Autumn equinox 2014 time

[Alexander Wolf]

This report was posted by Anonymous at the forum on SF: https://sourceforge.net/p/stellarium/discussion/278769/thread/4ee10fea/

According to available sources, the autumn equinox this year takes place on September 23rd at 2:29 UTC. However, Stellarium shows that the Sun reaches a declination of precisely 0 degrees at 2:16 UTC. What is the reason for this discrepancy?

[Alexander Wolf (alexwolf)]

Quick checks:
Time and Date website give September 23rd at 2:29 UTC
Stellarium give September 23rd at 2:16 UTC
SkyChart (Cartes du Ciel) give September 23rd at 2:21 UTC
KStars give September 23rd at 2:29 UTC

Any explanation of this effect?

[Alexander Wolf (alexwolf)]

See discussion for details.

[Worachate Boonplod (worachateb)]

Adding aberration and nutation to the 'equinox of date' position should fix the problem.

[Jon Seamans (silvy5)]

In addition to possible motion corrections, Geocentric Coordinates are necessary for ascertaining equinox data. After all, the equinox is defined wrt the Earth's center. Switching to a geocentric coordinate system (RA/Dec and/or Ecliptic) is a good idea. I believe this alone can account for most of the equinox timing errors.

[Alexander Wolf (alexwolf)]

Precession, nutation and planetocentric coordinates has been added by Georg Zotti. Current date for the autumn equinox of 2014 year takes place on September 23rd at 2:25:09 UTC.