A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert H. van Gent
Date: 2019 Sep 28, 11:05 +0000
Similar values (about 1 second later) can be found in the monthly newsletter (lettre d’information) of the IMCCE (Institut de mécanique céleste et de calcul des éphémérides) at Paris
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Rob van Gent
From: NavList@fer3.com <NavList@fer3.com>
On Behalf Of Paul Hirose
Sent: Fri 27 September 2019 18:46
To: Gent, R.H. van (Rob) <R.H.vanGent@uu.nl>
Subject: [NavList] Equinox, 2019 September
The four different "equinox" times (the third one is the formally
07:49:53 Sun declination = 0
07:50:04 ITRS latitude = 0
07:50:11 ecliptic longitude = 0
07:50:14 right ascension = 12 h
Times are UTC on 2019-09-23.
JPL DE431 ephemeris
IAU 2006 precession
IAU 2000B nutation
0.2029 0.3185 polar motion x, y (arc sec)
If the Sun were exactly on the ecliptic, the first, third, and fourth
events would be simultaneous.
If not for polar motion, declination and ITRS latitude would be zero at
the same time. But Earth's axis of rotation (the celestial pole), which
is the basis of declination, is offset a few tenths of an arc second
from the geodetic (ITRS / WGS84) north pole.
The polar motion parameters I used are from IERS Bulletin A. The numbers
constantly change since the celestial pole follows a quasi circular path
with respect to the crust. One circle takes about 400 days and the
circle is about 20 feet in diameter. It is not centered on the geodetic
north pole, which is well outside the circle.