Dear all,
Thank you Frank for your puzzle. 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 : https://www.imcce.fr/lettre-information/archives/160 Your data is compatible with the information on the picture. We need to distinguish the true ecliptic path from the apparent orbit of the Sun's center. The text says : By definition, the autumn equinox moment in the northern hemisphere is the time when the apparent geocentric longitude of the Sun’s center is 180 degrees. At this moment, the right ascension is not exactly equal to 12 hours and the declination of the Sun’s center is not zero, because the apparent latitude of the Sun’s center is not zero, but these last two values are close to zero. The direction of the center of the Sun is then very close to the direction opposite to the gamma point, intersection of the ecliptic and the celestial equator. The definition of this direction is therefore unique on the celestial sphere. We must not confuse the direction of the autumn equinox which is unique with the fact that the Sun passes through this direction. Thus, in the northern hemisphere, the beginning of autumn corresponds to the passage of the Sun in the direction of the autumn equinox, whereas this same phenomenon reflects the beginning of spring in the southern hemisphere. Kind regards Hervé Guillemet
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