" <sundial@uni-koeln.de>, "Frank King" < > f...@cl.cam.ac.uk> > *Envoyé: *Vendredi 4 Octobre 2019 14:58:54 > *Objet: *Re: Equinoctial Puzzle > > Dear Hervé, > > Congratulations on your comments on my > puzzle about the September Equinox last > month... > > > It seems that the answer to your question > > can be found in the attached picture > > inclosed in a recent information letter > > issued by the French IMCCE institute > > specialised in celestial mechanics and > > ephemerides calculations > > This gives us three times of interest on > 23 September 2009: > > 07:49:51.80 Right Ascension = 12h > > 07:50:11.81 Solar Longitude = 180 > > 07:50:15.58 Solar Declination = 0 > > Call this the FRENCH solution. > > If you have an Android cell 'phone you > can look at Sol et Umbra which gives > these times on 23 September 2009: > > 07:49:49.40 Solar Declination = 0 > > 07:50:09.25 Solar Longitude = 180 > > 07:50:32.50 Right Ascension = 12h > > Note that the events occur in reverse > order! Call this the ITALIAN solution. > > Now use the JPL Horizons program: > > https://ssd.jpl.nasa.gov/horizons.cgi > > I don't have a proper computer at the > moment but here are three values I found > (using my 'phone) for the single time: > > 23 September 2019 07:50:12.00 > > Solar Declination = 00:06:13.3 > > Solar Longitude = 180.0019964 > > Right Ascension = 11:59:01.94 > > The declination has not yet dropped to > zero. > > The longitude has gone past 180. > > The Right Ascension has not yet > reached 12h. > > Call this the U.S. solution. > > Moral: never believe a single source > of information :-) > > If you think you can see the pattern, > try using the Horizons program to > investigate the March Equinox in > 1718. Using the Gregorian Calendar, > we find: > > The Right Ascension went to zero > late on 16 March (just before > midnight). > > The declination went through zero > about the same time on 16 March. > > The solar longitude reached zero > on 21 March. FIVE DAYS LATER!!! > > So you see: there is still a little > bit more of my puzzle to unravel!! > > Very best wishes > Frank > > >
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
