NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: When is the Autumnal Equinox?
From: Paul Hirose
Date: 2011 Sep 04, 13:56 -0700
From: Paul Hirose
Date: 2011 Sep 04, 13:56 -0700
The equinoxes and solstices are the moments when the geocentric apparent Sun reaches 0, 90, 180, and 270 degrees of ecliptical longitude. See The Astronomical Almanac glossary: http://asa.usno.navy.mil/SecM/Section_M.html In general, declination isn't zero at an equinox. That would require the Sun to be exactly on the ecliptic. But in reality its geocentric coordinates can deviate from the ecliptic by a several tenths of an arc second. This happens because the ecliptic is defined by the motion of the Earth - Moon barycenter, which is far enough from the geocenter to cause noticeable parallax on an approximate monthly cycle. According to my program, the Sun will have 180°00'00.00" geocentric apparent true longitude on 2011 September 23 at 09:04:38 UTC. At that time its ecliptic latitude is -.37" (IAU 2006 precession and 2001A nutation, JPL DE422 ephemeris). Declination is zero at 09:04:17 UTC, 21 seconds earlier. JPL HORIZONS and MICA have different ephemerides and precession - nutation models, but agree with those figures within one second of time. With MICA, the TT time scale should be used, keeping in mind that TT is presently 34 + 32.184 = 66.184 s ahead of UTC. --
