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

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